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Postgres FD Implementation
Commit e68eb63e authored by Peter Eisentraut's avatar Peter Eisentraut
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Improve spelling of lock names, and some other editing.

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doc/src/sgml/mvcc.sgml
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$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">
......
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