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'\" t
.\" Title: LOCK
.\" Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\" Date: 2017-11-06
.\" Manual: PostgreSQL 9.2.24 Documentation
.\" Source: PostgreSQL 9.2.24
.\" Language: English
.\"
.TH "LOCK" "7" "2017-11-06" "PostgreSQL 9.2.24" "PostgreSQL 9.2.24 Documentation"
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.SH "NAME"
LOCK \- lock a table
.\" LOCK
.SH "SYNOPSIS"
.sp
.nf
LOCK [ TABLE ] [ ONLY ] \fIname\fR [ * ] [, \&.\&.\&.] [ IN \fIlockmode\fR MODE ] [ NOWAIT ]
where \fIlockmode\fR is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
.fi
.SH "DESCRIPTION"
.PP
\fBLOCK TABLE\fR
obtains a table\-level lock, waiting if necessary for any conflicting locks to be released\&. If
NOWAIT
is specified,
\fBLOCK TABLE\fR
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
\fBUNLOCK TABLE\fR
command; locks are always released at transaction end\&.)
.PP
When acquiring locks automatically for commands that reference tables,
PostgreSQL
always uses the least restrictive lock mode possible\&.
\fBLOCK TABLE\fR
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
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
\fBLOCK TABLE \fR\fB\fIname\fR\fR\fB IN SHARE MODE\fR
statement will wait until any concurrent holders of
ROW EXCLUSIVE
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\&.
.PP
To achieve a similar effect when running a transaction at the
REPEATABLE READ
or
SERIALIZABLE
isolation level, you have to execute the
\fBLOCK TABLE\fR
statement before executing any
\fBSELECT\fR
or data modification statement\&. A
REPEATABLE READ
or
SERIALIZABLE
transaction\*(Aqs view of data will be frozen when its first
\fBSELECT\fR
or data modification statement begins\&. A
\fBLOCK TABLE\fR
later in the transaction will still prevent concurrent writes \(em but it won\*(Aqt ensure that what the transaction reads corresponds to the latest committed values\&.
.PP
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\*(Aqs own locks never conflict, so a transaction can acquire
ROW EXCLUSIVE
mode when it holds
SHARE
mode \(em but not if anyone else holds
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\&.
.PP
More information about the lock modes and locking strategies can be found in
Section 13.3, \(lqExplicit Locking\(rq, in the documentation\&.
.SH "PARAMETERS"
.PP
\fIname\fR
.RS 4
The name (optionally schema\-qualified) of an existing table to lock\&. If
ONLY
is specified before the table name, only that table is locked\&. If
ONLY
is not specified, the table and all its descendant tables (if any) are locked\&. Optionally,
*
can be specified after the table name to explicitly indicate that descendant tables are included\&.
.sp
The command
LOCK TABLE a, b;
is equivalent to
LOCK TABLE a; LOCK TABLE b;\&. The tables are locked one\-by\-one in the order specified in the
\fBLOCK TABLE\fR
command\&.
.RE
.PP
\fIlockmode\fR
.RS 4
The lock mode specifies which locks this lock conflicts with\&. Lock modes are described in
Section 13.3, \(lqExplicit Locking\(rq, in the documentation\&.
.sp
If no lock mode is specified, then
ACCESS EXCLUSIVE, the most restrictive mode, is used\&.
.RE
.PP
NOWAIT
.RS 4
Specifies that
\fBLOCK TABLE\fR
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\&.
.RE
.SH "NOTES"
.PP
LOCK TABLE \&.\&.\&. IN ACCESS SHARE MODE
requires
SELECT
privileges on the target table\&. All other forms of
\fBLOCK\fR
require table\-level
UPDATE,
DELETE, or
TRUNCATE
privileges\&.
.PP
\fBLOCK TABLE\fR
is useless outside a transaction block: the lock would remain held only to the completion of the statement\&. Therefore
PostgreSQL
reports an error if
\fBLOCK\fR
is used outside a transaction block\&. Use
\fBBEGIN\fR(7)
and
\fBCOMMIT\fR(7)
(or
\fBROLLBACK\fR(7)) to define a transaction block\&.
.PP
\fBLOCK TABLE\fR
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 is a sharable table lock\&. Keep in mind that all the lock modes have identical semantics so far as
\fBLOCK TABLE\fR
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
Section 13.3.2, \(lqRow-level Locks\(rq, in the documentation
and the
FOR UPDATE/FOR SHARE Clause
in the
\fBSELECT\fR
reference documentation\&.
.SH "EXAMPLES"
.PP
Obtain a
SHARE
lock on a primary key table when going to perform inserts into a foreign key table:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
WHERE name = \*(AqStar Wars: Episode I \- The Phantom Menace\*(Aq;
\-\- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments VALUES
(_id_, \*(AqGREAT! I was waiting for it for so long!\*(Aq);
COMMIT WORK;
.fi
.if n \{\
.RE
.\}
.PP
Take a
SHARE ROW EXCLUSIVE
lock on a primary key table when going to perform a delete operation:
.sp
.if n \{\
.RS 4
.\}
.nf
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;
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
There is no
\fBLOCK TABLE\fR
in the SQL standard, which instead uses
\fBSET TRANSACTION\fR
to specify concurrency levels on transactions\&.
PostgreSQL
supports that too; see
SET TRANSACTION (\fBSET_TRANSACTION\fR(7))
for details\&.
.PP
Except for
ACCESS SHARE,
ACCESS EXCLUSIVE, and
SHARE UPDATE EXCLUSIVE
lock modes, the
PostgreSQL
lock modes and the
\fBLOCK TABLE\fR
syntax are compatible with those present in
Oracle\&.
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