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| -rw-r--r-- | doc/src/sgml/mvcc.sgml | 162 |
1 files changed, 82 insertions, 80 deletions
diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml index 69c73a67ff8..fddeb679c87 100644 --- a/doc/src/sgml/mvcc.sgml +++ b/doc/src/sgml/mvcc.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.23 2002/02/18 16:13:10 tgl Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.24 2002/03/27 02:36:51 petere Exp $ --> <chapter id="mvcc"> @@ -376,24 +376,28 @@ ERROR: Can't serialize access due to concurrent update </indexterm> <para> - <productname>PostgreSQL</productname> - provides various lock modes to control concurrent - access to data in tables. Some of these lock modes are acquired by - <productname>PostgreSQL</productname> - automatically before statement execution, while others are - provided to be used by applications. All lock modes acquired in a - transaction are held for the duration - of the transaction. + <productname>PostgreSQL</productname> provides various lock modes + to control concurrent access to data in tables. Users normally + need not be concerned about the different lock modes because + <productname>PostgreSQL</productname> commands automatically + acquire locks of appropriate modes to ensure data integrity while + permitting an appropriate level of concurrent access. + Nevertheless, a user can manually lock a table in any of the + available modes using the <command>LOCK TABLE</command> command. </para> - <sect2> - <title>Table-level locks</title> + <para> + The list below shows the available lock modes and the contexts in + which they are used. Remember that all of these lock modes are + table-level locks, even if the name contains the word + <quote>row</quote>. The names of the lock modes are historical. + </para> - <para> <variablelist> + <title>Table-level lock modes</title> <varlistentry> <term> - AccessShareLock + <literal>ACCESS SHARE</literal> </term> <listitem> <para> @@ -402,139 +406,141 @@ ERROR: Can't serialize access due to concurrent update </para> <para> - Conflicts with AccessExclusiveLock only. + Conflicts with the <literal>ACCESS EXCLUSIVE</literal> lock + mode only. </para> </listitem> </varlistentry> <varlistentry> <term> - RowShareLock + <literal>ROW SHARE</literal> </term> <listitem> <para> - Acquired by <command>SELECT FOR UPDATE</command> - and <command>LOCK TABLE</command> - <option>IN ROW SHARE MODE</option> statements. + The <command>SELECT FOR UPDATE</command> command acquires a + lock of this mode. </para> <para> - Conflicts with ExclusiveLock and AccessExclusiveLock modes. + Conflicts with the <literal>EXCLUSIVE</literal> and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - RowExclusiveLock + <literal>ROW EXCLUSIVE</literal> </term> <listitem> <para> - Acquired by <command>UPDATE</command>, <command>DELETE</command>, - <command>INSERT</command> and <command>LOCK TABLE</command> - <option>IN ROW EXCLUSIVE MODE</option> statements. + The commands <command>UPDATE</command>, + <command>DELETE</command>, and <command>INSERT</command> + automatically acquire this lock mode. </para> <para> - Conflicts with ShareLock, ShareRowExclusiveLock, ExclusiveLock and - AccessExclusiveLock modes. + Conflicts with the <literal>SHARE</literal>, <literal>SHARE ROW + EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - ShareUpdateExclusiveLock + <literal>SHARE UPDATE EXCLUSIVE</literal> </term> <listitem> <para> - Acquired by <command>VACUUM</command> (without <option>FULL</option>) - and <command>LOCK TABLE</command> - <option>IN SHARE UPDATE EXCLUSIVE MODE</option> - statements. + Acquired by <command>VACUUM</command> (without <option>FULL</option>). </para> <para> - Conflicts with ShareUpdateExclusiveLock, ShareLock, - ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock modes. + Conflicts with the <literal>SHARE UPDATE EXCLUSIVE</literal>, + <literal>SHARE</literal>, <literal>SHARE ROW + EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - ShareLock + <literal>SHARE</literal> </term> <listitem> <para> - Acquired by <command>CREATE INDEX</command> - and <command>LOCK TABLE</command> - <option>IN SHARE MODE</option> - statements. + Acquired by <command>CREATE INDEX</command>. </para> <para> - Conflicts with RowExclusiveLock, ShareUpdateExclusiveLock, - ShareRowExclusiveLock, - ExclusiveLock and AccessExclusiveLock modes. + Conflicts with the <literal>ROW EXCLUSIVE</literal>, + <literal>SHARE UPDATE EXCLUSIVE</literal>, <literal>SHARE ROW + EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - ShareRowExclusiveLock + <literal>SHARE ROW EXCLUSIVE</literal> </term> <listitem> <para> - Acquired by <command>LOCK TABLE</command> - <option>IN SHARE ROW EXCLUSIVE MODE</option> statements. + This lock mode is not automatically acquired by any command. </para> <para> - Conflicts with RowExclusiveLock, ShareUpdateExclusiveLock, - ShareLock, ShareRowExclusiveLock, - ExclusiveLock and AccessExclusiveLock modes. + Conflicts with the <literal>ROW EXCLUSIVE</literal>, + <literal>SHARE UPDATE EXCLUSIVE</literal>, + <literal>SHARE</literal>, <literal>SHARE ROW + EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - ExclusiveLock + <literal>EXCLUSIVE LOCK</literal> </term> <listitem> <para> - Acquired by <command>LOCK TABLE</command> - <option>IN EXCLUSIVE MODE</option> statements. + This lock mode is not automatically acquired by any command. </para> <para> - Conflicts with RowShareLock, RowExclusiveLock, - ShareUpdateExclusiveLock, ShareLock, - ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock - modes. + Conflicts with the <literal>ROW SHARE</literal>, <literal>ROW + EXCLUSIVE</literal>, <literal>SHARE UPDATE + EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE + ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal> lock modes. </para> </listitem> </varlistentry> <varlistentry> <term> - AccessExclusiveLock + <literal>ACCESS EXCLUSIVE</literal> </term> <listitem> <para> - Acquired by <command>ALTER TABLE</command>, - <command>DROP TABLE</command>, - <command>VACUUM FULL</command> and <command>LOCK TABLE</command> - <option>IN ACCESS EXCLUSIVE MODE</option> (or plain - <command>LOCK TABLE</command>) statements. + Acquired by the <command>ALTER TABLE</command>, <command>DROP + TABLE</command>, and <command>VACUUM FULL</command> commands. + This is also the default lock mode for <command>LOCK TABLE</command> + statements that do not specify a mode explicitly. </para> <para> - Conflicts with all modes (AccessShareLock, RowShareLock, - RowExclusiveLock, ShareUpdateExclusiveLock, ShareLock, - ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock). + Conflicts with locks of all modes ( <literal>ACCESS + SHARE</literal>, <literal>ROW SHARE</literal>, <literal>ROW + EXCLUSIVE</literal>, <literal>SHARE UPDATE + EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE + ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and + <literal>ACCESS EXCLUSIVE</literal>). </para> </listitem> </varlistentry> @@ -542,30 +548,27 @@ ERROR: Can't serialize access due to concurrent update <note> <para> - Only AccessExclusiveLock blocks <command>SELECT</command> (without - <option>FOR UPDATE</option>) statement. + Only an <literal>ACCESS EXCLUSIVE</literal> lock blocks a + <command>SELECT</command> (without <option>FOR UPDATE</option>) + statement. </para> </note> - </para> - </sect2> - - <sect2> - <title>Row-level locks</title> <para> - Row-level locks are acquired when rows are being updated (or deleted or - marked for update). - Row-level locks don't affect data querying. They block - writers to <emphasis>the same row</emphasis> only. + In addition to table-level locks, there are row-level locks. + Row-level locks are acquired when rows are being updated (or + deleted or marked for update). Row-level locks don't affect data + querying; they block writers to <emphasis>the same row</emphasis> + only. Row-level locks cannot be acquired explicitly by the user. </para> <para> - <productname>PostgreSQL</productname> - doesn't remember any information about modified rows in memory and - so has no limit to the number of rows locked at one time. However, - locking a row may cause a disk write; thus, for example, - <command>SELECT FOR UPDATE</command> will modify - selected rows to mark them and so will result in disk writes. + <productname>PostgreSQL</productname> doesn't remember any + information about modified rows in memory, so is has no limit to + the number of rows locked at one time. However, locking a row + may cause a disk write; thus, for example, <command>SELECT FOR + UPDATE</command> will modify selected rows to mark them and so + will result in disk writes. </para> <para> @@ -575,7 +578,6 @@ ERROR: Can't serialize access due to concurrent update updated. Application writers normally need not be concerned with page-level locks, but we mention them for completeness. </para> - </sect2> </sect1> <sect1 id="locking-indexes"> |