diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/ref/select.sgml | 26 | ||||
| -rw-r--r-- | doc/src/sgml/sql.sgml | 2 |
2 files changed, 18 insertions, 10 deletions
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index 940d1aa5c0d..473939ab4eb 100644 --- a/doc/src/sgml/ref/select.sgml +++ b/doc/src/sgml/ref/select.sgml @@ -45,7 +45,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac [ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ] [ OFFSET <replaceable class="parameter">start</replaceable> [ ROW | ROWS ] ] [ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } ONLY ] - [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ] [...] ] + [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ] <phrase>where <replaceable class="parameter">from_item</replaceable> can be one of:</phrase> @@ -1284,7 +1284,7 @@ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { The locking clause has the general form <synopsis> -FOR <replaceable>lock_strength</> [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ] +FOR <replaceable>lock_strength</> [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ] </synopsis> where <replaceable>lock_strength</> can be one of @@ -1360,9 +1360,15 @@ KEY SHARE <para> To prevent the operation from waiting for other transactions to commit, - use the <literal>NOWAIT</> option. With <literal>NOWAIT</>, the statement - reports an error, rather than waiting, if a selected row - cannot be locked immediately. Note that <literal>NOWAIT</> applies only + use either the <literal>NOWAIT</> or <literal>SKIP LOCKED</literal> + option. With <literal>NOWAIT</>, the statement reports an error, rather + than waiting, if a selected row cannot be locked immediately. + With <literal>SKIP LOCKED</literal>, any selected rows that cannot be + immediately locked are skipped. Skipping locked rows provides an + inconsistent view of the data, so this is not suitable for general purpose + work, but can be used to avoid lock contention with multiple consumers + accessing a queue-like table. + Note that <literal>NOWAIT</> and <literal>SKIP LOCKED</literal> apply only to the row-level lock(s) — the required <literal>ROW SHARE</literal> table-level lock is still taken in the ordinary way (see <xref linkend="mvcc">). You can use @@ -1394,7 +1400,9 @@ KEY SHARE then it is processed as if it was only specified by the strongest one. Similarly, a table is processed as <literal>NOWAIT</> if that is specified in any of the clauses - affecting it. + affecting it. Otherwise, it is processed + as <literal>SKIP LOCKED</literal> if that is specified in any of the + clauses affecting it. </para> <para> @@ -1931,9 +1939,9 @@ SELECT distributors.* WHERE distributors.name = 'Westward'; <productname>PostgreSQL</productname> allows it in any <command>SELECT</> query as well as in sub-<command>SELECT</>s, but this is an extension. The <literal>FOR NO KEY UPDATE</>, <literal>FOR SHARE</> and - <literal>FOR KEY SHARE</> variants, - as well as the <literal>NOWAIT</> option, - do not appear in the standard. + <literal>FOR KEY SHARE</> variants, as well as the <literal>NOWAIT</> + and <literal>SKIP LOCKED</literal> options, do not appear in the + standard. </para> </refsect2> diff --git a/doc/src/sgml/sql.sgml b/doc/src/sgml/sql.sgml index ba92607966c..57396d7c245 100644 --- a/doc/src/sgml/sql.sgml +++ b/doc/src/sgml/sql.sgml @@ -863,7 +863,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac [ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...] ] [ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } ] [ OFFSET <replaceable class="PARAMETER">start</replaceable> ] - [ FOR { UPDATE | SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ] [...] ] + [ FOR { UPDATE | SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ] </synopsis> </para> |