Postgres provides various lock modes to control concurrent access to data in tables. Some of these lock modes are acquired by Postgres 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.
A read-lock mode acquired automatically on tables being queried.
Conflicts with AccessExclusiveLock only.
Acquired by SELECT FOR UPDATE and LOCK TABLE for IN ROW SHARE MODE statements.
Conflicts with ExclusiveLock and AccessExclusiveLock modes.
Acquired by UPDATE, DELETE, INSERT and LOCK TABLE for IN ROW EXCLUSIVE MODE statements.
Conflicts with ShareLock, ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock modes.
Acquired by CREATE INDEX and LOCK TABLE table for IN SHARE MODE statements.
Conflicts with RowExclusiveLock, ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock modes.
Acquired by LOCK TABLE for IN SHARE ROW EXCLUSIVE MODE statements.
Conflicts with RowExclusiveLock, ShareLock, ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock modes.
Acquired by LOCK TABLE table for IN EXCLUSIVE MODE statements.
Conflicts with RowShareLock, RowExclusiveLock, ShareLock, ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock modes.
Acquired by ALTER TABLE, DROP TABLE, VACUUM and LOCK TABLE statements.
Conflicts with all modes (AccessShareLock, RowShareLock, RowExclusiveLock, ShareLock, ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock).
Note: Only AccessExclusiveLock blocks SELECT (without FOR UPDATE) statement.
These 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 the same row only.
Postgres 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, SELECT FOR UPDATE will modify selected rows to mark them and so will result in disk writes.
In addition to table and row locks, short-term 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.