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<title>LOCK</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
<a name="sql-lock"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2>Name</h2>
<p>LOCK — lock a table</p>
</div>
<a name="id778022"></a><div class="refsynopsisdiv">
<h2>Synopsis</h2>
<pre class="synopsis">LOCK [ TABLE ] <em class="replaceable"><code>name</code></em> [, ...] [ IN <em class="replaceable"><code>lockmode</code></em> MODE ] [ NOWAIT ]
where <em class="replaceable"><code>lockmode</code></em> is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE</pre>
</div>
<div class="refsect1" lang="en">
<a name="id778062"></a><h2>Description</h2>
<p> <code class="command">LOCK TABLE</code> obtains a table-level lock, waiting
if necessary for any conflicting locks to be released. If
<code class="literal">NOWAIT</code> is specified, <code class="command">LOCK
TABLE</code> 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 <code class="command">UNLOCK
TABLE</code> command; locks are always released at transaction
end.)
</p>
<p> When acquiring locks automatically for commands that reference
tables, <span class="productname">PostgreSQL</span> always uses the least
restrictive lock mode possible. <code class="command">LOCK TABLE</code>
provides for cases when you might need more restrictive locking.
For example, suppose an application runs a transaction at the
Read Committed 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 <code class="literal">SHARE</code> 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 <code class="literal">SHARE</code> lock mode conflicts with
the <code class="literal">ROW EXCLUSIVE</code> lock acquired by writers, and your
<code class="command">LOCK TABLE <em class="replaceable"><code>name</code></em> IN SHARE MODE</code>
statement will wait until any concurrent holders of <code class="literal">ROW
EXCLUSIVE</code> 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.
</p>
<p> To achieve a similar effect when running a transaction at the Serializable
isolation level, you have to execute the <code class="command">LOCK TABLE</code> statement
before executing any <code class="command">SELECT</code> or data modification statement.
A serializable transaction's view of data will be frozen when its first
<code class="command">SELECT</code> or data modification statement begins. A <code class="command">LOCK
TABLE</code> later in the transaction will still prevent concurrent writes
[mdash ] but it won't ensure that what the transaction reads corresponds to
the latest committed values.
</p>
<p> If a transaction of this sort is going to change the data in the
table, then it should use <code class="literal">SHARE ROW EXCLUSIVE</code> lock mode
instead of <code class="literal">SHARE</code> 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 <code class="literal">SHARE</code>
mode, and then be unable to also acquire <code class="literal">ROW EXCLUSIVE</code>
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire <code class="literal">ROW
EXCLUSIVE</code> mode when it holds <code class="literal">SHARE</code> mode [mdash ] but not
if anyone else holds <code class="literal">SHARE</code> 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.
</p>
<p> More information about the lock modes and locking strategies can be
found in <a href="explicit-locking.html" title="12.3.Explicit Locking">Section12.3, “Explicit Locking”</a>.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id778283"></a><h2>Parameters</h2>
<div class="variablelist"><dl>
<dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt>
<dd>
<p> The name (optionally schema-qualified) of an existing table to
lock.
</p>
<p> The command <code class="literal">LOCK TABLE a, b;</code> is equivalent to
<code class="literal">LOCK TABLE a; LOCK TABLE b;</code>. The tables are locked
one-by-one in the order specified in the <code class="command">LOCK
TABLE</code> command.
</p>
</dd>
<dt><span class="term"><em class="replaceable"><code>lockmode</code></em></span></dt>
<dd>
<p> The lock mode specifies which locks this lock conflicts with.
Lock modes are described in <a href="explicit-locking.html" title="12.3.Explicit Locking">Section12.3, “Explicit Locking”</a>.
</p>
<p> If no lock mode is specified, then <code class="literal">ACCESS
EXCLUSIVE</code>, the most restrictive mode, is used.
</p>
</dd>
<dt><span class="term"><code class="literal">NOWAIT</code></span></dt>
<dd><p> Specifies that <code class="command">LOCK TABLE</code> 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.
</p></dd>
</dl></div>
</div>
<div class="refsect1" lang="en">
<a name="id778378"></a><h2>Notes</h2>
<p> <code class="literal">LOCK TABLE ... IN ACCESS SHARE MODE</code> requires <code class="literal">SELECT</code>
privileges on the target table. All other forms of <code class="command">LOCK</code>
require <code class="literal">UPDATE</code> and/or <code class="literal">DELETE</code> privileges.
</p>
<p> <code class="command">LOCK TABLE</code> is useful only inside a transaction
block (<code class="command">BEGIN</code>/<code class="command">COMMIT</code> pair), since the lock
is dropped as soon as the transaction ends. A <code class="command">LOCK
TABLE</code> command appearing outside any transaction block forms a
self-contained transaction, so the lock will be dropped as soon as
it is obtained.
</p>
<p> <code class="command">LOCK TABLE</code> only deals with table-level locks, and so
the mode names involving <code class="literal">ROW</code> 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,
<code class="literal">ROW EXCLUSIVE</code> mode is a sharable table lock. Keep in
mind that all the lock modes have identical semantics so far as
<code class="command">LOCK TABLE</code> 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 <a href="explicit-locking.html#locking-rows" title="12.3.2.Row-Level Locks">Section12.3.2, “Row-Level Locks”</a>
and the <a href="sql-select.html#sql-for-update-share"><code class="literal">FOR UPDATE</code>/<code class="literal">FOR SHARE</code> Clause</a> in the <code class="command">SELECT</code>
reference documentation.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id778519"></a><h2>Examples</h2>
<p> Obtain a <code class="literal">SHARE</code> lock on a primary key table when going to perform
inserts into a foreign key table:
</p>
<pre class="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;</pre>
<p>
</p>
<p> Take a <code class="literal">SHARE ROW EXCLUSIVE</code> lock on a primary key table when going to perform
a delete operation:
</p>
<pre class="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;</pre>
<p>
</p>
</div>
<div class="refsect1" lang="en">
<a name="id778566"></a><h2>Compatibility</h2>
<p> There is no <code class="command">LOCK TABLE</code> in the SQL standard,
which instead uses <code class="command">SET TRANSACTION</code> to specify
concurrency levels on transactions. <span class="productname">PostgreSQL</span> supports that too;
see <a href="sql-set-transaction.html">SET TRANSACTION</a> for details.
</p>
<p> Except for <code class="literal">ACCESS SHARE</code>, <code class="literal">ACCESS EXCLUSIVE</code>,
and <code class="literal">SHARE UPDATE EXCLUSIVE</code> lock modes, the
<span class="productname">PostgreSQL</span> lock modes and the
<code class="command">LOCK TABLE</code> syntax are compatible with those
present in <span class="productname">Oracle</span>.
</p>
</div>
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