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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="EXPLICIT-LOCKING"
>13.3. Explicit Locking</A
></H1
><A
NAME="AEN19653"
></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, <TT
CLASS="COMMAND"
>ALTER TABLE</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 26</A
>.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCKING-TABLES"
>13.3.1. Table-Level Locks</A
></H2
><A
NAME="AEN19666"
></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
HREF="sql-lock.html"
><I
>LOCK</I
></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 &mdash; 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 reads 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 modifies the data 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
>, and <TT
CLASS="COMMAND"
>CREATE INDEX CONCURRENTLY</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.
        </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 user tables by any
         <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> command.  However it is
         acquired on certain system catalogs in some operations.
        </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"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>X</TD
></TR
><TR
><TD
>ROW SHARE</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>X</TD
><TD
ALIGN="CENTER"
>X</TD
></TR
><TR
><TD
>ROW EXCLUSIVE</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</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"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</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"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>X</TD
><TD
ALIGN="CENTER"
>X</TD
><TD
ALIGN="CENTER"
>&nbsp;</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"
>&nbsp;</TD
><TD
ALIGN="CENTER"
>&nbsp;</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"
>&nbsp;</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, in just the same way as for table-level locks.  Row-level locks do
     not affect data querying; they block <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>writers to the same
     row</I
></SPAN
> only.
    </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 it has no limit to
     the number of rows locked at one time.  However, locking a row
     might cause a disk write; thus, for example, <TT
CLASS="COMMAND"
>SELECT FOR
     UPDATE</TT
> will modify 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 we mention them for completeness.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCKING-DEADLOCKS"
>13.3.3. Deadlocks</A
></H2
><A
NAME="AEN19913"
></A
><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 on.)
    </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 there are two concurrent
     transactions modifying 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 highest 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 are aborted due to deadlock.
    </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
><A
NAME="AEN19929"
></A
><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 &mdash; 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.  Once acquired, an
     advisory lock is held until explicitly released or the session ends.
     Unlike standard locks, advisory locks 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.  The same lock can be acquired multiple times by
     its owning process: for each lock request there must be a corresponding
     unlock request before the lock is actually released.  (If a session
     already holds a given lock, additional requests will always succeed, even
     if other sessions are awaiting the lock.)  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
>     Advisory locks are allocated out of 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 not be able 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
>     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 MVCC bloat, and are automatically
     cleaned up by the server at the end of the session.
     In certain cases using this method, 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 &gt; 12345 LIMIT 100; -- danger!
SELECT pg_advisory_lock(q.id) FROM
(
  SELECT id FROM foo WHERE id &gt; 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"
>Table 9-56</A
>.
    </P
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