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diff --git a/doc/src/sgml/ref/lock.sgml b/doc/src/sgml/ref/lock.sgml new file mode 100644 index 0000000..0c46886 --- /dev/null +++ b/doc/src/sgml/ref/lock.sgml @@ -0,0 +1,261 @@ +<!-- +doc/src/sgml/ref/lock.sgml +PostgreSQL documentation +--> + +<refentry id="sql-lock"> + <indexterm zone="sql-lock"> + <primary>LOCK</primary> + </indexterm> + + <refmeta> + <refentrytitle>LOCK</refentrytitle> + <manvolnum>7</manvolnum> + <refmiscinfo>SQL - Language Statements</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>LOCK</refname> + <refpurpose>lock a table</refpurpose> + </refnamediv> + + <refsynopsisdiv> +<synopsis> +LOCK [ TABLE ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] [, ...] [ IN <replaceable class="parameter">lockmode</replaceable> MODE ] [ NOWAIT ] + +<phrase>where <replaceable class="parameter">lockmode</replaceable> is one of:</phrase> + + ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE + | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE +</synopsis> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para> + <command>LOCK TABLE</command> obtains a table-level lock, waiting + if necessary for any conflicting locks to be released. If + <literal>NOWAIT</literal> is specified, <command>LOCK + TABLE</command> does not wait to acquire the desired lock: if it + cannot be acquired immediately, the command is aborted and an + error is emitted. Once obtained, the lock is held for the + remainder of the current transaction. (There is no <command>UNLOCK + TABLE</command> command; locks are always released at transaction + end.) + </para> + + <para> + When a view is locked, all relations appearing in the view definition + query are also locked recursively with the same lock mode. + </para> + + <para> + When acquiring locks automatically for commands that reference + tables, <productname>PostgreSQL</productname> always uses the least + restrictive lock mode possible. <command>LOCK TABLE</command> + provides for cases when you might need more restrictive locking. + For example, suppose an application runs a transaction at the + <literal>READ COMMITTED</literal> isolation level and needs to ensure that + data in a table remains stable for the duration of the transaction. + To achieve this you could obtain <literal>SHARE</literal> lock mode over the + table before querying. This will prevent concurrent data changes + and ensure subsequent reads of the table see a stable view of + committed data, because <literal>SHARE</literal> lock mode conflicts with + the <literal>ROW EXCLUSIVE</literal> lock acquired by writers, and your + <command>LOCK TABLE <replaceable + class="parameter">name</replaceable> IN SHARE MODE</command> + statement will wait until any concurrent holders of <literal>ROW + EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you + obtain the lock, there are no uncommitted writes outstanding; + furthermore none can begin until you release the lock. + </para> + + <para> + To achieve a similar effect when running a transaction at the + <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal> + isolation level, you have to execute the <command>LOCK TABLE</command> statement + before executing any <command>SELECT</command> or data modification statement. + A <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal> transaction's + view of data will be frozen when its first + <command>SELECT</command> or data modification statement begins. A <command>LOCK + TABLE</command> later in the transaction will still prevent concurrent writes + — but it won't ensure that what the transaction reads corresponds to + the latest committed values. + </para> + + <para> + If a transaction of this sort is going to change the data in the + table, then it should use <literal>SHARE ROW EXCLUSIVE</literal> lock mode + instead of <literal>SHARE</literal> mode. This ensures that only one + transaction of this type runs at a time. Without this, a deadlock + is possible: two transactions might both acquire <literal>SHARE</literal> + mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</literal> + mode to actually perform their updates. (Note that a transaction's + own locks never conflict, so a transaction can acquire <literal>ROW + EXCLUSIVE</literal> mode when it holds <literal>SHARE</literal> mode — but not + if anyone else holds <literal>SHARE</literal> mode.) To avoid deadlocks, + make sure all transactions acquire locks on the same objects in the + same order, and if multiple lock modes are involved for a single + object, then transactions should always acquire the most + restrictive mode first. + </para> + + <para> + More information about the lock modes and locking strategies can be + found in <xref linkend="explicit-locking"/>. + </para> + </refsect1> + + <refsect1> + <title>Parameters</title> + + <variablelist> + <varlistentry> + <term><replaceable class="parameter">name</replaceable></term> + <listitem> + <para> + The name (optionally schema-qualified) of an existing table to + lock. If <literal>ONLY</literal> is specified before the table name, only that + table is locked. If <literal>ONLY</literal> is not specified, the table and all + its descendant tables (if any) are locked. Optionally, <literal>*</literal> + can be specified after the table name to explicitly indicate that + descendant tables are included. + </para> + + <para> + The command <literal>LOCK TABLE a, b;</literal> is equivalent to + <literal>LOCK TABLE a; LOCK TABLE b;</literal>. The tables are locked + one-by-one in the order specified in the <command>LOCK + TABLE</command> command. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">lockmode</replaceable></term> + <listitem> + <para> + The lock mode specifies which locks this lock conflicts with. + Lock modes are described in <xref linkend="explicit-locking"/>. + </para> + + <para> + If no lock mode is specified, then <literal>ACCESS + EXCLUSIVE</literal>, the most restrictive mode, is used. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>NOWAIT</literal></term> + <listitem> + <para> + Specifies that <command>LOCK TABLE</command> should not wait for + any conflicting locks to be released: if the specified lock(s) + cannot be acquired immediately without waiting, the transaction + is aborted. + </para> + </listitem> + </varlistentry> + </variablelist> + </refsect1> + + <refsect1> + <title>Notes</title> + + <para> + <literal>LOCK TABLE ... IN ACCESS SHARE MODE</literal> requires <literal>SELECT</literal> + privileges on the target table. <literal>LOCK TABLE ... IN ROW EXCLUSIVE + MODE</literal> requires <literal>INSERT</literal>, <literal>UPDATE</literal>, <literal>DELETE</literal>, + or <literal>TRUNCATE</literal> privileges on the target table. All other forms of + <command>LOCK</command> require table-level <literal>UPDATE</literal>, <literal>DELETE</literal>, + or <literal>TRUNCATE</literal> privileges. + </para> + + <para> + The user performing the lock on the view must have the corresponding privilege + on the view. In addition the view's owner must have the relevant privileges on + the underlying base relations, but the user performing the lock does + not need any permissions on the underlying base relations. + </para> + + <para> + <command>LOCK TABLE</command> is useless outside a transaction block: the lock + would remain held only to the completion of the statement. Therefore + <productname>PostgreSQL</productname> reports an error if <command>LOCK</command> + is used outside a transaction block. + Use + <xref linkend="sql-begin"/> and + <xref linkend="sql-commit"/> + (or <xref linkend="sql-rollback"/>) + to define a transaction block. + </para> + + <para> + <command>LOCK TABLE</command> only deals with table-level locks, and so + the mode names involving <literal>ROW</literal> are all misnomers. These + mode names should generally be read as indicating the intention of + the user to acquire row-level locks within the locked table. Also, + <literal>ROW EXCLUSIVE</literal> mode is a shareable table lock. Keep in + mind that all the lock modes have identical semantics so far as + <command>LOCK TABLE</command> is concerned, differing only in the rules + about which modes conflict with which. For information on how to + acquire an actual row-level lock, see <xref linkend="locking-rows"/> + and <xref linkend="sql-for-update-share"/> + in the <xref linkend="sql-select"/> documentation. + </para> + </refsect1> + + <refsect1> + <title>Examples</title> + + <para> + Obtain a <literal>SHARE</literal> lock on a primary key table when going to perform + inserts into a foreign key table: + +<programlisting> +BEGIN WORK; +LOCK TABLE films IN SHARE MODE; +SELECT id FROM films + WHERE name = 'Star Wars: Episode I - The Phantom Menace'; +-- Do ROLLBACK if record was not returned +INSERT INTO films_user_comments VALUES + (_id_, 'GREAT! I was waiting for it for so long!'); +COMMIT WORK; +</programlisting> + </para> + + <para> + Take a <literal>SHARE ROW EXCLUSIVE</literal> lock on a primary key table when going to perform + a delete operation: + +<programlisting> +BEGIN WORK; +LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE; +DELETE FROM films_user_comments WHERE id IN + (SELECT id FROM films WHERE rating < 5); +DELETE FROM films WHERE rating < 5; +COMMIT WORK; +</programlisting></para> + </refsect1> + + <refsect1> + <title>Compatibility</title> + + <para> + There is no <command>LOCK TABLE</command> in the SQL standard, + which instead uses <command>SET TRANSACTION</command> to specify + concurrency levels on transactions. <productname>PostgreSQL</productname> supports that too; + see <xref linkend="sql-set-transaction"/> for details. + </para> + + <para> + Except for <literal>ACCESS SHARE</literal>, <literal>ACCESS EXCLUSIVE</literal>, + and <literal>SHARE UPDATE EXCLUSIVE</literal> lock modes, the + <productname>PostgreSQL</productname> lock modes and the + <command>LOCK TABLE</command> syntax are compatible with those + present in <productname>Oracle</productname>. + </para> + </refsect1> +</refentry> |