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diff --git a/doc/src/sgml/ref/merge.sgml b/doc/src/sgml/ref/merge.sgml new file mode 100644 index 0000000..0995fe0 --- /dev/null +++ b/doc/src/sgml/ref/merge.sgml @@ -0,0 +1,634 @@ +<!-- +doc/src/sgml/ref/merge.sgml +PostgreSQL documentation +--> + +<refentry id="sql-merge"> + <indexterm zone="sql-merge"> + <primary>MERGE</primary> + </indexterm> + + <refmeta> + <refentrytitle>MERGE</refentrytitle> + <manvolnum>7</manvolnum> + <refmiscinfo>SQL - Language Statements</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>MERGE</refname> + <refpurpose>conditionally insert, update, or delete rows of a table</refpurpose> + </refnamediv> + + <refsynopsisdiv> +<synopsis> +[ WITH <replaceable class="parameter">with_query</replaceable> [, ...] ] +MERGE INTO [ ONLY ] <replaceable class="parameter">target_table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">target_alias</replaceable> ] +USING <replaceable class="parameter">data_source</replaceable> ON <replaceable class="parameter">join_condition</replaceable> +<replaceable class="parameter">when_clause</replaceable> [...] + +<phrase>where <replaceable class="parameter">data_source</replaceable> is:</phrase> + +{ [ ONLY ] <replaceable class="parameter">source_table_name</replaceable> [ * ] | ( <replaceable class="parameter">source_query</replaceable> ) } [ [ AS ] <replaceable class="parameter">source_alias</replaceable> ] + +<phrase>and <replaceable class="parameter">when_clause</replaceable> is:</phrase> + +{ WHEN MATCHED [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_update</replaceable> | <replaceable class="parameter">merge_delete</replaceable> | DO NOTHING } | + WHEN NOT MATCHED [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_insert</replaceable> | DO NOTHING } } + +<phrase>and <replaceable class="parameter">merge_insert</replaceable> is:</phrase> + +INSERT [( <replaceable class="parameter">column_name</replaceable> [, ...] )] +[ OVERRIDING { SYSTEM | USER } VALUE ] +{ VALUES ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) | DEFAULT VALUES } + +<phrase>and <replaceable class="parameter">merge_update</replaceable> is:</phrase> + +UPDATE SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } | + ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) } [, ...] + +<phrase>and <replaceable class="parameter">merge_delete</replaceable> is:</phrase> + +DELETE +</synopsis> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para> + <command>MERGE</command> performs actions that modify rows in the + <replaceable class="parameter">target_table_name</replaceable>, + using the <replaceable class="parameter">data_source</replaceable>. + <command>MERGE</command> provides a single <acronym>SQL</acronym> + statement that can conditionally <command>INSERT</command>, + <command>UPDATE</command> or <command>DELETE</command> rows, a task + that would otherwise require multiple procedural language statements. + </para> + + <para> + First, the <command>MERGE</command> command performs a join + from <replaceable class="parameter">data_source</replaceable> to + <replaceable class="parameter">target_table_name</replaceable> + producing zero or more candidate change rows. For each candidate change + row, the status of <literal>MATCHED</literal> or <literal>NOT MATCHED</literal> + is set just once, after which <literal>WHEN</literal> clauses are evaluated + in the order specified. For each candidate change row, the first clause to + evaluate as true is executed. No more than one <literal>WHEN</literal> + clause is executed for any candidate change row. + </para> + + <para> + <command>MERGE</command> actions have the same effect as + regular <command>UPDATE</command>, <command>INSERT</command>, or + <command>DELETE</command> commands of the same names. The syntax of + those commands is different, notably that there is no <literal>WHERE</literal> + clause and no table name is specified. All actions refer to the + <replaceable class="parameter">target_table_name</replaceable>, + though modifications to other tables may be made using triggers. + </para> + + <para> + When <literal>DO NOTHING</literal> is specified, the source row is + skipped. Since actions are evaluated in their specified order, <literal>DO + NOTHING</literal> can be handy to skip non-interesting source rows before + more fine-grained handling. + </para> + + <para> + There is no separate <literal>MERGE</literal> privilege. + If you specify an update action, you must have the + <literal>UPDATE</literal> privilege on the column(s) + of the <replaceable class="parameter">target_table_name</replaceable> + that are referred to in the <literal>SET</literal> clause. + If you specify an insert action, you must have the <literal>INSERT</literal> + privilege on the <replaceable class="parameter">target_table_name</replaceable>. + If you specify an delete action, you must have the <literal>DELETE</literal> + privilege on the <replaceable class="parameter">target_table_name</replaceable>. + Privileges are tested once at statement start and are checked + whether or not particular <literal>WHEN</literal> clauses are executed. + You will require the <literal>SELECT</literal> privilege on the + <replaceable class="parameter">data_source</replaceable> and any column(s) + of the <replaceable class="parameter">target_table_name</replaceable> + referred to in a <literal>condition</literal>. + </para> + + <para> + <command>MERGE</command> is not supported if the + <replaceable class="parameter">target_table_name</replaceable> is a + materialized view, foreign table, or if it has any + rules defined on it. + </para> + </refsect1> + + <refsect1> + <title>Parameters</title> + + <variablelist> + <varlistentry> + <term><replaceable class="parameter">target_table_name</replaceable></term> + <listitem> + <para> + The name (optionally schema-qualified) of the target table to merge into. + If <literal>ONLY</literal> is specified before the table name, matching + rows are updated or deleted in the named table only. If + <literal>ONLY</literal> is not specified, matching rows are also updated + or deleted in any tables inheriting from the named table. Optionally, + <literal>*</literal> can be specified after the table name to explicitly + indicate that descendant tables are included. The + <literal>ONLY</literal> keyword and <literal>*</literal> option do not + affect insert actions, which always insert into the named table only. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">target_alias</replaceable></term> + <listitem> + <para> + A substitute name for the target table. When an alias is + provided, it completely hides the actual name of the table. For + example, given <literal>MERGE INTO foo AS f</literal>, the remainder of the + <command>MERGE</command> statement must refer to this table as + <literal>f</literal> not <literal>foo</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">source_table_name</replaceable></term> + <listitem> + <para> + The name (optionally schema-qualified) of the source table, view, or + transition table. If <literal>ONLY</literal> is specified before the + table name, matching rows are included from the named table only. If + <literal>ONLY</literal> is not specified, matching rows are also included + from any tables inheriting from the named table. 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">source_query</replaceable></term> + <listitem> + <para> + A query (<command>SELECT</command> statement or <command>VALUES</command> + statement) that supplies the rows to be merged into the + <replaceable class="parameter">target_table_name</replaceable>. + Refer to the <xref linkend="sql-select"/> + statement or <xref linkend="sql-values"/> + statement for a description of the syntax. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">source_alias</replaceable></term> + <listitem> + <para> + A substitute name for the data source. When an alias is + provided, it completely hides the actual name of the table or the fact + that a query was issued. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">join_condition</replaceable></term> + <listitem> + <para> + <replaceable class="parameter">join_condition</replaceable> is + an expression resulting in a value of type + <type>boolean</type> (similar to a <literal>WHERE</literal> + clause) that specifies which rows in the + <replaceable class="parameter">data_source</replaceable> + match rows in the + <replaceable class="parameter">target_table_name</replaceable>. + </para> + <warning> + <para> + Only columns from <replaceable class="parameter">target_table_name</replaceable> + that attempt to match <replaceable class="parameter">data_source</replaceable> + rows should appear in <replaceable class="parameter">join_condition</replaceable>. + <replaceable class="parameter">join_condition</replaceable> subexpressions that + only reference <replaceable class="parameter">target_table_name</replaceable> + columns can affect which action is taken, often in surprising ways. + </para> + </warning> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">when_clause</replaceable></term> + <listitem> + <para> + At least one <literal>WHEN</literal> clause is required. + </para> + <para> + If the <literal>WHEN</literal> clause specifies <literal>WHEN MATCHED</literal> + and the candidate change row matches a row in the + <replaceable class="parameter">target_table_name</replaceable>, + the <literal>WHEN</literal> clause is executed if the + <replaceable class="parameter">condition</replaceable> is + absent or it evaluates to <literal>true</literal>. + </para> + <para> + Conversely, if the <literal>WHEN</literal> clause specifies + <literal>WHEN NOT MATCHED</literal> + and the candidate change row does not match a row in the + <replaceable class="parameter">target_table_name</replaceable>, + the <literal>WHEN</literal> clause is executed if the + <replaceable class="parameter">condition</replaceable> is + absent or it evaluates to <literal>true</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">condition</replaceable></term> + <listitem> + <para> + An expression that returns a value of type <type>boolean</type>. + If this expression for a <literal>WHEN</literal> clause + returns <literal>true</literal>, then the action for that clause + is executed for that row. + </para> + <para> + A condition on a <literal>WHEN MATCHED</literal> clause can refer to columns + in both the source and the target relations. A condition on a + <literal>WHEN NOT MATCHED</literal> clause can only refer to columns from + the source relation, since by definition there is no matching target row. + Only the system attributes from the target table are accessible. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">merge_insert</replaceable></term> + <listitem> + <para> + The specification of an <literal>INSERT</literal> action that inserts + one row into the target table. + The target column names can be listed in any order. If no list of + column names is given at all, the default is all the columns of the + table in their declared order. + </para> + <para> + Each column not present in the explicit or implicit column list will be + filled with a default value, either its declared default value + or null if there is none. + </para> + <para> + If <replaceable class="parameter">target_table_name</replaceable> + is a partitioned table, each row is routed to the appropriate partition + and inserted into it. + If <replaceable class="parameter">target_table_name</replaceable> + is a partition, an error will occur if any input row violates the + partition constraint. + </para> + <para> + Column names may not be specified more than once. + <command>INSERT</command> actions cannot contain sub-selects. + </para> + <para> + Only one <literal>VALUES</literal> clause can be specified. + The <literal>VALUES</literal> clause can only refer to columns from + the source relation, since by definition there is no matching target row. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">merge_update</replaceable></term> + <listitem> + <para> + The specification of an <literal>UPDATE</literal> action that updates + the current row of the <replaceable class="parameter">target_table_name</replaceable>. + Column names may not be specified more than once. + </para> + <para> + Neither a table name nor a <literal>WHERE</literal> clause are allowed. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">merge_delete</replaceable></term> + <listitem> + <para> + Specifies a <literal>DELETE</literal> action that deletes the current row + of the <replaceable class="parameter">target_table_name</replaceable>. + Do not include the table name or any other clauses, as you would normally + do with a <xref linkend="sql-delete"/> command. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">column_name</replaceable></term> + <listitem> + <para> + The name of a column in the <replaceable + class="parameter">target_table_name</replaceable>. The column name + can be qualified with a subfield name or array subscript, if + needed. (Inserting into only some fields of a composite + column leaves the other fields null.) + Do not include the table's name in the specification + of a target column. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>OVERRIDING SYSTEM VALUE</literal></term> + <listitem> + <para> + Without this clause, it is an error to specify an explicit value + (other than <literal>DEFAULT</literal>) for an identity column defined + as <literal>GENERATED ALWAYS</literal>. This clause overrides that + restriction. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>OVERRIDING USER VALUE</literal></term> + <listitem> + <para> + If this clause is specified, then any values supplied for identity + columns defined as <literal>GENERATED BY DEFAULT</literal> are ignored + and the default sequence-generated values are applied. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DEFAULT VALUES</literal></term> + <listitem> + <para> + All columns will be filled with their default values. + (An <literal>OVERRIDING</literal> clause is not permitted in this + form.) + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">expression</replaceable></term> + <listitem> + <para> + An expression to assign to the column. If used in a + <literal>WHEN MATCHED</literal> clause, the expression can use values + from the original row in the target table, and values from the + <literal>data_source</literal> row. + If used in a <literal>WHEN NOT MATCHED</literal> clause, the + expression can use values from the <literal>data_source</literal>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>DEFAULT</literal></term> + <listitem> + <para> + Set the column to its default value (which will be <literal>NULL</literal> + if no specific default expression has been assigned to it). + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">with_query</replaceable></term> + <listitem> + <para> + The <literal>WITH</literal> clause allows you to specify one or more + subqueries that can be referenced by name in the <command>MERGE</command> + query. See <xref linkend="queries-with"/> and <xref linkend="sql-select"/> + for details. + </para> + </listitem> + </varlistentry> + + </variablelist> + </refsect1> + + <refsect1> + <title>Outputs</title> + + <para> + On successful completion, a <command>MERGE</command> command returns a command + tag of the form +<screen> +MERGE <replaceable class="parameter">total_count</replaceable> +</screen> + The <replaceable class="parameter">total_count</replaceable> is the total + number of rows changed (whether inserted, updated, or deleted). + If <replaceable class="parameter">total_count</replaceable> is 0, no rows + were changed in any way. + </para> + + </refsect1> + + <refsect1> + <title>Notes</title> + + <para> + The following steps take place during the execution of + <command>MERGE</command>. + <orderedlist> + <listitem> + <para> + Perform any <literal>BEFORE STATEMENT</literal> triggers for all + actions specified, whether or not their <literal>WHEN</literal> + clauses match. + </para> + </listitem> + <listitem> + <para> + Perform a join from source to target table. + The resulting query will be optimized normally and will produce + a set of candidate change rows. For each candidate change row, + <orderedlist> + <listitem> + <para> + Evaluate whether each row is <literal>MATCHED</literal> or + <literal>NOT MATCHED</literal>. + </para> + </listitem> + <listitem> + <para> + Test each <literal>WHEN</literal> condition in the order + specified until one returns true. + </para> + </listitem> + <listitem> + <para> + When a condition returns true, perform the following actions: + <orderedlist> + <listitem> + <para> + Perform any <literal>BEFORE ROW</literal> triggers that fire + for the action's event type. + </para> + </listitem> + <listitem> + <para> + Perform the specified action, invoking any check constraints on the + target table. + </para> + </listitem> + <listitem> + <para> + Perform any <literal>AFTER ROW</literal> triggers that fire for + the action's event type. + </para> + </listitem> + </orderedlist></para> + </listitem> + </orderedlist></para> + </listitem> + <listitem> + <para> + Perform any <literal>AFTER STATEMENT</literal> triggers for actions + specified, whether or not they actually occur. This is similar to the + behavior of an <command>UPDATE</command> statement that modifies no rows. + </para> + </listitem> + </orderedlist> + In summary, statement triggers for an event type (say, + <command>INSERT</command>) will be fired whenever we + <emphasis>specify</emphasis> an action of that kind. + In contrast, row-level triggers will fire only for the specific event type + being <emphasis>executed</emphasis>. + So a <command>MERGE</command> command might fire statement triggers for both + <command>UPDATE</command> and <command>INSERT</command>, even though only + <command>UPDATE</command> row triggers were fired. + </para> + + <para> + You should ensure that the join produces at most one candidate change row + for each target row. In other words, a target row shouldn't join to more + than one data source row. If it does, then only one of the candidate change + rows will be used to modify the target row; later attempts to modify the + row will cause an error. + This can also occur if row triggers make changes to the target table + and the rows so modified are then subsequently also modified by + <command>MERGE</command>. + If the repeated action is an <command>INSERT</command>, this will + cause a uniqueness violation, while a repeated <command>UPDATE</command> + or <command>DELETE</command> will cause a cardinality violation; the + latter behavior is required by the <acronym>SQL</acronym> standard. + This differs from historical <productname>PostgreSQL</productname> + behavior of joins in <command>UPDATE</command> and + <command>DELETE</command> statements where second and subsequent + attempts to modify the same row are simply ignored. + </para> + + <para> + If a <literal>WHEN</literal> clause omits an <literal>AND</literal> + sub-clause, it becomes the final reachable clause of that + kind (<literal>MATCHED</literal> or <literal>NOT MATCHED</literal>). + If a later <literal>WHEN</literal> clause of that kind + is specified it would be provably unreachable and an error is raised. + If no final reachable clause is specified of either kind, it is + possible that no action will be taken for a candidate change row. + </para> + + <para> + The order in which rows are generated from the data source is + indeterminate by default. + A <replaceable class="parameter">source_query</replaceable> can be + used to specify a consistent ordering, if required, which might be + needed to avoid deadlocks between concurrent transactions. + </para> + + <para> + There is no <literal>RETURNING</literal> clause with + <command>MERGE</command>. Actions of <command>INSERT</command>, + <command>UPDATE</command> and <command>DELETE</command> cannot contain + <literal>RETURNING</literal> or <literal>WITH</literal> clauses. + </para> + + <para> + When <command>MERGE</command> is run concurrently with other commands + that modify the target table, the usual transaction isolation rules + apply; see <xref linkend="transaction-iso"/> for an explanation + on the behavior at each isolation level. + You may also wish to consider using <command>INSERT ... ON CONFLICT</command> + as an alternative statement which offers the ability to run an + <command>UPDATE</command> if a concurrent <command>INSERT</command> + occurs. There are a variety of differences and restrictions between + the two statement types and they are not interchangeable. + </para> + </refsect1> + + <refsect1> + <title>Examples</title> + + <para> + Perform maintenance on <literal>customer_accounts</literal> based + upon new <literal>recent_transactions</literal>. + +<programlisting> +MERGE INTO customer_account ca +USING recent_transactions t +ON t.customer_id = ca.customer_id +WHEN MATCHED THEN + UPDATE SET balance = balance + transaction_value +WHEN NOT MATCHED THEN + INSERT (customer_id, balance) + VALUES (t.customer_id, t.transaction_value); +</programlisting> + </para> + + <para> + Notice that this would be exactly equivalent to the following + statement because the <literal>MATCHED</literal> result does not change + during execution. + +<programlisting> +MERGE INTO customer_account ca +USING (SELECT customer_id, transaction_value FROM recent_transactions) AS t +ON t.customer_id = ca.customer_id +WHEN MATCHED THEN + UPDATE SET balance = balance + transaction_value +WHEN NOT MATCHED THEN + INSERT (customer_id, balance) + VALUES (t.customer_id, t.transaction_value); +</programlisting> + </para> + + <para> + Attempt to insert a new stock item along with the quantity of stock. If + the item already exists, instead update the stock count of the existing + item. Don't allow entries that have zero stock. +<programlisting> +MERGE INTO wines w +USING wine_stock_changes s +ON s.winename = w.winename +WHEN NOT MATCHED AND s.stock_delta > 0 THEN + INSERT VALUES(s.winename, s.stock_delta) +WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN + UPDATE SET stock = w.stock + s.stock_delta +WHEN MATCHED THEN + DELETE; +</programlisting> + + The <literal>wine_stock_changes</literal> table might be, for example, a + temporary table recently loaded into the database. + </para> + + </refsect1> + + <refsect1> + <title>Compatibility</title> + <para> + This command conforms to the <acronym>SQL</acronym> standard. + </para> + <para> + The WITH clause and <literal>DO NOTHING</literal> action are extensions to + the <acronym>SQL</acronym> standard. + </para> + </refsect1> +</refentry> |