PostgreSQL provides various lock modes
to control concurrent access to data in tables. These modes can be
used for application-controlled locking in situations where MVCC
does not give the desired behavior. Also, most
PostgreSQL commands automatically
acquire locks of appropriate modes to ensure that referenced tables
are not dropped or modified in incompatible ways while the command
executes. (For example, ALTER TABLE cannot be executed
concurrently with other operations on the same table.)
The list below shows the available lock modes and the contexts in
which they are used automatically by
PostgreSQL.
Remember that all of these lock modes are table-level locks,
even if the name contains the word
"row". The names of the lock modes are historical.
To some extent the names reflect the typical usage of each lock
mode --- but the semantics are all the same. The only real difference
between one lock mode and another is the set of lock modes with
which each conflicts. Two transactions cannot hold locks of conflicting
modes on the same table at the same time. (However, a transaction
never conflicts with itself --- for example, it may acquire
ACCESS EXCLUSIVE lock and later acquire
ACCESS SHARE lock on the same table.) Non-conflicting
lock modes may be held concurrently by many transactions. Notice in
particular that some lock modes are self-conflicting (for example,
ACCESS EXCLUSIVE cannot be held by more than one
transaction at a time) while others are not self-conflicting (for example,
ACCESS SHARE can be held by multiple transactions).
Once acquired, a lock mode is held till end of transaction.
To examine a list of the currently outstanding locks in a
database server, use the pg_locks system
view. For more information on monitoring the status of the lock
manager subsystem, refer to the PostgreSQL 7.3 Administrator's Guide.
Table-level lock modes
ACCESS SHARE
Conflicts with the ACCESS EXCLUSIVE lock
mode only.
The SELECT 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.
ROW SHARE
Conflicts with the EXCLUSIVE and
ACCESS EXCLUSIVE lock modes.
The SELECT FOR UPDATE command acquires a
lock of this mode on the target table(s) (in addition to
ACCESS SHARE locks on any other tables
that are referenced but not selected FOR UPDATE).
ROW EXCLUSIVE
Conflicts with the SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock modes.
The commands UPDATE,
DELETE, and INSERT
acquire this lock mode on the target table (in addition to
ACCESS SHARE locks on any other referenced
tables). In general, this lock mode will be acquired by any
query that modifies the data in a table.
SHARE UPDATE EXCLUSIVE
Conflicts with the SHARE UPDATE EXCLUSIVE,
SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock modes.
This mode protects a table against
concurrent schema changes and VACUUM runs.
Acquired by VACUUM (without FULL).
SHARE
Conflicts with the ROW EXCLUSIVE,
SHARE UPDATE EXCLUSIVE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock modes.
This mode protects a table against concurrent data changes.
Acquired by CREATE INDEX.
SHARE ROW EXCLUSIVE
Conflicts with the ROW EXCLUSIVE,
SHARE UPDATE EXCLUSIVE,
SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock modes.
This lock mode is not automatically acquired by any
PostgreSQL command.
EXCLUSIVE
Conflicts with the ROW SHARE, ROW
EXCLUSIVE, SHARE UPDATE
EXCLUSIVE, SHARE, SHARE
ROW EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock 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.
This lock mode is not automatically acquired by any
PostgreSQL command.
ACCESS EXCLUSIVE
Conflicts with locks of all modes (ACCESS
SHARE, ROW SHARE, ROW
EXCLUSIVE, SHARE UPDATE
EXCLUSIVE, SHARE, SHARE
ROW EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE).
This mode guarantees that the
holder is the only transaction accessing the table in any way.
Acquired by the ALTER TABLE, DROP
TABLE, and VACUUM FULL commands.
This is also the default lock mode for LOCK TABLE
statements that do not specify a mode explicitly.
Note: Only an ACCESS EXCLUSIVE lock blocks a
SELECT (without FOR UPDATE)
statement.
In addition to table-level locks, there are row-level locks.
A row-level lock on a specific row is automatically acquired when the
row is updated (or deleted or marked for update). The lock is held
until the transaction commits or rolls back.
Row-level locks don't affect data
querying; they block writers to the same row
only. To acquire a row-level lock on a row without actually
modifying the row, select the row with SELECT FOR
UPDATE. Note that once a particular row-level lock is
acquired, the transaction may update the row multiple times without
fear of conflicts.
PostgreSQL doesn't remember any
information about modified rows in memory, so it has no limit to
the number of rows locked at one time. However, locking a row
may cause a disk write; thus, for example, SELECT FOR
UPDATE will modify selected rows to mark them and so
will result in disk writes.
In addition to table and row locks, page-level share/exclusive locks are
used to control read/write access to table pages in the shared buffer
pool. These locks are released immediately after a tuple is fetched or
updated. Application writers normally need not be concerned with
page-level locks, but we mention them for completeness.
Use of explicit locking can cause deadlocks, wherein
two (or more) transactions each hold locks that the other wants.
For example, if transaction 1 acquires an exclusive lock on table A
and then tries to acquire an exclusive lock on table B, while transaction
2 has already exclusive-locked table B and now wants an exclusive lock
on table A, then neither one can proceed.
PostgreSQL automatically detects deadlock
situations and resolves them by aborting one of the transactions
involved, allowing the other(s) to complete. (Exactly which transaction
will be aborted is difficult to predict and should not be relied on.)
The best defense against deadlocks is generally to avoid them by being
certain that all applications using a database acquire locks on multiple
objects in a consistent order. One should also ensure that the first
lock acquired on an object in a transaction is the highest mode that
will be needed for that object. If it is not feasible to verify this
in advance, then deadlocks may be handled on-the-fly by retrying
transactions that are aborted due to deadlock.
So long as no deadlock situation is detected, a transaction seeking
either a table-level or row-level lock will wait indefinitely for
conflicting locks to be released. This means it is a bad idea for
applications to hold transactions open for long periods of time
(e.g., while waiting for user input).
