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><H1
><A
NAME="SQL-LOCK"
></A
>LOCK</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN76324"
></A
><H2
>Name</H2
>LOCK -- lock a table</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN76329"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>LOCK [ TABLE ] [ ONLY ] <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [ * ] [, ...] [ IN <TT
CLASS="REPLACEABLE"
><I
>lockmode</I
></TT
> MODE ] [ NOWAIT ]
<SPAN
CLASS="phrase"
><SPAN
CLASS="PHRASE"
>where <TT
CLASS="REPLACEABLE"
><I
>lockmode</I
></TT
> is one of:</SPAN
></SPAN
>
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN76335"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> obtains a table-level lock, waiting
if necessary for any conflicting locks to be released. If
<TT
CLASS="LITERAL"
>NOWAIT</TT
> is specified, <TT
CLASS="COMMAND"
>LOCK
TABLE</TT
> does not wait to acquire the desired lock: if it
cannot be acquired immediately, the command is aborted and an
error is emitted. Once obtained, the lock is held for the
remainder of the current transaction. (There is no <TT
CLASS="COMMAND"
>UNLOCK
TABLE</TT
> command; locks are always released at transaction
end.)
</P
><P
> When acquiring locks automatically for commands that reference
tables, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> always uses the least
restrictive lock mode possible. <TT
CLASS="COMMAND"
>LOCK TABLE</TT
>
provides for cases when you might need more restrictive locking.
For example, suppose an application runs a transaction at the
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 <TT
CLASS="LITERAL"
>SHARE</TT
> 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 <TT
CLASS="LITERAL"
>SHARE</TT
> lock mode conflicts with
the <TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
> lock acquired by writers, and your
<TT
CLASS="COMMAND"
>LOCK TABLE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> IN SHARE MODE</TT
>
statement will wait until any concurrent holders of <TT
CLASS="LITERAL"
>ROW
EXCLUSIVE</TT
> 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.
</P
><P
> To achieve a similar effect when running a transaction at the
<TT
CLASS="LITERAL"
>REPEATABLE READ</TT
> or <TT
CLASS="LITERAL"
>SERIALIZABLE</TT
>
isolation level, you have to execute the <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> statement
before executing any <TT
CLASS="COMMAND"
>SELECT</TT
> or data modification statement.
A <TT
CLASS="LITERAL"
>REPEATABLE READ</TT
> or <TT
CLASS="LITERAL"
>SERIALIZABLE</TT
> transaction's
view of data will be frozen when its first
<TT
CLASS="COMMAND"
>SELECT</TT
> or data modification statement begins. A <TT
CLASS="COMMAND"
>LOCK
TABLE</TT
> later in the transaction will still prevent concurrent writes
— but it won't ensure that what the transaction reads corresponds to
the latest committed values.
</P
><P
> If a transaction of this sort is going to change the data in the
table, then it should use <TT
CLASS="LITERAL"
>SHARE ROW EXCLUSIVE</TT
> lock mode
instead of <TT
CLASS="LITERAL"
>SHARE</TT
> 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 <TT
CLASS="LITERAL"
>SHARE</TT
>
mode, and then be unable to also acquire <TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
>
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire <TT
CLASS="LITERAL"
>ROW
EXCLUSIVE</TT
> mode when it holds <TT
CLASS="LITERAL"
>SHARE</TT
> mode — but not
if anyone else holds <TT
CLASS="LITERAL"
>SHARE</TT
> 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.
</P
><P
> More information about the lock modes and locking strategies can be
found in <A
HREF="explicit-locking.html"
>Section 13.3</A
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN76370"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></DT
><DD
><P
> The name (optionally schema-qualified) of an existing table to
lock. If <TT
CLASS="LITERAL"
>ONLY</TT
> is specified before the table name, only that
table is locked. If <TT
CLASS="LITERAL"
>ONLY</TT
> is not specified, the table and all
its descendant tables (if any) are locked. Optionally, <TT
CLASS="LITERAL"
>*</TT
>
can be specified after the table name to explicitly indicate that
descendant tables are included.
</P
><P
> The command <TT
CLASS="LITERAL"
>LOCK TABLE a, b;</TT
> is equivalent to
<TT
CLASS="LITERAL"
>LOCK TABLE a; LOCK TABLE b;</TT
>. The tables are locked
one-by-one in the order specified in the <TT
CLASS="COMMAND"
>LOCK
TABLE</TT
> command.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>lockmode</I
></TT
></DT
><DD
><P
> The lock mode specifies which locks this lock conflicts with.
Lock modes are described in <A
HREF="explicit-locking.html"
>Section 13.3</A
>.
</P
><P
> If no lock mode is specified, then <TT
CLASS="LITERAL"
>ACCESS
EXCLUSIVE</TT
>, the most restrictive mode, is used.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NOWAIT</TT
></DT
><DD
><P
> Specifies that <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> should not wait for
any conflicting locks to be released: if the specified lock(s)
cannot be acquired immediately without waiting, the transaction
is aborted.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN76399"
></A
><H2
>Notes</H2
><P
> <TT
CLASS="LITERAL"
>LOCK TABLE ... IN ACCESS SHARE MODE</TT
> requires <TT
CLASS="LITERAL"
>SELECT</TT
>
privileges on the target table. All other forms of <TT
CLASS="COMMAND"
>LOCK</TT
>
require table-level <TT
CLASS="LITERAL"
>UPDATE</TT
>, <TT
CLASS="LITERAL"
>DELETE</TT
>, or
<TT
CLASS="LITERAL"
>TRUNCATE</TT
> privileges.
</P
><P
> <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> is useless outside a transaction block: the lock
would remain held only to the completion of the statement. Therefore
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> reports an error if <TT
CLASS="COMMAND"
>LOCK</TT
>
is used outside a transaction block.
Use
<A
HREF="sql-begin.html"
>BEGIN</A
> and
<A
HREF="sql-commit.html"
>COMMIT</A
>
(or <A
HREF="sql-rollback.html"
>ROLLBACK</A
>)
to define a transaction block.
</P
><P
> <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> only deals with table-level locks, and so
the mode names involving <TT
CLASS="LITERAL"
>ROW</TT
> 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,
<TT
CLASS="LITERAL"
>ROW EXCLUSIVE</TT
> mode is a sharable table lock. Keep in
mind that all the lock modes have identical semantics so far as
<TT
CLASS="COMMAND"
>LOCK TABLE</TT
> is concerned, differing only in the rules
about which modes conflict with which. For information on how to
acquire an actual row-level lock, see <A
HREF="explicit-locking.html#LOCKING-ROWS"
>Section 13.3.2</A
>
and the <A
HREF="sql-select.html#SQL-FOR-UPDATE-SHARE"
><I
><I
>FOR UPDATE</I
>/<I
>FOR SHARE</I
> Clause</I
></A
> in the <TT
CLASS="COMMAND"
>SELECT</TT
>
reference documentation.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN76423"
></A
><H2
>Examples</H2
><P
> Obtain a <TT
CLASS="LITERAL"
>SHARE</TT
> lock on a primary key table when going to perform
inserts into a foreign key table:
</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
WHERE name = 'Star Wars: Episode I - The Phantom Menace';
-- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments VALUES
(_id_, 'GREAT! I was waiting for it for so long!');
COMMIT WORK;</PRE
><P>
</P
><P
> Take a <TT
CLASS="LITERAL"
>SHARE ROW EXCLUSIVE</TT
> lock on a primary key table when going to perform
a delete operation:
</P><PRE
CLASS="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;</PRE
><P></P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN76431"
></A
><H2
>Compatibility</H2
><P
> There is no <TT
CLASS="COMMAND"
>LOCK TABLE</TT
> in the SQL standard,
which instead uses <TT
CLASS="COMMAND"
>SET TRANSACTION</TT
> to specify
concurrency levels on transactions. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports that too;
see <A
HREF="sql-set-transaction.html"
>SET TRANSACTION</A
> for details.
</P
><P
> Except for <TT
CLASS="LITERAL"
>ACCESS SHARE</TT
>, <TT
CLASS="LITERAL"
>ACCESS EXCLUSIVE</TT
>,
and <TT
CLASS="LITERAL"
>SHARE UPDATE EXCLUSIVE</TT
> lock modes, the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> lock modes and the
<TT
CLASS="COMMAND"
>LOCK TABLE</TT
> syntax are compatible with those
present in <SPAN
CLASS="PRODUCTNAME"
>Oracle</SPAN
>.
</P
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