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diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml new file mode 100644 index 0000000..beae516 --- /dev/null +++ b/doc/src/sgml/ref/alter_table.sgml @@ -0,0 +1,1644 @@ +<!-- +doc/src/sgml/ref/alter_table.sgml +PostgreSQL documentation +--> + +<refentry id="sql-altertable"> + <indexterm zone="sql-altertable"> + <primary>ALTER TABLE</primary> + </indexterm> + + <refmeta> + <refentrytitle>ALTER TABLE</refentrytitle> + <manvolnum>7</manvolnum> + <refmiscinfo>SQL - Language Statements</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>ALTER TABLE</refname> + <refpurpose>change the definition of a table</refpurpose> + </refnamediv> + + <refsynopsisdiv> +<synopsis> +ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] + <replaceable class="parameter">action</replaceable> [, ... ] +ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] + RENAME [ COLUMN ] <replaceable class="parameter">column_name</replaceable> TO <replaceable class="parameter">new_column_name</replaceable> +ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] + RENAME CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> TO <replaceable class="parameter">new_constraint_name</replaceable> +ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> + RENAME TO <replaceable class="parameter">new_name</replaceable> +ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> + SET SCHEMA <replaceable class="parameter">new_schema</replaceable> +ALTER TABLE ALL IN TABLESPACE <replaceable class="parameter">name</replaceable> [ OWNED BY <replaceable class="parameter">role_name</replaceable> [, ... ] ] + SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable> [ NOWAIT ] +ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> + ATTACH PARTITION <replaceable class="parameter">partition_name</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT } +ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> + DETACH PARTITION <replaceable class="parameter">partition_name</replaceable> + +<phrase>where <replaceable class="parameter">action</replaceable> is one of:</phrase> + + ADD [ COLUMN ] [ IF NOT EXISTS ] <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">data_type</replaceable> [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">column_constraint</replaceable> [ ... ] ] + DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="parameter">column_name</replaceable> [ RESTRICT | CASCADE ] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="parameter">data_type</replaceable> [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ USING <replaceable class="parameter">expression</replaceable> ] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET DEFAULT <replaceable class="parameter">expression</replaceable> + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP DEFAULT + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET | DROP } NOT NULL + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP EXPRESSION [ IF EXISTS ] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <replaceable>sequence_option</replaceable> | RESTART [ [ WITH ] <replaceable class="parameter">restart</replaceable> ] } [...] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP IDENTITY [ IF EXISTS ] + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STATISTICS <replaceable class="parameter">integer</replaceable> + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] ) + ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN } + ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ] + ADD <replaceable class="parameter">table_constraint_using_index</replaceable> + ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] + VALIDATE CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> + DROP CONSTRAINT [ IF EXISTS ] <replaceable class="parameter">constraint_name</replaceable> [ RESTRICT | CASCADE ] + DISABLE TRIGGER [ <replaceable class="parameter">trigger_name</replaceable> | ALL | USER ] + ENABLE TRIGGER [ <replaceable class="parameter">trigger_name</replaceable> | ALL | USER ] + ENABLE REPLICA TRIGGER <replaceable class="parameter">trigger_name</replaceable> + ENABLE ALWAYS TRIGGER <replaceable class="parameter">trigger_name</replaceable> + DISABLE RULE <replaceable class="parameter">rewrite_rule_name</replaceable> + ENABLE RULE <replaceable class="parameter">rewrite_rule_name</replaceable> + ENABLE REPLICA RULE <replaceable class="parameter">rewrite_rule_name</replaceable> + ENABLE ALWAYS RULE <replaceable class="parameter">rewrite_rule_name</replaceable> + DISABLE ROW LEVEL SECURITY + ENABLE ROW LEVEL SECURITY + FORCE ROW LEVEL SECURITY + NO FORCE ROW LEVEL SECURITY + CLUSTER ON <replaceable class="parameter">index_name</replaceable> + SET WITHOUT CLUSTER + SET WITHOUT OIDS + SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable> + SET { LOGGED | UNLOGGED } + SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) + RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] ) + INHERIT <replaceable class="parameter">parent_table</replaceable> + NO INHERIT <replaceable class="parameter">parent_table</replaceable> + OF <replaceable class="parameter">type_name</replaceable> + NOT OF + OWNER TO { <replaceable class="parameter">new_owner</replaceable> | CURRENT_USER | SESSION_USER } + REPLICA IDENTITY { DEFAULT | USING INDEX <replaceable class="parameter">index_name</replaceable> | FULL | NOTHING } + +<phrase>and <replaceable class="parameter">partition_bound_spec</replaceable> is:</phrase> + +IN ( <replaceable class="parameter">partition_bound_expr</replaceable> [, ...] ) | +FROM ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] ) + TO ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] ) | +WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REMAINDER <replaceable class="parameter">numeric_literal</replaceable> ) + +<phrase>and <replaceable class="parameter">column_constraint</replaceable> is:</phrase> + +[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ] +{ NOT NULL | + NULL | + CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] | + DEFAULT <replaceable>default_expr</replaceable> | + GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED | + GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] | + UNIQUE <replaceable class="parameter">index_parameters</replaceable> | + PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> | + REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] + [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] } +[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] + +<phrase>and <replaceable class="parameter">table_constraint</replaceable> is:</phrase> + +[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ] +{ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] | + UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> | + PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> | + EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] | + FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ] + [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] } +[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] + +<phrase>and <replaceable class="parameter">table_constraint_using_index</replaceable> is:</phrase> + + [ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ] + { UNIQUE | PRIMARY KEY } USING INDEX <replaceable class="parameter">index_name</replaceable> + [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] + +<phrase><replaceable class="parameter">index_parameters</replaceable> in <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and <literal>EXCLUDE</literal> constraints are:</phrase> + +[ INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] +[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ] +[ USING INDEX TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> ] + +<phrase><replaceable class="parameter">exclude_element</replaceable> in an <literal>EXCLUDE</literal> constraint is:</phrase> + +{ <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] +</synopsis> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para> + <command>ALTER TABLE</command> changes the definition of an existing table. + There are several subforms described below. Note that the lock level required + may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is + acquired unless explicitly noted. When multiple subcommands are given, the + lock acquired will be the strictest one required by any subcommand. + + <variablelist> + <varlistentry> + <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term> + <listitem> + <para> + This form adds a new column to the table, using the same syntax as + <xref linkend="sql-createtable"/>. If <literal>IF NOT EXISTS</literal> + is specified and a column already exists with this name, + no error is thrown. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DROP COLUMN [ IF EXISTS ]</literal></term> + <listitem> + <para> + This form drops a column from a table. Indexes and + table constraints involving the column will be automatically + dropped as well. + Multivariate statistics referencing the dropped column will also be + removed if the removal of the column would cause the statistics to + contain data for only a single column. + You will need to say <literal>CASCADE</literal> if anything outside the table + depends on the column, for example, foreign key references or views. + If <literal>IF EXISTS</literal> is specified and the column + does not exist, no error is thrown. In this case a notice + is issued instead. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET DATA TYPE</literal></term> + <listitem> + <para> + This form changes the type of a column of a table. Indexes and + simple table constraints involving the column will be automatically + converted to use the new column type by reparsing the originally + supplied expression. + The optional <literal>COLLATE</literal> clause specifies a collation + for the new column; if omitted, the collation is the default for the + new column type. + The optional <literal>USING</literal> + clause specifies how to compute the new column value from the old; + if omitted, the default conversion is the same as an assignment + cast from old data type to new. A <literal>USING</literal> + clause must be provided if there is no implicit or assignment + cast from old to new type. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term> + <listitem> + <para> + These forms set or remove the default value for a column (where + removal is equivalent to setting the default value to NULL). The new + default value will only apply in subsequent <command>INSERT</command> + or <command>UPDATE</command> commands; it does not cause rows already + in the table to change. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term> + <listitem> + <para> + These forms change whether a column is marked to allow null + values or to reject null values. + </para> + + <para> + <literal>SET NOT NULL</literal> may only be applied to a column + provided none of the records in the table contain a + <literal>NULL</literal> value for the column. Ordinarily this is + checked during the <literal>ALTER TABLE</literal> by scanning the + entire table; however, if a valid <literal>CHECK</literal> constraint is + found which proves no <literal>NULL</literal> can exist, then the + table scan is skipped. + </para> + + <para> + If this table is a partition, one cannot perform <literal>DROP NOT NULL</literal> + on a column if it is marked <literal>NOT NULL</literal> in the parent + table. To drop the <literal>NOT NULL</literal> constraint from all the + partitions, perform <literal>DROP NOT NULL</literal> on the parent + table. Even if there is no <literal>NOT NULL</literal> constraint on the + parent, such a constraint can still be added to individual partitions, + if desired; that is, the children can disallow nulls even if the parent + allows them, but not the other way around. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DROP EXPRESSION [ IF EXISTS ]</literal></term> + <listitem> + <para> + This form turns a stored generated column into a normal base column. + Existing data in the columns is retained, but future changes will no + longer apply the generation expression. + </para> + + <para> + If <literal>DROP EXPRESSION IF EXISTS</literal> is specified and the + column is not a stored generated column, no error is thrown. In this + case a notice is issued instead. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY</literal></term> + <term><literal>SET GENERATED { ALWAYS | BY DEFAULT }</literal></term> + <term><literal>DROP IDENTITY [ IF EXISTS ]</literal></term> + <listitem> + <para> + These forms change whether a column is an identity column or change the + generation attribute of an existing identity column. + See <xref linkend="sql-createtable"/> for details. + Like <literal>SET DEFAULT</literal>, these forms only affect the + behavior of subsequent <command>INSERT</command> + and <command>UPDATE</command> commands; they do not cause rows + already in the table to change. + </para> + + <para> + If <literal>DROP IDENTITY IF EXISTS</literal> is specified and the + column is not an identity column, no error is thrown. In this case a + notice is issued instead. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET <replaceable>sequence_option</replaceable></literal></term> + <term><literal>RESTART</literal></term> + <listitem> + <para> + These forms alter the sequence that underlies an existing identity + column. <replaceable>sequence_option</replaceable> is an option + supported by <xref linkend="sql-altersequence"/> such + as <literal>INCREMENT BY</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET STATISTICS</literal></term> + <listitem> + <para> + This form + sets the per-column statistics-gathering target for subsequent + <xref linkend="sql-analyze"/> operations. + The target can be set in the range 0 to 10000; alternatively, set it + to -1 to revert to using the system default statistics + target (<xref linkend="guc-default-statistics-target"/>). + For more information on the use of statistics by the + <productname>PostgreSQL</productname> query planner, refer to + <xref linkend="planner-stats"/>. + </para> + <para> + <literal>SET STATISTICS</literal> acquires a + <literal>SHARE UPDATE EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )</literal></term> + <term><literal>RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )</literal></term> + <listitem> + <para> + This form sets or resets per-attribute options. Currently, the only + defined per-attribute options are <literal>n_distinct</literal> and + <literal>n_distinct_inherited</literal>, which override the + number-of-distinct-values estimates made by subsequent + <xref linkend="sql-analyze"/> + operations. <literal>n_distinct</literal> affects the statistics for the table + itself, while <literal>n_distinct_inherited</literal> affects the statistics + gathered for the table plus its inheritance children. When set to a + positive value, <command>ANALYZE</command> will assume that the column contains + exactly the specified number of distinct nonnull values. When set to a + negative value, which must be greater + than or equal to -1, <command>ANALYZE</command> will assume that the number of + distinct nonnull values in the column is linear in the size of the + table; the exact count is to be computed by multiplying the estimated + table size by the absolute value of the given number. For example, + a value of -1 implies that all values in the column are distinct, while + a value of -0.5 implies that each value appears twice on the average. + This can be useful when the size of the table changes over time, since + the multiplication by the number of rows in the table is not performed + until query planning time. Specify a value of 0 to revert to estimating + the number of distinct values normally. For more information on the use + of statistics by the <productname>PostgreSQL</productname> query + planner, refer to <xref linkend="planner-stats"/>. + </para> + <para> + Changing per-attribute options acquires a + <literal>SHARE UPDATE EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term> + <literal>SET STORAGE</literal> + <indexterm> + <primary>TOAST</primary> + <secondary>per-column storage settings</secondary> + </indexterm> + </term> + <listitem> + <para> + This form sets the storage mode for a column. This controls whether this + column is held inline or in a secondary <acronym>TOAST</acronym> table, and + whether the data + should be compressed or not. <literal>PLAIN</literal> must be used + for fixed-length values such as <type>integer</type> and is + inline, uncompressed. <literal>MAIN</literal> is for inline, + compressible data. <literal>EXTERNAL</literal> is for external, + uncompressed data, and <literal>EXTENDED</literal> is for external, + compressed data. <literal>EXTENDED</literal> is the default for most + data types that support non-<literal>PLAIN</literal> storage. + Use of <literal>EXTERNAL</literal> will make substring operations on + very large <type>text</type> and <type>bytea</type> values run faster, + at the penalty of increased storage space. Note that + <literal>SET STORAGE</literal> doesn't itself change anything in the table, + it just sets the strategy to be pursued during future table updates. + See <xref linkend="storage-toast"/> for more information. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]</literal></term> + <listitem> + <para> + This form adds a new constraint to a table using the same constraint + syntax as <xref linkend="sql-createtable"/>, plus the option <literal>NOT + VALID</literal>, which is currently only allowed for foreign key + and CHECK constraints. + </para> + + <para> + Normally, this form will cause a scan of the table to verify that all + existing rows in the table satisfy the new constraint. But if + the <literal>NOT VALID</literal> option is used, this + potentially-lengthy scan is skipped. The constraint will still be + enforced against subsequent inserts or updates (that is, they'll fail + unless there is a matching row in the referenced table, in the case + of foreign keys, or they'll fail unless the new row matches the + specified check condition). But the + database will not assume that the constraint holds for all rows in + the table, until it is validated by using the <literal>VALIDATE + CONSTRAINT</literal> option. + See <xref linkend="sql-altertable-notes"/> below for more information + about using the <literal>NOT VALID</literal> option. + </para> + + <para> + Although most forms of <literal>ADD + <replaceable class="parameter">table_constraint</replaceable></literal> + require an <literal>ACCESS EXCLUSIVE</literal> lock, <literal>ADD + FOREIGN KEY</literal> requires only a <literal>SHARE ROW + EXCLUSIVE</literal> lock. Note that <literal>ADD FOREIGN KEY</literal> + also acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock on the + referenced table, in addition to the lock on the table on which the + constraint is declared. + </para> + + <para> + Additional restrictions apply when unique or primary key constraints + are added to partitioned tables; see <xref linkend="sql-createtable"/>. + Also, foreign key constraints on partitioned + tables may not be declared <literal>NOT VALID</literal> at present. + </para> + + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ADD <replaceable class="parameter">table_constraint_using_index</replaceable></literal></term> + <listitem> + <para> + This form adds a new <literal>PRIMARY KEY</literal> or <literal>UNIQUE</literal> + constraint to a table based on an existing unique index. All the + columns of the index will be included in the constraint. + </para> + + <para> + The index cannot have expression columns nor be a partial index. + Also, it must be a b-tree index with default sort ordering. These + restrictions ensure that the index is equivalent to one that would be + built by a regular <literal>ADD PRIMARY KEY</literal> or <literal>ADD UNIQUE</literal> + command. + </para> + + <para> + If <literal>PRIMARY KEY</literal> is specified, and the index's columns are not + already marked <literal>NOT NULL</literal>, then this command will attempt to + do <literal>ALTER COLUMN SET NOT NULL</literal> against each such column. + That requires a full table scan to verify the column(s) contain no + nulls. In all other cases, this is a fast operation. + </para> + + <para> + If a constraint name is provided then the index will be renamed to match + the constraint name. Otherwise the constraint will be named the same as + the index. + </para> + + <para> + After this command is executed, the index is <quote>owned</quote> by the + constraint, in the same way as if the index had been built by + a regular <literal>ADD PRIMARY KEY</literal> or <literal>ADD UNIQUE</literal> + command. In particular, dropping the constraint will make the index + disappear too. + </para> + + <para> + This form is not currently supported on partitioned tables. + </para> + + <note> + <para> + Adding a constraint using an existing index can be helpful in + situations where a new constraint needs to be added without blocking + table updates for a long time. To do that, create the index using + <command>CREATE INDEX CONCURRENTLY</command>, and then install it as an + official constraint using this syntax. See the example below. + </para> + </note> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ALTER CONSTRAINT</literal></term> + <listitem> + <para> + This form alters the attributes of a constraint that was previously + created. Currently only foreign key constraints may be altered. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>VALIDATE CONSTRAINT</literal></term> + <listitem> + <para> + This form validates a foreign key or check constraint that was + previously created as <literal>NOT VALID</literal>, by scanning the + table to ensure there are no rows for which the constraint is not + satisfied. Nothing happens if the constraint is already marked valid. + (See <xref linkend="sql-altertable-notes"/> below for an explanation + of the usefulness of this command.) + </para> + <para> + This command acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DROP CONSTRAINT [ IF EXISTS ]</literal></term> + <listitem> + <para> + This form drops the specified constraint on a table, along with + any index underlying the constraint. + If <literal>IF EXISTS</literal> is specified and the constraint + does not exist, no error is thrown. In this case a notice is issued instead. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] TRIGGER</literal></term> + <listitem> + <para> + These forms configure the firing of trigger(s) belonging to the table. + A disabled trigger is still known to the system, but is not executed + when its triggering event occurs. For a deferred trigger, the enable + status is checked when the event occurs, not when the trigger function + is actually executed. One can disable or enable a single + trigger specified by name, or all triggers on the table, or only + user triggers (this option excludes internally generated constraint + triggers such as those that are used to implement foreign key + constraints or deferrable uniqueness and exclusion constraints). + Disabling or enabling internally generated constraint triggers + requires superuser privileges; it should be done with caution since + of course the integrity of the constraint cannot be guaranteed if the + triggers are not executed. + </para> + + <para> + The trigger firing mechanism is also affected by the configuration + variable <xref linkend="guc-session-replication-role"/>. Simply enabled + triggers (the default) will fire when the replication role is <quote>origin</quote> + (the default) or <quote>local</quote>. Triggers configured as <literal>ENABLE + REPLICA</literal> will only fire if the session is in <quote>replica</quote> + mode, and triggers configured as <literal>ENABLE ALWAYS</literal> will + fire regardless of the current replication role. + </para> + + <para> + The effect of this mechanism is that in the default configuration, + triggers do not fire on replicas. This is useful because if a trigger + is used on the origin to propagate data between tables, then the + replication system will also replicate the propagated data, and the + trigger should not fire a second time on the replica, because that would + lead to duplication. However, if a trigger is used for another purpose + such as creating external alerts, then it might be appropriate to set it + to <literal>ENABLE ALWAYS</literal> so that it is also fired on + replicas. + </para> + + <para> + This command acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] RULE</literal></term> + <listitem> + <para> + These forms configure the firing of rewrite rules belonging to the table. + A disabled rule is still known to the system, but is not applied + during query rewriting. The semantics are as for disabled/enabled + triggers. This configuration is ignored for <literal>ON SELECT</literal> rules, which + are always applied in order to keep views working even if the current + session is in a non-default replication role. + </para> + + <para> + The rule firing mechanism is also affected by the configuration variable + <xref linkend="guc-session-replication-role"/>, analogous to triggers as + described above. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DISABLE</literal>/<literal>ENABLE ROW LEVEL SECURITY</literal></term> + <listitem> + <para> + These forms control the application of row security policies belonging + to the table. If enabled and no policies exist for the table, then a + default-deny policy is applied. Note that policies can exist for a table + even if row level security is disabled. In this case, the policies will + <emphasis>not</emphasis> be applied and the policies will be ignored. + See also + <xref linkend="sql-createpolicy"/>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>NO FORCE</literal>/<literal>FORCE ROW LEVEL SECURITY</literal></term> + <listitem> + <para> + These forms control the application of row security policies belonging + to the table when the user is the table owner. If enabled, row level + security policies will be applied when the user is the table owner. If + disabled (the default) then row level security will not be applied when + the user is the table owner. + See also + <xref linkend="sql-createpolicy"/>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>CLUSTER ON</literal></term> + <listitem> + <para> + This form selects the default index for future + <xref linkend="sql-cluster"/> + operations. It does not actually re-cluster the table. + </para> + <para> + Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET WITHOUT CLUSTER</literal></term> + <listitem> + <para> + This form removes the most recently used + <xref linkend="sql-cluster"/> + index specification from the table. This affects + future cluster operations that don't specify an index. + </para> + <para> + Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET WITHOUT OIDS</literal></term> + <listitem> + <para> + Backward-compatible syntax for removing the <literal>oid</literal> + system column. As <literal>oid</literal> system columns cannot be + added anymore, this never has an effect. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET TABLESPACE</literal></term> + <listitem> + <para> + This form changes the table's tablespace to the specified tablespace and + moves the data file(s) associated with the table to the new tablespace. + Indexes on the table, if any, are not moved; but they can be moved + separately with additional <literal>SET TABLESPACE</literal> commands. + When applied to a partitioned table, nothing is moved, but any + partitions created afterwards with + <command>CREATE TABLE PARTITION OF</command> will use that tablespace, + unless overridden by a <literal>TABLESPACE</literal> clause. + </para> + + <para> + All tables in the current database in a tablespace can be moved by using + the <literal>ALL IN TABLESPACE</literal> form, which will lock all tables + to be moved first and then move each one. This form also supports + <literal>OWNED BY</literal>, which will only move tables owned by the + roles specified. If the <literal>NOWAIT</literal> option is specified + then the command will fail if it is unable to acquire all of the locks + required immediately. Note that system catalogs are not moved by this + command; use <command>ALTER DATABASE</command> or explicit + <command>ALTER TABLE</command> invocations instead if desired. The + <literal>information_schema</literal> relations are not considered part + of the system catalogs and will be moved. + See also + <xref linkend="sql-createtablespace"/>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET { LOGGED | UNLOGGED }</literal></term> + <listitem> + <para> + This form changes the table from unlogged to logged or vice-versa + (see <xref linkend="sql-createtable-unlogged"/>). It cannot be applied + to a temporary table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )</literal></term> + <listitem> + <para> + This form changes one or more storage parameters for the table. See + <xref linkend="sql-createtable-storage-parameters"/> in the + <xref linkend="sql-createtable"/> documentation + for details on the available parameters. Note that the table contents + will not be modified immediately by this command; depending on the + parameter you might need to rewrite the table to get the desired effects. + That can be done with <link linkend="sql-vacuum">VACUUM + FULL</link>, <xref linkend="sql-cluster"/> or one of the forms + of <command>ALTER TABLE</command> that forces a table rewrite. + For planner related parameters, changes will take effect from the next + time the table is locked so currently executing queries will not be + affected. + </para> + + <para> + <literal>SHARE UPDATE EXCLUSIVE</literal> lock will be taken for + fillfactor, toast and autovacuum storage parameters, as well as the + planner parameter <varname>parallel_workers</varname>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )</literal></term> + <listitem> + <para> + This form resets one or more storage parameters to their + defaults. As with <literal>SET</literal>, a table rewrite might be + needed to update the table entirely. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>INHERIT <replaceable class="parameter">parent_table</replaceable></literal></term> + <listitem> + <para> + This form adds the target table as a new child of the specified parent + table. Subsequently, queries against the parent will include records + of the target table. To be added as a child, the target table must + already contain all the same columns as the parent (it could have + additional columns, too). The columns must have matching data types, + and if they have <literal>NOT NULL</literal> constraints in the parent + then they must also have <literal>NOT NULL</literal> constraints in the + child. + </para> + + <para> + There must also be matching child-table constraints for all + <literal>CHECK</literal> constraints of the parent, except those + marked non-inheritable (that is, created with <literal>ALTER TABLE ... ADD CONSTRAINT ... NO INHERIT</literal>) + in the parent, which are ignored; all child-table constraints matched + must not be marked non-inheritable. + Currently + <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and + <literal>FOREIGN KEY</literal> constraints are not considered, but + this might change in the future. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>NO INHERIT <replaceable class="parameter">parent_table</replaceable></literal></term> + <listitem> + <para> + This form removes the target table from the list of children of the + specified parent table. + Queries against the parent table will no longer include records drawn + from the target table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>OF <replaceable class="parameter">type_name</replaceable></literal></term> + <listitem> + <para> + This form links the table to a composite type as though <command>CREATE + TABLE OF</command> had formed it. The table's list of column names and types + must precisely match that of the composite type. The table must + not inherit from any other table. These restrictions ensure + that <command>CREATE TABLE OF</command> would permit an equivalent table + definition. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>NOT OF</literal></term> + <listitem> + <para> + This form dissociates a typed table from its type. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>OWNER TO</literal></term> + <listitem> + <para> + This form changes the owner of the table, sequence, view, materialized view, + or foreign table to the specified user. + </para> + </listitem> + </varlistentry> + + <varlistentry id="sql-createtable-replica-identity"> + <term><literal>REPLICA IDENTITY</literal></term> + <listitem> + <para> + This form changes the information which is written to the write-ahead log + to identify rows which are updated or deleted. This option has no effect + except when logical replication is in use. <literal>DEFAULT</literal> + (the default for non-system tables) records the + old values of the columns of the primary key, if any. <literal>USING INDEX</literal> + records the old values of the columns covered by the named index, which + must be unique, not partial, not deferrable, and include only columns marked + <literal>NOT NULL</literal>. <literal>FULL</literal> records the old values of all columns + in the row. <literal>NOTHING</literal> records no information about the old row. + (This is the default for system tables.) + In all cases, no old values are logged unless at least one of the columns + that would be logged differs between the old and new versions of the row. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>RENAME</literal></term> + <listitem> + <para> + The <literal>RENAME</literal> forms change the name of a table + (or an index, sequence, view, materialized view, or foreign table), the + name of an individual column in a table, or the name of a constraint of + the table. When renaming a constraint that has an underlying index, + the index is renamed as well. + There is no effect on the stored data. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>SET SCHEMA</literal></term> + <listitem> + <para> + This form moves the table into another schema. Associated indexes, + constraints, and sequences owned by table columns are moved as well. + </para> + </listitem> + </varlistentry> + + <varlistentry id="sql-altertable-attach-partition"> + <term><literal>ATTACH PARTITION <replaceable class="parameter">partition_name</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }</literal></term> + <listitem> + <para> + This form attaches an existing table (which might itself be partitioned) + as a partition of the target table. The table can be attached + as a partition for specific values using <literal>FOR VALUES</literal> + or as a default partition by using <literal>DEFAULT</literal>. + For each index in the target table, a corresponding + one will be created in the attached table; or, if an equivalent + index already exists, it will be attached to the target table's index, + as if <command>ALTER INDEX ATTACH PARTITION</command> had been executed. + Note that if the existing table is a foreign table, it is currently not + allowed to attach the table as a partition of the target table if there + are <literal>UNIQUE</literal> indexes on the target table. (See also + <xref linkend="sql-createforeigntable"/>.) For each user-defined + row-level trigger that exists in the target table, a corresponding one + is created in the attached table. + </para> + + <para> + A partition using <literal>FOR VALUES</literal> uses same syntax for + <replaceable class="parameter">partition_bound_spec</replaceable> as + <xref linkend="sql-createtable"/>. The partition bound specification + must correspond to the partitioning strategy and partition key of the + target table. The table to be attached must have all the same columns + as the target table and no more; moreover, the column types must also + match. Also, it must have all the <literal>NOT NULL</literal> and + <literal>CHECK</literal> constraints of the target table. Currently + <literal>FOREIGN KEY</literal> constraints are not considered. + <literal>UNIQUE</literal> and <literal>PRIMARY KEY</literal> constraints + from the parent table will be created in the partition, if they don't + already exist. + If any of the <literal>CHECK</literal> constraints of the table being + attached are marked <literal>NO INHERIT</literal>, the command will fail; + such constraints must be recreated without the + <literal>NO INHERIT</literal> clause. + </para> + + <para> + If the new partition is a regular table, a full table scan is performed + to check that existing rows in the table do not violate the partition + constraint. It is possible to avoid this scan by adding a valid + <literal>CHECK</literal> constraint to the table that allows only + rows satisfying the desired partition constraint before running this + command. The <literal>CHECK</literal> constraint will be used to + determine that the table need not be scanned to validate the partition + constraint. This does not work, however, if any of the partition keys + is an expression and the partition does not accept + <literal>NULL</literal> values. If attaching a list partition that will + not accept <literal>NULL</literal> values, also add + <literal>NOT NULL</literal> constraint to the partition key column, + unless it's an expression. + </para> + + <para> + If the new partition is a foreign table, nothing is done to verify + that all the rows in the foreign table obey the partition constraint. + (See the discussion in <xref linkend="sql-createforeigntable"/> about + constraints on the foreign table.) + </para> + + <para> + When a table has a default partition, defining a new partition changes + the partition constraint for the default partition. The default + partition can't contain any rows that would need to be moved to the new + partition, and will be scanned to verify that none are present. This + scan, like the scan of the new partition, can be avoided if an + appropriate <literal>CHECK</literal> constraint is present. Also like + the scan of the new partition, it is always skipped when the default + partition is a foreign table. + </para> + + <para> + Attaching a partition acquires a + <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table, + in addition to the <literal>ACCESS EXCLUSIVE</literal> locks on the table + being attached and on the default partition (if any). + </para> + + <para> + Further locks must also be held on all sub-partitions if the table being + attached is itself a partitioned table. Likewise if the default + partition is itself a partitioned table. The locking of the + sub-partitions can be avoided by adding a <literal>CHECK</literal> + constraint as described in + <xref linkend="ddl-partitioning-declarative-maintenance"/>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DETACH PARTITION</literal> <replaceable class="parameter">partition_name</replaceable></term> + <listitem> + <para> + This form detaches the specified partition of the target table. The detached + partition continues to exist as a standalone table, but no longer has any + ties to the table from which it was detached. Any indexes that were + attached to the target table's indexes are detached. Any triggers that + were created as clones of those in the target table are removed. + <literal>SHARE</literal> lock is obtained on any tables that reference + this partitioned table in foreign key constraints. + </para> + </listitem> + </varlistentry> + + </variablelist> + </para> + + <para> + All the forms of ALTER TABLE that act on a single table, except + <literal>RENAME</literal>, <literal>SET SCHEMA</literal>, + <literal>ATTACH PARTITION</literal>, and + <literal>DETACH PARTITION</literal> can be combined into + a list of multiple alterations to be applied together. For example, it + is possible to add several columns and/or alter the type of several + columns in a single command. This is particularly useful with large + tables, since only one pass over the table need be made. + </para> + + <para> + You must own the table to use <command>ALTER TABLE</command>. + To change the schema or tablespace of a table, you must also have + <literal>CREATE</literal> privilege on the new schema or tablespace. + To add the table as a new child of a parent table, you must own the parent + table as well. Also, to attach a table as a new partition of the table, + you must own the table being attached. + To alter the owner, you must also be a direct or indirect member of the new + owning role, and that role must have <literal>CREATE</literal> privilege on + the table's schema. (These restrictions enforce that altering the owner + doesn't do anything you couldn't do by dropping and recreating the table. + However, a superuser can alter ownership of any table anyway.) + To add a column or alter a column type or use the <literal>OF</literal> + clause, you must also have <literal>USAGE</literal> privilege on the data + type. + </para> + </refsect1> + + <refsect1> + <title>Parameters</title> + + <variablelist> + + <varlistentry> + <term><literal>IF EXISTS</literal></term> + <listitem> + <para> + Do not throw an error if the table does not exist. A notice is issued + in this case. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">name</replaceable></term> + <listitem> + <para> + The name (optionally schema-qualified) of an existing table to + alter. If <literal>ONLY</literal> is specified before the table name, only + that table is altered. If <literal>ONLY</literal> is not specified, the table + and all its descendant tables (if any) are altered. Optionally, + <literal>*</literal> can be specified after the table name to explicitly + indicate that descendant tables are included. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">column_name</replaceable></term> + <listitem> + <para> + Name of a new or existing column. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">new_column_name</replaceable></term> + <listitem> + <para> + New name for an existing column. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">new_name</replaceable></term> + <listitem> + <para> + New name for the table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">data_type</replaceable></term> + <listitem> + <para> + Data type of the new column, or new data type for an existing + column. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">table_constraint</replaceable></term> + <listitem> + <para> + New table constraint for the table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">constraint_name</replaceable></term> + <listitem> + <para> + Name of a new or existing constraint. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>CASCADE</literal></term> + <listitem> + <para> + Automatically drop objects that depend on the dropped column + or constraint (for example, views referencing the column), + and in turn all objects that depend on those objects + (see <xref linkend="ddl-depend"/>). + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>RESTRICT</literal></term> + <listitem> + <para> + Refuse to drop the column or constraint if there are any dependent + objects. This is the default behavior. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">trigger_name</replaceable></term> + <listitem> + <para> + Name of a single trigger to disable or enable. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>ALL</literal></term> + <listitem> + <para> + Disable or enable all triggers belonging to the table. + (This requires superuser privilege if any of the triggers are + internally generated constraint triggers such as those that are used + to implement foreign key constraints or deferrable uniqueness and + exclusion constraints.) + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>USER</literal></term> + <listitem> + <para> + Disable or enable all triggers belonging to the table except for + internally generated constraint triggers such as those that are used + to implement foreign key constraints or deferrable uniqueness and + exclusion constraints. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">index_name</replaceable></term> + <listitem> + <para> + The name of an existing index. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">storage_parameter</replaceable></term> + <listitem> + <para> + The name of a table storage parameter. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">value</replaceable></term> + <listitem> + <para> + The new value for a table storage parameter. + This might be a number or a word depending on the parameter. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">parent_table</replaceable></term> + <listitem> + <para> + A parent table to associate or de-associate with this table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">new_owner</replaceable></term> + <listitem> + <para> + The user name of the new owner of the table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">new_tablespace</replaceable></term> + <listitem> + <para> + The name of the tablespace to which the table will be moved. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">new_schema</replaceable></term> + <listitem> + <para> + The name of the schema to which the table will be moved. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">partition_name</replaceable></term> + <listitem> + <para> + The name of the table to attach as a new partition or to detach from this table. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">partition_bound_spec</replaceable></term> + <listitem> + <para> + The partition bound specification for a new partition. Refer to + <xref linkend="sql-createtable"/> for more details on the syntax of the same. + </para> + </listitem> + </varlistentry> + + </variablelist> + </refsect1> + + <refsect1 id="sql-altertable-notes" xreflabel="Notes"> + <title>Notes</title> + + <para> + The key word <literal>COLUMN</literal> is noise and can be omitted. + </para> + + <para> + When a column is added with <literal>ADD COLUMN</literal> and a + non-volatile <literal>DEFAULT</literal> is specified, the default is + evaluated at the time of the statement and the result stored in the + table's metadata. That value will be used for the column for all existing + rows. If no <literal>DEFAULT</literal> is specified, NULL is used. In + neither case is a rewrite of the table required. + </para> + + <para> + Adding a column with a volatile <literal>DEFAULT</literal> or + changing the type of an existing column will require the entire table and + its indexes to be rewritten. As an exception, when changing the type of an + existing column, if the <literal>USING</literal> clause does not change + the column contents and the old type is either binary coercible to the new + type or an unconstrained domain over the new type, a table rewrite is not + needed; but any indexes on the affected columns must still be rebuilt. + Table and/or index rebuilds may take a + significant amount of time for a large table; and will temporarily require + as much as double the disk space. + </para> + + <para> + Adding a <literal>CHECK</literal> or <literal>NOT NULL</literal> constraint requires + scanning the table to verify that existing rows meet the constraint, + but does not require a table rewrite. + </para> + + <para> + Similarly, when attaching a new partition it may be scanned to verify that + existing rows meet the partition constraint. + </para> + + <para> + The main reason for providing the option to specify multiple changes + in a single <command>ALTER TABLE</command> is that multiple table scans or + rewrites can thereby be combined into a single pass over the table. + </para> + + <para> + Scanning a large table to verify a new foreign key or check constraint + can take a long time, and other updates to the table are locked out + until the <command>ALTER TABLE ADD CONSTRAINT</command> command is + committed. The main purpose of the <literal>NOT VALID</literal> + constraint option is to reduce the impact of adding a constraint on + concurrent updates. With <literal>NOT VALID</literal>, + the <command>ADD CONSTRAINT</command> command does not scan the table + and can be committed immediately. After that, a <literal>VALIDATE + CONSTRAINT</literal> command can be issued to verify that existing rows + satisfy the constraint. The validation step does not need to lock out + concurrent updates, since it knows that other transactions will be + enforcing the constraint for rows that they insert or update; only + pre-existing rows need to be checked. Hence, validation acquires only + a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the table being + altered. (If the constraint is a foreign key then a <literal>ROW + SHARE</literal> lock is also required on the table referenced by the + constraint.) In addition to improving concurrency, it can be useful to + use <literal>NOT VALID</literal> and <literal>VALIDATE + CONSTRAINT</literal> in cases where the table is known to contain + pre-existing violations. Once the constraint is in place, no new + violations can be inserted, and the existing problems can be corrected + at leisure until <literal>VALIDATE CONSTRAINT</literal> finally + succeeds. + </para> + + <para> + The <literal>DROP COLUMN</literal> form does not physically remove + the column, but simply makes it invisible to SQL operations. Subsequent + insert and update operations in the table will store a null value for the + column. Thus, dropping a column is quick but it will not immediately + reduce the on-disk size of your table, as the space occupied + by the dropped column is not reclaimed. The space will be + reclaimed over time as existing rows are updated. + </para> + + <para> + To force immediate reclamation of space occupied by a dropped column, + you can execute one of the forms of <command>ALTER TABLE</command> that + performs a rewrite of the whole table. This results in reconstructing + each row with the dropped column replaced by a null value. + </para> + + <para> + The rewriting forms of <command>ALTER TABLE</command> are not MVCC-safe. + After a table rewrite, the table will appear empty to concurrent + transactions, if they are using a snapshot taken before the rewrite + occurred. See <xref linkend="mvcc-caveats"/> for more details. + </para> + + <para> + The <literal>USING</literal> option of <literal>SET DATA TYPE</literal> can actually + specify any expression involving the old values of the row; that is, it + can refer to other columns as well as the one being converted. This allows + very general conversions to be done with the <literal>SET DATA TYPE</literal> + syntax. Because of this flexibility, the <literal>USING</literal> + expression is not applied to the column's default value (if any); the + result might not be a constant expression as required for a default. + This means that when there is no implicit or assignment cast from old to + new type, <literal>SET DATA TYPE</literal> might fail to convert the default even + though a <literal>USING</literal> clause is supplied. In such cases, + drop the default with <literal>DROP DEFAULT</literal>, perform the <literal>ALTER + TYPE</literal>, and then use <literal>SET DEFAULT</literal> to add a suitable new + default. Similar considerations apply to indexes and constraints involving + the column. + </para> + + <para> + If a table has any descendant tables, it is not permitted to add, + rename, or change the type of a column in the parent table without doing + the same to the descendants. This ensures that the descendants always + have columns matching the parent. Similarly, a <literal>CHECK</literal> + constraint cannot be renamed in the parent without also renaming it in + all descendants, so that <literal>CHECK</literal> constraints also match + between the parent and its descendants. (That restriction does not apply + to index-based constraints, however.) + Also, because selecting from the parent also selects from its descendants, + a constraint on the parent cannot be marked valid unless it is also marked + valid for those descendants. In all of these cases, <command>ALTER TABLE + ONLY</command> will be rejected. + </para> + + <para> + A recursive <literal>DROP COLUMN</literal> operation will remove a + descendant table's column only if the descendant does not inherit + that column from any other parents and never had an independent + definition of the column. A nonrecursive <literal>DROP + COLUMN</literal> (i.e., <command>ALTER TABLE ONLY ... DROP + COLUMN</command>) never removes any descendant columns, but + instead marks them as independently defined rather than inherited. + A nonrecursive <literal>DROP COLUMN</literal> command will fail for a + partitioned table, because all partitions of a table must have the same + columns as the partitioning root. + </para> + + <para> + The actions for identity columns (<literal>ADD + GENERATED</literal>, <literal>SET</literal> etc., <literal>DROP + IDENTITY</literal>), as well as the actions + <literal>TRIGGER</literal>, <literal>CLUSTER</literal>, <literal>OWNER</literal>, + and <literal>TABLESPACE</literal> never recurse to descendant tables; + that is, they always act as though <literal>ONLY</literal> were specified. + Adding a constraint recurses only for <literal>CHECK</literal> constraints + that are not marked <literal>NO INHERIT</literal>. + </para> + + <para> + Changing any part of a system catalog table is not permitted. + </para> + + <para> + Refer to <xref linkend="sql-createtable"/> for a further description of valid + parameters. <xref linkend="ddl"/> has further information on + inheritance. + </para> + </refsect1> + + <refsect1> + <title>Examples</title> + + <para> + To add a column of type <type>varchar</type> to a table: +<programlisting> +ALTER TABLE distributors ADD COLUMN address varchar(30); +</programlisting> + That will cause all existing rows in the table to be filled with null + values for the new column. + </para> + + <para> + To add a column with a non-null default: +<programlisting> +ALTER TABLE measurements + ADD COLUMN mtime timestamp with time zone DEFAULT now(); +</programlisting> + Existing rows will be filled with the current time as the value of the + new column, and then new rows will receive the time of their insertion. + </para> + + <para> + To add a column and fill it with a value different from the default to + be used later: +<programlisting> +ALTER TABLE transactions + ADD COLUMN status varchar(30) DEFAULT 'old', + ALTER COLUMN status SET default 'current'; +</programlisting> + Existing rows will be filled with <literal>old</literal>, but then + the default for subsequent commands will be <literal>current</literal>. + The effects are the same as if the two sub-commands had been issued + in separate <command>ALTER TABLE</command> commands. + </para> + + <para> + To drop a column from a table: +<programlisting> +ALTER TABLE distributors DROP COLUMN address RESTRICT; +</programlisting> + </para> + + <para> + To change the types of two existing columns in one operation: +<programlisting> +ALTER TABLE distributors + ALTER COLUMN address TYPE varchar(80), + ALTER COLUMN name TYPE varchar(100); +</programlisting> + </para> + + <para> + To change an integer column containing Unix timestamps to <type>timestamp + with time zone</type> via a <literal>USING</literal> clause: +<programlisting> +ALTER TABLE foo + ALTER COLUMN foo_timestamp SET DATA TYPE timestamp with time zone + USING + timestamp with time zone 'epoch' + foo_timestamp * interval '1 second'; +</programlisting> + </para> + + <para> + The same, when the column has a default expression that won't automatically + cast to the new data type: +<programlisting> +ALTER TABLE foo + ALTER COLUMN foo_timestamp DROP DEFAULT, + ALTER COLUMN foo_timestamp TYPE timestamp with time zone + USING + timestamp with time zone 'epoch' + foo_timestamp * interval '1 second', + ALTER COLUMN foo_timestamp SET DEFAULT now(); +</programlisting> + </para> + + <para> + To rename an existing column: +<programlisting> +ALTER TABLE distributors RENAME COLUMN address TO city; +</programlisting> + </para> + + <para> + To rename an existing table: +<programlisting> +ALTER TABLE distributors RENAME TO suppliers; +</programlisting> + </para> + + <para> + To rename an existing constraint: +<programlisting> +ALTER TABLE distributors RENAME CONSTRAINT zipchk TO zip_check; +</programlisting> + </para> + + <para> + To add a not-null constraint to a column: +<programlisting> +ALTER TABLE distributors ALTER COLUMN street SET NOT NULL; +</programlisting> + To remove a not-null constraint from a column: +<programlisting> +ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL; +</programlisting> + </para> + + <para> + To add a check constraint to a table and all its children: +<programlisting> +ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5); +</programlisting> + </para> + + <para> + To add a check constraint only to a table and not to its children: +<programlisting> +ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5) NO INHERIT; +</programlisting> + (The check constraint will not be inherited by future children, either.) + </para> + + <para> + To remove a check constraint from a table and all its children: +<programlisting> +ALTER TABLE distributors DROP CONSTRAINT zipchk; +</programlisting> + </para> + + <para> + To remove a check constraint from one table only: +<programlisting> +ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk; +</programlisting> + (The check constraint remains in place for any child tables.) + </para> + + <para> + To add a foreign key constraint to a table: +<programlisting> +ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address); +</programlisting> + </para> + + <para> + To add a foreign key constraint to a table with the least impact on other work: +<programlisting> +ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID; +ALTER TABLE distributors VALIDATE CONSTRAINT distfk; +</programlisting> + </para> + + <para> + To add a (multicolumn) unique constraint to a table: +<programlisting> +ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode); +</programlisting> + </para> + + <para> + To add an automatically named primary key constraint to a table, noting + that a table can only ever have one primary key: +<programlisting> +ALTER TABLE distributors ADD PRIMARY KEY (dist_id); +</programlisting> + </para> + + <para> + To move a table to a different tablespace: +<programlisting> +ALTER TABLE distributors SET TABLESPACE fasttablespace; +</programlisting> + </para> + + <para> + To move a table to a different schema: +<programlisting> +ALTER TABLE myschema.distributors SET SCHEMA yourschema; +</programlisting> + </para> + + <para> + To recreate a primary key constraint, without blocking updates while the + index is rebuilt: +<programlisting> +CREATE UNIQUE INDEX CONCURRENTLY dist_id_temp_idx ON distributors (dist_id); +ALTER TABLE distributors DROP CONSTRAINT distributors_pkey, + ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx; +</programlisting></para> + + <para> + To attach a partition to a range-partitioned table: +<programlisting> +ALTER TABLE measurement + ATTACH PARTITION measurement_y2016m07 FOR VALUES FROM ('2016-07-01') TO ('2016-08-01'); +</programlisting></para> + + <para> + To attach a partition to a list-partitioned table: +<programlisting> +ALTER TABLE cities + ATTACH PARTITION cities_ab FOR VALUES IN ('a', 'b'); +</programlisting></para> + + <para> + To attach a partition to a hash-partitioned table: +<programlisting> +ALTER TABLE orders + ATTACH PARTITION orders_p4 FOR VALUES WITH (MODULUS 4, REMAINDER 3); +</programlisting></para> + + <para> + To attach a default partition to a partitioned table: +<programlisting> +ALTER TABLE cities + ATTACH PARTITION cities_partdef DEFAULT; +</programlisting></para> + + <para> + To detach a partition from a partitioned table: +<programlisting> +ALTER TABLE measurement + DETACH PARTITION measurement_y2015m12; +</programlisting></para> + + </refsect1> + + <refsect1> + <title>Compatibility</title> + + <para> + The forms <literal>ADD</literal> (without <literal>USING INDEX</literal>), + <literal>DROP [COLUMN]</literal>, <literal>DROP IDENTITY</literal>, <literal>RESTART</literal>, + <literal>SET DEFAULT</literal>, <literal>SET DATA TYPE</literal> (without <literal>USING</literal>), + <literal>SET GENERATED</literal>, and <literal>SET <replaceable>sequence_option</replaceable></literal> + conform with the SQL standard. The other forms are + <productname>PostgreSQL</productname> extensions of the SQL standard. + Also, the ability to specify more than one manipulation in a single + <command>ALTER TABLE</command> command is an extension. + </para> + + <para> + <command>ALTER TABLE DROP COLUMN</command> can be used to drop the only + column of a table, leaving a zero-column table. This is an + extension of SQL, which disallows zero-column tables. + </para> + </refsect1> + + <refsect1> + <title>See Also</title> + + <simplelist type="inline"> + <member><xref linkend="sql-createtable"/></member> + </simplelist> + </refsect1> +</refentry> |