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doc/src/sgml/ref/lock.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.34 2002/09/21 18:32:54 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.35 2003/04/26 23:56:51 petere Exp $
PostgreSQL documentation
-->
@@ -8,251 +8,25 @@ PostgreSQL documentation
<refentrytitle id="sql-lock-title">LOCK</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
LOCK
</refname>
<refpurpose>
explicitly lock a table
</refpurpose>
<refname>LOCK</refname>
<refpurpose>lock a table</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>2001-07-09</date>
</refsynopsisdivinfo>
<synopsis>
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...]
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] IN <replaceable class="PARAMETER">lockmode</replaceable> MODE
<synopsis>
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ IN <replaceable class="PARAMETER">lockmode</replaceable> MODE ]
where <replaceable class="PARAMETER">lockmode</replaceable> is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE |
SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
</synopsis>
<refsect2 id="R2-SQL-LOCK-1">
<refsect2info>
<date>1999-06-09</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of an existing table to lock.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ACCESS SHARE MODE</term>
<listitem>
<para>
This is the least restrictive lock mode. It conflicts only with
ACCESS EXCLUSIVE mode. It is used to protect a table from being
modified by concurrent <command>ALTER TABLE</command>,
<command>DROP TABLE</command> and <command>VACUUM FULL</command>
commands.
</para>
<note>
<para>
The <command>SELECT</command> 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.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>ROW SHARE MODE</term>
<listitem>
<para>
Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.
</para>
<note>
<para>
The <command>SELECT FOR UPDATE</command> command acquires a
lock of this mode on the target table(s) (in addition to
<literal>ACCESS SHARE</literal> locks on any other tables
that are referenced but not selected <option>FOR UPDATE</option>).
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>ROW EXCLUSIVE MODE</term>
<listitem>
<para>
Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and
ACCESS EXCLUSIVE modes.
</para>
<note>
<para>
The commands <command>UPDATE</command>,
<command>DELETE</command>, and <command>INSERT</command>
acquire this lock mode on the target table (in addition to
<literal>ACCESS SHARE</literal> locks on any other referenced
tables). In general, this lock mode will be acquired by any
query that modifies the data in a table.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>SHARE UPDATE EXCLUSIVE MODE</term>
<listitem>
<para>
Conflicts with SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE,
EXCLUSIVE and
ACCESS EXCLUSIVE modes. This mode protects a table against
concurrent schema changes and <command>VACUUM</> runs.
</para>
<note>
<para>
Acquired by <command>VACUUM</command> (without
<option>FULL</option>).
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>SHARE MODE</term>
<listitem>
<para>
Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
SHARE ROW EXCLUSIVE, EXCLUSIVE and
ACCESS EXCLUSIVE modes. This mode protects a table against
concurrent data changes.
</para>
<note>
<para>
Acquired by <command>CREATE INDEX</command>.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>SHARE ROW EXCLUSIVE MODE</term>
<listitem>
<para>
Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE,
SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.
</para>
<note>
<para>
This lock mode is not automatically acquired by any
<productname>PostgreSQL</productname> command.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>EXCLUSIVE MODE</term>
<listitem>
<para>
Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
SHARE, SHARE ROW EXCLUSIVE,
EXCLUSIVE and ACCESS EXCLUSIVE modes.
This mode allows only concurrent ACCESS SHARE, i.e., only reads
from the table can proceed in parallel with a transaction holding
this lock mode.
</para>
<note>
<para>
This lock mode is not automatically acquired by any
<productname>PostgreSQL</productname> command.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>ACCESS EXCLUSIVE MODE</term>
<listitem>
<para>
Conflicts with all lock modes. This mode guarantees that the
holder is the only transaction accessing the table in any way.
</para>
<note>
<para>
Acquired by <command>ALTER TABLE</command>,
<command>DROP TABLE</command>, and <command>VACUUM FULL</command>
statements.
This is also the default lock mode for <command>LOCK TABLE</command>
statements that do not specify a mode explicitly.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="R2-SQL-LOCK-2">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
LOCK TABLE
</computeroutput></term>
<listitem>
<para>
The lock was successfully acquired.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
</computeroutput></term>
<listitem>
<para>
Message returned if <replaceable class="PARAMETER">name</replaceable>
does not exist.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
</synopsis>
</refsynopsisdiv>
<refsect1 id="R1-SQL-LOCK-1">
<refsect1info>
<date>1998-09-24</date>
</refsect1info>
<title>
Description
</title>
<refsect1>
<title>Description</title>
<para>
<command>LOCK TABLE</command> obtains a table-level lock, waiting if
@@ -263,109 +37,114 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
</para>
<para>
When acquiring locks automatically for commands that reference tables,
<productname>PostgreSQL</productname> always uses the least restrictive
lock mode possible. <command>LOCK TABLE</command>
When acquiring locks automatically for commands that reference
tables, <productname>PostgreSQL</productname> always uses the least
restrictive lock mode possible. <command>LOCK TABLE</command>
provides for cases when you might need more restrictive locking.
</para>
<para>
For example, suppose an application runs a transaction at READ COMMITTED
isolation level and needs to ensure that data in a table remains stable
for the duration of the
transaction. To achieve this you could obtain SHARE lock mode over the
For example, suppose an application runs a transaction at the
isolation level read committed and needs to ensure that data in a
table remains stable for the duration of the transaction. To
achieve this you could obtain <literal>SHARE</> lock mode over the
table before querying. This will prevent concurrent data changes
and ensure subsequent reads of the table see a stable
view of committed data, because SHARE lock mode conflicts with the ROW
EXCLUSIVE lock acquired by writers, and your
<command>LOCK TABLE <replaceable class="PARAMETER">name</replaceable> IN SHARE MODE</command>
statement will wait until any concurrent holders of ROW EXCLUSIVE mode
commit or roll back. Thus, once you obtain the lock, there are no
uncommitted writes outstanding; furthermore none can begin until you
release the lock.
and ensure subsequent reads of the table see a stable view of
committed data, because <literal>SHARE</> lock mode conflicts with
the <literal>ROW EXCLUSIVE</> lock acquired by writers, and your
<command>LOCK TABLE <replaceable
class="PARAMETER">name</replaceable> IN SHARE MODE</command>
statement will wait until any concurrent holders of <literal>ROW
EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you
obtain the lock, there are no uncommitted writes outstanding;
furthermore none can begin until you release the lock.
</para>
<note>
<para>
To achieve a similar effect when running a transaction
at the SERIALIZABLE isolation level, you have to execute the
<command>LOCK TABLE</>
statement before executing any <acronym>DML</> statement. A serializable
transaction's view of data will be frozen when its first <acronym>DML</> statement
begins. A later <command>LOCK</> will still prevent concurrent writes
--- but it
won't ensure that what the transaction reads corresponds to the latest
committed values.
</para>
</note>
<para>
To achieve a similar effect when running a transaction at the
isolation level serializable, you have to execute the <command>LOCK
TABLE</> statement before executing any data modification
statement. A serializable transaction's view of data will be
frozen when its first data modification statement begins. A later
<command>LOCK TABLE</> will still prevent concurrent writes --- but it
won't ensure that what the transaction reads corresponds to the
latest committed values.
</para>
<para>
If a transaction of this sort is going to
change the data in the table, then it should use SHARE ROW EXCLUSIVE lock
mode instead of SHARE mode. This ensures that only one transaction of
this type runs at a time. Without this, a deadlock is possible: two
transactions might both acquire SHARE mode, and then be unable to also
acquire ROW EXCLUSIVE mode to actually perform their updates. (Note that
a transaction's own locks never conflict, so a transaction can acquire
ROW EXCLUSIVE mode when it holds SHARE mode --- but not if anyone else
holds SHARE mode.)
</para>
<para>
Two general rules may be followed to prevent deadlock conditions:
</para>
<itemizedlist>
<listitem>
<para>
Transactions have to acquire locks on the same objects in the same order.
</para>
<para>
For example, if one application updates row R1 and than updates
row R2 (in the same transaction) then the second application shouldn't
update row R2 if it's going to update row R1 later (in a single transaction).
Instead, it should update rows R1 and R2 in the same order as the first
application.
</para>
</listitem>
<listitem>
<para>
If multiple lock modes are involved for a single object,
then transactions should always acquire the most restrictive mode first.
</para>
<para>
An example for this rule was given previously when discussing the
use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.
</para>
</listitem>
</itemizedlist>
<para>
<productname>PostgreSQL</productname> does detect deadlocks and will
rollback at least one waiting transaction to resolve the deadlock.
If it is not practical to code an application to follow the above rules
strictly, an alternative solution is to be prepared to retry transactions
when they are aborted by deadlocks.
If a transaction of this sort is going to change the data in the
table, then it should use <literal>SHARE ROW EXCLUSIVE</> lock mode
instead of <literal>SHARE</> mode. This ensures that only one
transaction of this type runs at a time. Without this, a deadlock
is possible: two transactions might both acquire <literal>SHARE</>
mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</>
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire <literal>ROW
EXCLUSIVE</> mode when it holds <literal>SHARE</> mode --- but not
if anyone else holds <literal>SHARE</> mode.) To avoid deadlocks,
make sure all transactions acquire locks on the same objects in the
same order, and if multiple lock modes are involved for a single
object, then transactions should always acquire the most
restrictive mode first.
</para>
<para>
When locking multiple tables, the command <literal>LOCK a, b;</> is
equivalent to <literal>LOCK a; LOCK b;</>. The tables are locked one-by-one
in the order specified in the
<command>LOCK</command> command.
More information about the lock modes and locking strategies can be
found in <xref linkend="explicit-locking">.
</para>
</refsect1>
<refsect2 id="R2-SQL-LOCK-3">
<refsect2info>
<date>1999-06-08</date>
</refsect2info>
<title>
Notes
</title>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of an existing table to
lock.
</para>
<para>
The command <literal>LOCK a, b;</> is equivalent to
<literal>LOCK a; LOCK b;</>. The tables are locked one-by-one in
the order specified in the <command>LOCK</command> command.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">lockmode</replaceable></term>
<listitem>
<para>
The lock mode specifies which locks this lock conflicts with.
Lock modes are described in <xref linkend="explicit-locking">.
</para>
<para>
If no lock mode is specified, then <literal>ACCESS
EXCLUSIVE</literal>, the most restrictive mode, is used.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Diagnostics</title>
<variablelist>
<varlistentry>
<term><computeroutput>LOCK TABLE</computeroutput></term>
<listitem>
<para>
Message returned if the lock was successfully acquired.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
<literal>LOCK ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
@@ -375,81 +154,32 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
<para>
<command>LOCK</command> is useful only inside a transaction block
(<command>BEGIN</>...<command>COMMIT</>), since the lock is dropped
(<command>BEGIN</>/<command>COMMIT</> pair), since the lock is dropped
as soon as the transaction ends. A <command>LOCK</> command appearing
outside any transaction block forms a self-contained transaction, so the
lock will be dropped as soon as it is obtained.
</para>
<para>
<acronym>RDBMS</acronym> locking uses the following standard terminology:
<variablelist>
<varlistentry>
<term>EXCLUSIVE</term>
<listitem>
<para>
An exclusive lock prevents other locks of the same type from being
granted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SHARE</term>
<listitem>
<para>
A shared lock allows others to also hold the same type of lock,
but prevents the corresponding EXCLUSIVE lock from being granted.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ACCESS</term>
<listitem>
<para>
Locks table schema.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ROW</term>
<listitem>
<para>
Locks individual rows.
</para>
</listitem>
</varlistentry>
</variablelist>
<command>LOCK TABLE</> only deals with table-level locks, and so
the mode names involving <literal>ROW</> are all misnomers. These
mode names should generally be read as indicating the intention of
the user to acquire row-level locks within the locked table. Also,
<literal>ROW EXCLUSIVE</> mode is a sharable table lock. Keep in
mind that all the lock modes have identical semantics so far as
<command>LOCK TABLE</> is concerned, differing only in the rules
about which modes conflict with which.
</para>
<para>
<productname>PostgreSQL</productname> does not follow this terminology
exactly. <command>LOCK TABLE</> only deals with table-level locks, and
so the mode names involving ROW are all misnomers. These mode names
should generally be read as indicating the intention of the user to
acquire row-level locks within the locked table. Also,
ROW EXCLUSIVE mode does not follow this naming convention accurately,
since it is a sharable table lock. Keep in mind that all the lock modes
have identical semantics so far as <command>LOCK TABLE</> is concerned,
differing only in the rules about which modes conflict with which.
</para>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-LOCK-2">
<title>
Usage
</title>
<refsect1>
<title>Examples</title>
<para>
Obtain a SHARE lock on a primary key table when going to perform
Obtain a <literal>SHARE</> lock on a primary key table when going to perform
inserts into a foreign key table:
<programlisting>
<programlisting>
BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
@@ -458,50 +188,42 @@ SELECT id FROM films
INSERT INTO films_user_comments VALUES
(_id_, 'GREAT! I was waiting for it for so long!');
COMMIT WORK;
</programlisting>
</programlisting>
</para>
<para>
Take a SHARE ROW EXCLUSIVE lock on a primary key table when going to perform
Take a <literal>SHARE ROW EXCLUSIVE</> lock on a primary key table when going to perform
a delete operation:
<programlisting>
<programlisting>
BEGIN WORK;
LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
DELETE FROM films_user_comments WHERE id IN
(SELECT id FROM films WHERE rating < 5);
DELETE FROM films WHERE rating < 5;
COMMIT WORK;
</programlisting>
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-LOCK-3">
<title>
Compatibility
</title>
<refsect1>
<title>Compatibility</title>
<refsect2 id="R2-SQL-LOCK-4">
<refsect2info>
<date>1998-09-24</date>
</refsect2info>
<title>
SQL92
</title>
<para>
There is no <command>LOCK TABLE</command> in <acronym>SQL92</acronym>,
which instead uses <command>SET TRANSACTION</command> to specify
concurrency levels on transactions. We support that too; see
<xref linkend="SQL-SET-TRANSACTION" endterm="SQL-SET-TRANSACTION-TITLE"> for details.
</para>
<para>
There is no <command>LOCK TABLE</command> in the SQL standard,
which instead uses <command>SET TRANSACTION</command> to specify
concurrency levels on transactions. PostgreSQL supports that too;
see <xref linkend="SQL-SET-TRANSACTION"
endterm="SQL-SET-TRANSACTION-TITLE"> for details.
</para>
<para>
Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE lock
modes, the <productname>PostgreSQL</productname> lock modes and the
<command>LOCK TABLE</command> syntax are compatible with those
present in <productname>Oracle</productname>(TM).
</para>
</refsect2>
<para>
Except for <literal>ACCESS SHARE</>, <literal>ACCESS EXCLUSIVE</>,
and <literal>SHARE UPDATE EXCLUSIVE</> lock modes, the
<productname>PostgreSQL</productname> lock modes and the
<command>LOCK TABLE</command> syntax are compatible with those
present in <productname>Oracle</productname>.
</para>
</refsect1>
</refentry>