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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>13.3. Explicit Locking</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="transaction-iso.html" title="13.2. Transaction Isolation" /><link rel="next" href="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">13.3. Explicit Locking</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="transaction-iso.html" title="13.2. Transaction Isolation">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="mvcc.html" title="Chapter 13. Concurrency Control">Up</a></td><th width="60%" align="center">Chapter 13. Concurrency Control</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level">Next</a></td></tr></table><hr /></div><div class="sect1" id="EXPLICIT-LOCKING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">13.3. Explicit Locking <a href="#EXPLICIT-LOCKING" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="explicit-locking.html#LOCKING-TABLES">13.3.1. Table-Level Locks</a></span></dt><dt><span class="sect2"><a href="explicit-locking.html#LOCKING-ROWS">13.3.2. Row-Level Locks</a></span></dt><dt><span class="sect2"><a href="explicit-locking.html#LOCKING-PAGES">13.3.3. Page-Level Locks</a></span></dt><dt><span class="sect2"><a href="explicit-locking.html#LOCKING-DEADLOCKS">13.3.4. Deadlocks</a></span></dt><dt><span class="sect2"><a href="explicit-locking.html#ADVISORY-LOCKS">13.3.5. Advisory Locks</a></span></dt></dl></div><a id="id-1.5.12.6.2" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> provides various lock modes
to control concurrent access to data in tables. These modes can
be used for application-controlled locking in situations where
<acronym class="acronym">MVCC</acronym> does not give the desired behavior. Also,
most <span class="productname">PostgreSQL</span> commands automatically
acquire locks of appropriate modes to ensure that referenced
tables are not dropped or modified in incompatible ways while the
command executes. (For example, <code class="command">TRUNCATE</code> cannot safely be
executed concurrently with other operations on the same table, so it
obtains an <code class="literal">ACCESS EXCLUSIVE</code> lock on the table to
enforce that.)
</p><p>
To examine a list of the currently outstanding locks in a database
server, use the
<a class="link" href="view-pg-locks.html" title="54.12. pg_locks"><code class="structname">pg_locks</code></a>
system view. For more information on monitoring the status of the lock
manager subsystem, refer to <a class="xref" href="monitoring.html" title="Chapter 28. Monitoring Database Activity">Chapter 28</a>.
</p><div class="sect2" id="LOCKING-TABLES"><div class="titlepage"><div><div><h3 class="title">13.3.1. Table-Level Locks <a href="#LOCKING-TABLES" class="id_link">#</a></h3></div></div></div><a id="id-1.5.12.6.5.2" class="indexterm"></a><p>
The list below shows the available lock modes and the contexts in
which they are used automatically by
<span class="productname">PostgreSQL</span>. You can also acquire any
of these locks explicitly with the command <a class="xref" href="sql-lock.html" title="LOCK"><span class="refentrytitle">LOCK</span></a>.
Remember that all of these lock modes are table-level locks,
even if the name contains the word
<span class="quote">“<span class="quote">row</span>”</span>; the names of the lock modes are historical.
To some extent the names reflect the typical usage of each lock
mode — but the semantics are all the same. The only real difference
between one lock mode and another is the set of lock modes with
which each conflicts (see <a class="xref" href="explicit-locking.html#TABLE-LOCK-COMPATIBILITY" title="Table 13.2. Conflicting Lock Modes">Table 13.2</a>).
Two transactions cannot hold locks of conflicting
modes on the same table at the same time. (However, a transaction
never conflicts with itself. For example, it might acquire
<code class="literal">ACCESS EXCLUSIVE</code> lock and later acquire
<code class="literal">ACCESS SHARE</code> lock on the same table.) Non-conflicting
lock modes can be held concurrently by many transactions. Notice in
particular that some lock modes are self-conflicting (for example,
an <code class="literal">ACCESS EXCLUSIVE</code> lock cannot be held by more than one
transaction at a time) while others are not self-conflicting (for example,
an <code class="literal">ACCESS SHARE</code> lock can be held by multiple transactions).
</p><div class="variablelist"><p class="title"><strong>Table-Level Lock Modes</strong></p><dl class="variablelist"><dt><span class="term">
<code class="literal">ACCESS SHARE</code> (<code class="literal">AccessShareLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">ACCESS EXCLUSIVE</code> lock
mode only.
</p><p>
The <code class="command">SELECT</code> command acquires a lock of this mode on
referenced tables. In general, any query that only <span class="emphasis"><em>reads</em></span> a table
and does not modify it will acquire this lock mode.
</p></dd><dt><span class="term">
<code class="literal">ROW SHARE</code> (<code class="literal">RowShareLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">EXCLUSIVE</code> and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
</p><p>
The <code class="command">SELECT</code> command acquires a lock of this mode
on all tables on which one of the <code class="option">FOR UPDATE</code>,
<code class="option">FOR NO KEY UPDATE</code>,
<code class="option">FOR SHARE</code>, or
<code class="option">FOR KEY SHARE</code> options is specified
(in addition to <code class="literal">ACCESS SHARE</code> locks on any other
tables that are referenced without any explicit
<code class="option">FOR ...</code> locking option).
</p></dd><dt><span class="term">
<code class="literal">ROW EXCLUSIVE</code> (<code class="literal">RowExclusiveLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">SHARE</code>, <code class="literal">SHARE ROW
EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
</p><p>
The commands <code class="command">UPDATE</code>,
<code class="command">DELETE</code>, <code class="command">INSERT</code>, and
<code class="command">MERGE</code>
acquire this lock mode on the target table (in addition to
<code class="literal">ACCESS SHARE</code> locks on any other referenced
tables). In general, this lock mode will be acquired by any
command that <span class="emphasis"><em>modifies data</em></span> in a table.
</p></dd><dt><span class="term">
<code class="literal">SHARE UPDATE EXCLUSIVE</code> (<code class="literal">ShareUpdateExclusiveLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">SHARE UPDATE EXCLUSIVE</code>,
<code class="literal">SHARE</code>, <code class="literal">SHARE ROW
EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
This mode protects a table against
concurrent schema changes and <code class="command">VACUUM</code> runs.
</p><p>
Acquired by <code class="command">VACUUM</code> (without <code class="option">FULL</code>),
<code class="command">ANALYZE</code>, <code class="command">CREATE INDEX CONCURRENTLY</code>,
<code class="command">CREATE STATISTICS</code>, <code class="command">COMMENT ON</code>,
<code class="command">REINDEX CONCURRENTLY</code>,
and certain <a class="link" href="sql-alterindex.html" title="ALTER INDEX"><code class="command">ALTER INDEX</code></a>
and <a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE</code></a> variants
(for full details see the documentation of these commands).
</p></dd><dt><span class="term">
<code class="literal">SHARE</code> (<code class="literal">ShareLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">ROW EXCLUSIVE</code>,
<code class="literal">SHARE UPDATE EXCLUSIVE</code>, <code class="literal">SHARE ROW
EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
This mode protects a table against concurrent data changes.
</p><p>
Acquired by <code class="command">CREATE INDEX</code>
(without <code class="option">CONCURRENTLY</code>).
</p></dd><dt><span class="term">
<code class="literal">SHARE ROW EXCLUSIVE</code> (<code class="literal">ShareRowExclusiveLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">ROW EXCLUSIVE</code>,
<code class="literal">SHARE UPDATE EXCLUSIVE</code>,
<code class="literal">SHARE</code>, <code class="literal">SHARE ROW
EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
This mode protects a table against concurrent data changes, and
is self-exclusive so that only one session can hold it at a time.
</p><p>
Acquired by <code class="command">CREATE TRIGGER</code> and some forms of
<a class="link" href="sql-altertable.html" title="ALTER TABLE"><code class="command">ALTER TABLE</code></a>.
</p></dd><dt><span class="term">
<code class="literal">EXCLUSIVE</code> (<code class="literal">ExclusiveLock</code>)
</span></dt><dd><p>
Conflicts with the <code class="literal">ROW SHARE</code>, <code class="literal">ROW
EXCLUSIVE</code>, <code class="literal">SHARE UPDATE
EXCLUSIVE</code>, <code class="literal">SHARE</code>, <code class="literal">SHARE
ROW EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code> lock modes.
This mode allows only concurrent <code class="literal">ACCESS SHARE</code> locks,
i.e., only reads from the table can proceed in parallel with a
transaction holding this lock mode.
</p><p>
Acquired by <code class="command">REFRESH MATERIALIZED VIEW CONCURRENTLY</code>.
</p></dd><dt><span class="term">
<code class="literal">ACCESS EXCLUSIVE</code> (<code class="literal">AccessExclusiveLock</code>)
</span></dt><dd><p>
Conflicts with locks of all modes (<code class="literal">ACCESS
SHARE</code>, <code class="literal">ROW SHARE</code>, <code class="literal">ROW
EXCLUSIVE</code>, <code class="literal">SHARE UPDATE
EXCLUSIVE</code>, <code class="literal">SHARE</code>, <code class="literal">SHARE
ROW EXCLUSIVE</code>, <code class="literal">EXCLUSIVE</code>, and
<code class="literal">ACCESS EXCLUSIVE</code>).
This mode guarantees that the
holder is the only transaction accessing the table in any way.
</p><p>
Acquired by the <code class="command">DROP TABLE</code>,
<code class="command">TRUNCATE</code>, <code class="command">REINDEX</code>,
<code class="command">CLUSTER</code>, <code class="command">VACUUM FULL</code>,
and <code class="command">REFRESH MATERIALIZED VIEW</code> (without
<code class="option">CONCURRENTLY</code>)
commands. Many forms of <code class="command">ALTER INDEX</code> and <code class="command">ALTER TABLE</code> also acquire
a lock at this level. This is also the default lock mode for
<code class="command">LOCK TABLE</code> statements that do not specify
a mode explicitly.
</p></dd></dl></div><div class="tip"><h3 class="title">Tip</h3><p>
Only an <code class="literal">ACCESS EXCLUSIVE</code> lock blocks a
<code class="command">SELECT</code> (without <code class="option">FOR UPDATE/SHARE</code>)
statement.
</p></div><p>
Once acquired, a lock is normally held until the end of the transaction. But if a
lock is acquired after establishing a savepoint, the lock is released
immediately if the savepoint is rolled back to. This is consistent with
the principle that <code class="command">ROLLBACK</code> cancels all effects of the
commands since the savepoint. The same holds for locks acquired within a
<span class="application">PL/pgSQL</span> exception block: an error escape from the block
releases locks acquired within it.
</p><div class="table" id="TABLE-LOCK-COMPATIBILITY"><p class="title"><strong>Table 13.2. Conflicting Lock Modes</strong></p><div class="table-contents"><table class="table" summary="Conflicting Lock Modes" border="1"><colgroup><col /><col class="lockst" /><col /><col /><col /><col /><col /><col /><col class="lockend" /></colgroup><thead><tr><th rowspan="2">Requested Lock Mode</th><th colspan="8" align="center">Existing Lock Mode</th></tr><tr><th><code class="literal">ACCESS SHARE</code></th><th><code class="literal">ROW SHARE</code></th><th><code class="literal">ROW EXCL.</code></th><th><code class="literal">SHARE UPDATE EXCL.</code></th><th><code class="literal">SHARE</code></th><th><code class="literal">SHARE ROW EXCL.</code></th><th><code class="literal">EXCL.</code></th><th><code class="literal">ACCESS EXCL.</code></th></tr></thead><tbody><tr><td><code class="literal">ACCESS SHARE</code></td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center">X</td></tr><tr><td><code class="literal">ROW SHARE</code></td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">ROW EXCL.</code></td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">SHARE UPDATE EXCL.</code></td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">SHARE</code></td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">SHARE ROW EXCL.</code></td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">EXCL.</code></td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td><code class="literal">ACCESS EXCL.</code></td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr></tbody></table></div></div><br class="table-break" /></div><div class="sect2" id="LOCKING-ROWS"><div class="titlepage"><div><div><h3 class="title">13.3.2. Row-Level Locks <a href="#LOCKING-ROWS" class="id_link">#</a></h3></div></div></div><p>
In addition to table-level locks, there are row-level locks, which
are listed as below with the contexts in which they are used
automatically by <span class="productname">PostgreSQL</span>. See
<a class="xref" href="explicit-locking.html#ROW-LOCK-COMPATIBILITY" title="Table 13.3. Conflicting Row-Level Locks">Table 13.3</a> for a complete table of
row-level lock conflicts. Note that a transaction can hold
conflicting locks on the same row, even in different subtransactions;
but other than that, two transactions can never hold conflicting locks
on the same row. Row-level locks do not affect data querying; they
block only <span class="emphasis"><em>writers and lockers</em></span> to the same
row. Row-level locks are released at transaction end or during
savepoint rollback, just like table-level locks.
</p><div class="variablelist"><p class="title"><strong>Row-Level Lock Modes</strong></p><dl class="variablelist"><dt><span class="term">
<code class="literal">FOR UPDATE</code>
</span></dt><dd><p>
<code class="literal">FOR UPDATE</code> causes the rows retrieved by the
<code class="command">SELECT</code> statement to be locked as though for
update. This prevents them from being locked, modified or deleted by
other transactions until the current transaction ends. That is,
other transactions that attempt <code class="command">UPDATE</code>,
<code class="command">DELETE</code>,
<code class="command">SELECT FOR UPDATE</code>,
<code class="command">SELECT FOR NO KEY UPDATE</code>,
<code class="command">SELECT FOR SHARE</code> or
<code class="command">SELECT FOR KEY SHARE</code>
of these rows will be blocked until the current transaction ends;
conversely, <code class="command">SELECT FOR UPDATE</code> will wait for a
concurrent transaction that has run any of those commands on the
same row,
and will then lock and return the updated row (or no row, if the
row was deleted). Within a <code class="literal">REPEATABLE READ</code> or
<code class="literal">SERIALIZABLE</code> transaction,
however, an error will be thrown if a row to be locked has changed
since the transaction started. For further discussion see
<a class="xref" href="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level">Section 13.4</a>.
</p><p>
The <code class="literal">FOR UPDATE</code> lock mode
is also acquired by any <code class="command">DELETE</code> on a row, and also by an
<code class="command">UPDATE</code> that modifies the values of certain columns. Currently,
the set of columns considered for the <code class="command">UPDATE</code> case are those that
have a unique index on them that can be used in a foreign key (so partial
indexes and expressional indexes are not considered), but this may change
in the future.
</p></dd><dt><span class="term">
<code class="literal">FOR NO KEY UPDATE</code>
</span></dt><dd><p>
Behaves similarly to <code class="literal">FOR UPDATE</code>, except that the lock
acquired is weaker: this lock will not block
<code class="literal">SELECT FOR KEY SHARE</code> commands that attempt to acquire
a lock on the same rows. This lock mode is also acquired by any
<code class="command">UPDATE</code> that does not acquire a <code class="literal">FOR UPDATE</code> lock.
</p></dd><dt><span class="term">
<code class="literal">FOR SHARE</code>
</span></dt><dd><p>
Behaves similarly to <code class="literal">FOR NO KEY UPDATE</code>, except that it
acquires a shared lock rather than exclusive lock on each retrieved
row. A shared lock blocks other transactions from performing
<code class="command">UPDATE</code>, <code class="command">DELETE</code>,
<code class="command">SELECT FOR UPDATE</code> or
<code class="command">SELECT FOR NO KEY UPDATE</code> on these rows, but it does not
prevent them from performing <code class="command">SELECT FOR SHARE</code> or
<code class="command">SELECT FOR KEY SHARE</code>.
</p></dd><dt><span class="term">
<code class="literal">FOR KEY SHARE</code>
</span></dt><dd><p>
Behaves similarly to <code class="literal">FOR SHARE</code>, except that the
lock is weaker: <code class="literal">SELECT FOR UPDATE</code> is blocked, but not
<code class="literal">SELECT FOR NO KEY UPDATE</code>. A key-shared lock blocks
other transactions from performing <code class="command">DELETE</code> or
any <code class="command">UPDATE</code> that changes the key values, but not
other <code class="command">UPDATE</code>, and neither does it prevent
<code class="command">SELECT FOR NO KEY UPDATE</code>, <code class="command">SELECT FOR SHARE</code>,
or <code class="command">SELECT FOR KEY SHARE</code>.
</p></dd></dl></div><p>
<span class="productname">PostgreSQL</span> doesn't remember any
information about modified rows in memory, so there is no limit on
the number of rows locked at one time. However, locking a row
might cause a disk write, e.g., <code class="command">SELECT FOR
UPDATE</code> modifies selected rows to mark them locked, and so
will result in disk writes.
</p><div class="table" id="ROW-LOCK-COMPATIBILITY"><p class="title"><strong>Table 13.3. Conflicting Row-Level Locks</strong></p><div class="table-contents"><table class="table" summary="Conflicting Row-Level Locks" border="1"><colgroup><col class="col1" /><col class="lockst" /><col class="col3" /><col class="col4" /><col class="lockend" /></colgroup><thead><tr><th rowspan="2">Requested Lock Mode</th><th colspan="4">Current Lock Mode</th></tr><tr><th>FOR KEY SHARE</th><th>FOR SHARE</th><th>FOR NO KEY UPDATE</th><th>FOR UPDATE</th></tr></thead><tbody><tr><td>FOR KEY SHARE</td><td align="center"> </td><td align="center"> </td><td align="center"> </td><td align="center">X</td></tr><tr><td>FOR SHARE</td><td align="center"> </td><td align="center"> </td><td align="center">X</td><td align="center">X</td></tr><tr><td>FOR NO KEY UPDATE</td><td align="center"> </td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr><tr><td>FOR UPDATE</td><td align="center">X</td><td align="center">X</td><td align="center">X</td><td align="center">X</td></tr></tbody></table></div></div><br class="table-break" /></div><div class="sect2" id="LOCKING-PAGES"><div class="titlepage"><div><div><h3 class="title">13.3.3. Page-Level Locks <a href="#LOCKING-PAGES" class="id_link">#</a></h3></div></div></div><p>
In addition to table and row locks, page-level share/exclusive locks are
used to control read/write access to table pages in the shared buffer
pool. These locks are released immediately after a row is fetched or
updated. Application developers normally need not be concerned with
page-level locks, but they are mentioned here for completeness.
</p></div><div class="sect2" id="LOCKING-DEADLOCKS"><div class="titlepage"><div><div><h3 class="title">13.3.4. Deadlocks <a href="#LOCKING-DEADLOCKS" class="id_link">#</a></h3></div></div></div><a id="id-1.5.12.6.8.2" class="indexterm"></a><p>
The use of explicit locking can increase the likelihood of
<em class="firstterm">deadlocks</em>, wherein two (or more) transactions each
hold locks that the other wants. For example, if transaction 1
acquires an exclusive lock on table A and then tries to acquire
an exclusive lock on table B, while transaction 2 has already
exclusive-locked table B and now wants an exclusive lock on table
A, then neither one can proceed.
<span class="productname">PostgreSQL</span> automatically detects
deadlock situations and resolves them by aborting one of the
transactions involved, allowing the other(s) to complete.
(Exactly which transaction will be aborted is difficult to
predict and should not be relied upon.)
</p><p>
Note that deadlocks can also occur as the result of row-level
locks (and thus, they can occur even if explicit locking is not
used). Consider the case in which two concurrent
transactions modify a table. The first transaction executes:
</p><pre class="screen">
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 11111;
</pre><p>
This acquires a row-level lock on the row with the specified
account number. Then, the second transaction executes:
</p><pre class="screen">
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 22222;
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 11111;
</pre><p>
The first <code class="command">UPDATE</code> statement successfully
acquires a row-level lock on the specified row, so it succeeds in
updating that row. However, the second <code class="command">UPDATE</code>
statement finds that the row it is attempting to update has
already been locked, so it waits for the transaction that
acquired the lock to complete. Transaction two is now waiting on
transaction one to complete before it continues execution. Now,
transaction one executes:
</p><pre class="screen">
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
</pre><p>
Transaction one attempts to acquire a row-level lock on the
specified row, but it cannot: transaction two already holds such
a lock. So it waits for transaction two to complete. Thus,
transaction one is blocked on transaction two, and transaction
two is blocked on transaction one: a deadlock
condition. <span class="productname">PostgreSQL</span> will detect this
situation and abort one of the transactions.
</p><p>
The best defense against deadlocks is generally to avoid them by
being certain that all applications using a database acquire
locks on multiple objects in a consistent order. In the example
above, if both transactions
had updated the rows in the same order, no deadlock would have
occurred. One should also ensure that the first lock acquired on
an object in a transaction is the most restrictive mode that will be
needed for that object. If it is not feasible to verify this in
advance, then deadlocks can be handled on-the-fly by retrying
transactions that abort due to deadlocks.
</p><p>
So long as no deadlock situation is detected, a transaction seeking
either a table-level or row-level lock will wait indefinitely for
conflicting locks to be released. This means it is a bad idea for
applications to hold transactions open for long periods of time
(e.g., while waiting for user input).
</p></div><div class="sect2" id="ADVISORY-LOCKS"><div class="titlepage"><div><div><h3 class="title">13.3.5. Advisory Locks <a href="#ADVISORY-LOCKS" class="id_link">#</a></h3></div></div></div><a id="id-1.5.12.6.9.2" class="indexterm"></a><a id="id-1.5.12.6.9.3" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> provides a means for
creating locks that have application-defined meanings. These are
called <em class="firstterm">advisory locks</em>, because the system does not
enforce their use — it is up to the application to use them
correctly. Advisory locks can be useful for locking strategies
that are an awkward fit for the MVCC model.
For example, a common use of advisory locks is to emulate pessimistic
locking strategies typical of so-called <span class="quote">“<span class="quote">flat file</span>”</span> data
management systems.
While a flag stored in a table could be used for the same purpose,
advisory locks are faster, avoid table bloat, and are automatically
cleaned up by the server at the end of the session.
</p><p>
There are two ways to acquire an advisory lock in
<span class="productname">PostgreSQL</span>: at session level or at
transaction level.
Once acquired at session level, an advisory lock is held until
explicitly released or the session ends. Unlike standard lock requests,
session-level advisory lock requests do not honor transaction semantics:
a lock acquired during a transaction that is later rolled back will still
be held following the rollback, and likewise an unlock is effective even
if the calling transaction fails later. A lock can be acquired multiple
times by its owning process; for each completed lock request there must
be a corresponding unlock request before the lock is actually released.
Transaction-level lock requests, on the other hand, behave more like
regular lock requests: they are automatically released at the end of the
transaction, and there is no explicit unlock operation. This behavior
is often more convenient than the session-level behavior for short-term
usage of an advisory lock.
Session-level and transaction-level lock requests for the same advisory
lock identifier will block each other in the expected way.
If a session already holds a given advisory lock, additional requests by
it will always succeed, even if other sessions are awaiting the lock; this
statement is true regardless of whether the existing lock hold and new
request are at session level or transaction level.
</p><p>
Like all locks in
<span class="productname">PostgreSQL</span>, a complete list of advisory locks
currently held by any session can be found in the <a class="link" href="view-pg-locks.html" title="54.12. pg_locks"><code class="structname">pg_locks</code></a> system
view.
</p><p>
Both advisory locks and regular locks are stored in a shared memory
pool whose size is defined by the configuration variables
<a class="xref" href="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION">max_locks_per_transaction</a> and
<a class="xref" href="runtime-config-connection.html#GUC-MAX-CONNECTIONS">max_connections</a>.
Care must be taken not to exhaust this
memory or the server will be unable to grant any locks at all.
This imposes an upper limit on the number of advisory locks
grantable by the server, typically in the tens to hundreds of thousands
depending on how the server is configured.
</p><p>
In certain cases using advisory locking methods, especially in queries
involving explicit ordering and <code class="literal">LIMIT</code> clauses, care must be
taken to control the locks acquired because of the order in which SQL
expressions are evaluated. For example:
</p><pre class="screen">
SELECT pg_advisory_lock(id) FROM foo WHERE id = 12345; -- ok
SELECT pg_advisory_lock(id) FROM foo WHERE id > 12345 LIMIT 100; -- danger!
SELECT pg_advisory_lock(q.id) FROM
(
SELECT id FROM foo WHERE id > 12345 LIMIT 100
) q; -- ok
</pre><p>
In the above queries, the second form is dangerous because the
<code class="literal">LIMIT</code> is not guaranteed to be applied before the locking
function is executed. This might cause some locks to be acquired
that the application was not expecting, and hence would fail to release
(until it ends the session).
From the point of view of the application, such locks
would be dangling, although still viewable in
<code class="structname">pg_locks</code>.
</p><p>
The functions provided to manipulate advisory locks are described in
<a class="xref" href="functions-admin.html#FUNCTIONS-ADVISORY-LOCKS" title="9.27.10. Advisory Lock Functions">Section 9.27.10</a>.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="transaction-iso.html" title="13.2. Transaction Isolation">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mvcc.html" title="Chapter 13. Concurrency Control">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level">Next</a></td></tr><tr><td width="40%" align="left" valign="top">13.2. Transaction Isolation </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 13.4. Data Consistency Checks at the Application Level</td></tr></table></div></body></html>
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