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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.38 2003/09/11 21:42:20 momjian Exp $
PostgreSQL documentation
-->
<refentry id="SQL-LOCK">
<refmeta>
<refentrytitle id="sql-lock-title">LOCK</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>LOCK</refname>
<refpurpose>lock a table</refpurpose>
</refnamediv>
<indexterm zone="sql-lock">
<primary>LOCK</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ IN <replaceable class="PARAMETER">lockmode</replaceable> MODE ]
where <replaceable class="PARAMETER">lockmode</replaceable> is one of:
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. 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 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
isolation level read committed 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</> 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</> lock mode conflicts with
the <literal>ROW EXCLUSIVE</> 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
isolation level serializable, you have to execute the <command>LOCK
TABLE</> statement before executing any data modification
statement. A serializable transaction's view of data will be
frozen when its first data modification statement begins. A later
<command>LOCK TABLE</> 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</> lock mode
instead of <literal>SHARE</> 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</>
mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</>
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire <literal>ROW
EXCLUSIVE</> mode when it holds <literal>SHARE</> mode --- but not
if anyone else holds <literal>SHARE</> 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.
</para>
<para>
The command <literal>LOCK a, b;</> is equivalent to
<literal>LOCK a; LOCK b;</>. The tables are locked one-by-one in
the order specified in the <command>LOCK</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>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
<literal>LOCK ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
privileges on the target table. All other forms of <command>LOCK</>
require <literal>UPDATE</> and/or <literal>DELETE</> privileges.
</para>
<para>
<command>LOCK</command> is useful only inside a transaction block
(<command>BEGIN</>/<command>COMMIT</> pair), since the lock is dropped
as soon as the transaction ends. A <command>LOCK</> command appearing
outside any transaction block forms a self-contained transaction, so the
lock will be dropped as soon as it is obtained.
</para>
<para>
<command>LOCK TABLE</> only deals with table-level locks, and so
the mode names involving <literal>ROW</> 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</> mode is a sharable table lock. Keep in
mind that all the lock modes have identical semantics so far as
<command>LOCK TABLE</> is concerned, differing only in the rules
about which modes conflict with which.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Obtain a <literal>SHARE</> 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</> 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"
endterm="SQL-SET-TRANSACTION-TITLE"> for details.
</para>
<para>
Except for <literal>ACCESS SHARE</>, <literal>ACCESS EXCLUSIVE</>,
and <literal>SHARE UPDATE EXCLUSIVE</> 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>
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