PostgreSQL Locks Test

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. The common lock modes are as follows:

  • 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 and SELECT FOR SHARE commands acquire 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/FOR SHARE).

  • 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 command 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), ANALYZE, and CREATE INDEX CONCURRENTLY.

  • 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 (without CONCURRENTLY).

  • 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 locks, 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 on user tables by any PostgreSQL command. However it is acquired on certain system catalogs in some operations.

  • 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, TRUNCATE, REINDEX, CLUSTER, and VACUUM FULL commands. This is also the default lock mode for LOCK TABLE statements that do not specify a mode explicitly.

The locking activity of a database server must be monitored carefully because an application holding a specific lock for a long time could cause a number of other transactions relying on the same lock to fail. The PostgreSQL Locks test does just that. For every lock mode that is currently active on the database server, this test reports the total number of locks that are in that mode.

Target of the test : PostgreSQL server

Agent deploying the test: An internal/remote agent

Outputs of the test : One set of results for each lock mode currently held on the target PostgreSQL server

Configurable parameters for the test
  1. TEST PERIOD – How often should the test be executed.
  2. Host – The IP address of the server.
  3. Port – The port on which the server is listening. The default port is 5432.
  4. User – In order to monitor a PostgreSQL server, you need to manually create a special database user account in every PostgreSQL database instance that requires monitoring. When doing so, ensure that this user is vested with the superuser privileges. The sample script we recommend for user creation for eG monitoring is:

    CREATE ROLE eguser LOGIN

    ENCRYPTED PASSWORD {‘eguser password’}

    SUPERUSER NOINHERIT NOCREATEDB NOCREATEROLE;

    The name of this user has to be specified in the USERNAME text box.

  5. Password- The password associated with the above user name (can be ‘NULL’). Here, ‘NULL’ means that the user does not have any password.
  6. Confirm password – Confirm the password (if any) by retyping it here.
  7. dbname - The name of the database to connect to. The default is “postgres”.
  8. ssl - The name of this user has to be specified in the USERNAME text box.
  9. To make diagnosis more efficient and accurate, the eG Enterprise embeds an optional detailed diagnostic capability. With this capability, the eG agents can be configured to run detailed, more elaborate tests as and when specific problems are detected. To enable the detailed diagnosis capability of this test for a particular server, choose the On option. To disable the capability, click on the Off option.

    The option to selectively enable/disable the detailed diagnosis capability will be available only if the following conditions are fulfilled:

    • The eG manager license should allow the detailed diagnosis capability
    • Both the normal and abnormal frequencies configured for the detailed diagnosis measures should not be 0.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Number of locks:

Indicates the total number of locks that are currently held on the database server.

Number

A high value may indicate one of the following:

  • Too many transactions happening
  • Locked resources not being released properly
  • Locks are being held unnecessarily.

With the help of the detailed diagnosis of this measure, you can determine the query that is waiting for the lock, the user who executed that query, the query that is blocking, and the user who is executing the blocking query. Once the blocked and blocking queries are isolated, you can then proceed to do what’s required to release unnecessary locks.