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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="EXPLICIT-LOCKING"
>13.3. Explicit Locking</A
></H1
><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, <TT
CLASS="COMMAND"
>TRUNCATE</TT
> cannot safely be
executed concurrently with other operations on the same table, so it
obtains an exclusive 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
HREF="view-pg-locks.html"
><TT
CLASS="STRUCTNAME"
>pg_locks</TT
></A
>
system view. For more information on monitoring the status of the lock
manager subsystem, refer to <A
HREF="monitoring.html"
>Chapter 27</A
>.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCKING-TABLES"
>13.3.1. Table-level Locks</A
></H2
><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
HREF="sql-lock.html"
>LOCK</A
>.
Remember that all of these lock modes are table-level locks,
even if the name contains the word
<SPAN
CLASS="QUOTE"
>"row"</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
HREF="explicit-locking.html#TABLE-LOCK-COMPATIBILITY"
>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
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock and later acquire
<TT
CLASS="LITERAL"
>ACCESS SHARE</TT
> 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 <TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock cannot be held by more than one
transaction at a time) while others are not self-conflicting (for example,
an <TT
CLASS="LITERAL"
>ACCESS SHARE</TT
> lock can be held by multiple transactions).
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
>Table-level Lock Modes</B
></P
><DL
><DT
><TT
CLASS="LITERAL"
>ACCESS SHARE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock
mode only.
</P
><P
> The <TT
CLASS="COMMAND"
>SELECT</TT
> command acquires a lock of this mode on
referenced tables. In general, any query that only <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>reads</I
></SPAN
> a table
and does not modify it will acquire this lock mode.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>ROW SHARE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
> and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock modes.
</P
><P
> The <TT
CLASS="COMMAND"
>SELECT FOR UPDATE</TT
> and
<TT
CLASS="COMMAND"
>SELECT FOR SHARE</TT
> commands acquire a
lock of this mode on the target table(s) (in addition to
<TT
CLASS="LITERAL"
>ACCESS SHARE</TT
> locks on any other tables
that are referenced but not selected
<TT
CLASS="OPTION"
>FOR UPDATE/FOR SHARE</TT
>).
</P
></DD
><DT
><TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>SHARE</TT
>, <TT
CLASS="LITERAL"
>SHARE ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock modes.
</P
><P
> The commands <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, and <TT
CLASS="COMMAND"
>INSERT</TT
>
acquire this lock mode on the target table (in addition to
<TT
CLASS="LITERAL"
>ACCESS SHARE</TT
> locks on any other referenced
tables). In general, this lock mode will be acquired by any
command that <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>modifies data</I
></SPAN
> in a table.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SHARE UPDATE EXCLUSIVE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>SHARE UPDATE EXCLUSIVE</TT
>,
<TT
CLASS="LITERAL"
>SHARE</TT
>, <TT
CLASS="LITERAL"
>SHARE ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock modes.
This mode protects a table against
concurrent schema changes and <TT
CLASS="COMMAND"
>VACUUM</TT
> runs.
</P
><P
> Acquired by <TT
CLASS="COMMAND"
>VACUUM</TT
> (without <TT
CLASS="OPTION"
>FULL</TT
>),
<TT
CLASS="COMMAND"
>ANALYZE</TT
>, <TT
CLASS="COMMAND"
>CREATE INDEX CONCURRENTLY</TT
>, and
some forms of <TT
CLASS="COMMAND"
>ALTER TABLE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SHARE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
>,
<TT
CLASS="LITERAL"
>SHARE UPDATE EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>SHARE ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock modes.
This mode protects a table against concurrent data changes.
</P
><P
> Acquired by <TT
CLASS="COMMAND"
>CREATE INDEX</TT
>
(without <TT
CLASS="OPTION"
>CONCURRENTLY</TT
>).
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SHARE ROW EXCLUSIVE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
>,
<TT
CLASS="LITERAL"
>SHARE UPDATE EXCLUSIVE</TT
>,
<TT
CLASS="LITERAL"
>SHARE</TT
>, <TT
CLASS="LITERAL"
>SHARE ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> 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
> This lock mode is not automatically acquired by any
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> command.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXCLUSIVE</TT
></DT
><DD
><P
> Conflicts with the <TT
CLASS="LITERAL"
>ROW SHARE</TT
>, <TT
CLASS="LITERAL"
>ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>SHARE UPDATE
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>SHARE</TT
>, <TT
CLASS="LITERAL"
>SHARE
ROW EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock modes.
This mode allows only concurrent <TT
CLASS="LITERAL"
>ACCESS SHARE</TT
> locks,
i.e., only reads from the table can proceed in parallel with a
transaction holding this lock mode.
</P
><P
> This lock mode is not automatically acquired on tables by any
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> command.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
></DT
><DD
><P
> Conflicts with locks of all modes (<TT
CLASS="LITERAL"
>ACCESS
SHARE</TT
>, <TT
CLASS="LITERAL"
>ROW SHARE</TT
>, <TT
CLASS="LITERAL"
>ROW
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>SHARE UPDATE
EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>SHARE</TT
>, <TT
CLASS="LITERAL"
>SHARE
ROW EXCLUSIVE</TT
>, <TT
CLASS="LITERAL"
>EXCLUSIVE</TT
>, and
<TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
>).
This mode guarantees that the
holder is the only transaction accessing the table in any way.
</P
><P
> Acquired by the <TT
CLASS="COMMAND"
>ALTER TABLE</TT
>, <TT
CLASS="COMMAND"
>DROP TABLE</TT
>,
<TT
CLASS="COMMAND"
>TRUNCATE</TT
>, <TT
CLASS="COMMAND"
>REINDEX</TT
>,
<TT
CLASS="COMMAND"
>CLUSTER</TT
>, and <TT
CLASS="COMMAND"
>VACUUM FULL</TT
>
commands.
This is also the default lock mode for <TT
CLASS="COMMAND"
>LOCK TABLE</TT
>
statements that do not specify a mode explicitly.
</P
></DD
></DL
></DIV
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Only an <TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
> lock blocks a
<TT
CLASS="COMMAND"
>SELECT</TT
> (without <TT
CLASS="OPTION"
>FOR UPDATE/SHARE</TT
>)
statement.
</P
></BLOCKQUOTE
></DIV
><P
> Once acquired, a lock is normally held till end of 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 <TT
CLASS="COMMAND"
>ROLLBACK</TT
> 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"
><A
NAME="TABLE-LOCK-COMPATIBILITY"
></A
><P
><B
>Table 13-2. Conflicting Lock Modes</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL
WIDTH="1*"
TITLE="lockst"><COL><COL><COL><COL><COL><COL><COL
WIDTH="1*"
TITLE="lockend"><THEAD
><TR
><TH
ROWSPAN="2"
>Requested Lock Mode</TH
><TH
COLSPAN="8"
>Current Lock Mode</TH
></TR
><TR
><TH
>ACCESS SHARE</TH
><TH
>ROW SHARE</TH
><TH
>ROW EXCLUSIVE</TH
><TH
>SHARE UPDATE EXCLUSIVE</TH
><TH
>SHARE</TH
><TH
>SHARE ROW EXCLUSIVE</TH
><TH
>EXCLUSIVE</TH
><TH
>ACCESS EXCLUSIVE</TH
></TR
></THEAD
><TBODY
><TR
><TD
>ACCESS SHARE</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
>ROW SHARE</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
>ROW EXCLUSIVE</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
>SHARE UPDATE EXCLUSIVE</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
>SHARE</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
>SHARE ROW EXCLUSIVE</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
>EXCLUSIVE</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
>ACCESS EXCLUSIVE</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
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCKING-ROWS"
>13.3.2. Row-level Locks</A
></H2
><P
> In addition to table-level locks, there are row-level locks, which
can be exclusive or shared locks. An exclusive row-level lock on a
specific row is automatically acquired when the row is updated or
deleted. The lock is held until the transaction commits or rolls
back, just like table-level locks. Row-level locks do
not affect data querying; they block only <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>writers to the same
row</I
></SPAN
>.
</P
><P
> To acquire an exclusive row-level lock on a row without actually
modifying the row, select the row with <TT
CLASS="COMMAND"
>SELECT FOR
UPDATE</TT
>. Note that once the row-level lock is acquired,
the transaction can update the row multiple times without
fear of conflicts.
</P
><P
> To acquire a shared row-level lock on a row, select the row with
<TT
CLASS="COMMAND"
>SELECT FOR SHARE</TT
>. A shared lock does not prevent
other transactions from acquiring the same shared lock. However,
no transaction is allowed to update, delete, or exclusively lock a
row on which any other transaction holds a shared lock. Any attempt
to do so will block until the shared lock(s) have been released.
</P
><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., <TT
CLASS="COMMAND"
>SELECT FOR
UPDATE</TT
> modifies selected rows to mark them locked, and so
will result in disk writes.
</P
><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"
><H2
CLASS="SECT2"
><A
NAME="LOCKING-DEADLOCKS"
>13.3.3. Deadlocks</A
></H2
><P
> The use of explicit locking can increase the likelihood of
<I
CLASS="FIRSTTERM"
>deadlocks</I
>, 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 <TT
CLASS="COMMAND"
>UPDATE</TT
> statement successfully
acquires a row-level lock on the specified row, so it succeeds in
updating that row. However, the second <TT
CLASS="COMMAND"
>UPDATE</TT
>
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"
><H2
CLASS="SECT2"
><A
NAME="ADVISORY-LOCKS"
>13.3.4. Advisory Locks</A
></H2
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides a means for
creating locks that have application-defined meanings. These are
called <I
CLASS="FIRSTTERM"
>advisory locks</I
>, 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"
>"flat file"</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
HREF="view-pg-locks.html"
><TT
CLASS="STRUCTNAME"
>pg_locks</TT
></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
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
> and
<A
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 <TT
CLASS="LITERAL"
>LIMIT</TT
> 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
<TT
CLASS="LITERAL"
>LIMIT</TT
> 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
<TT
CLASS="STRUCTNAME"
>pg_locks</TT
>.
</P
><P
> The functions provided to manipulate advisory locks are described in
<A
HREF="functions-admin.html#FUNCTIONS-ADVISORY-LOCKS"
>Section 9.26.8</A
>.
</P
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