PostgreSQL Lock Monitoring Identify & Resolve Blocking Application Locks

calendar Sep 1, 2024 / · 3 min read · PostgreSQL SQL Database Performance Troubleshooting pg_locks pg_stat_activity  ·
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PostgreSQL Lock Monitoring: Identify Blocking Locks and Resolve Application Conflicts

Purpose

In a busy PostgreSQL environment, transactions can sometimes become blocked, waiting for other transactions to release locks. This can slow performance and even cause stalls. This article dissects a powerful PostgreSQL script designed to expose blocked processes, offering insights to help you troubleshoot and optimize performance.

Sample Code from Command Line

 1SELECT
 2  blocked_locks.pid                   AS blocked_pid,
 3  blocked_activity.usename            AS blocked_user,
 4  blocking_locks.pid                  AS blocking_pid,
 5  blocking_activity.usename           AS blocking_user,
 6  blocked_activity.query              AS blocked_statement,
 7  blocking_activity.query             AS current_statement_in_blocking_process,
 8  blocked_activity.application_name   AS blocked_application,
 9  blocking_activity.application_name  AS blocking_application
10FROM
11  pg_catalog.pg_locks           blocked_locks
12JOIN
13  pg_catalog.pg_stat_activity   blocked_activity
14ON
15  blocked_activity.pid = blocked_locks.pid
16JOIN
17  pg_catalog.pg_locks           blocking_locks
18ON
19  blocking_locks.locktype = blocked_locks.locktype
20  AND blocking_locks.DATABASE       IS NOT DISTINCT FROM blocked_locks.DATABASE
21  AND blocking_locks.relation       IS NOT DISTINCT FROM blocked_locks.relation
22  AND blocking_locks.page           IS NOT DISTINCT FROM blocked_locks.page
23  AND blocking_locks.tuple          IS NOT DISTINCT FROM blocked_locks.tuple
24  AND blocking_locks.virtualxid     IS NOT DISTINCT FROM blocked_locks.virtualxid
25  AND blocking_locks.transactionid  IS NOT DISTINCT FROM blocked_locks.transactionid
26  AND blocking_locks.classid        IS NOT DISTINCT FROM blocked_locks.classid
27  AND blocking_locks.objid          IS NOT DISTINCT FROM blocked_locks.objid
28  AND blocking_locks.objsubid       IS NOT DISTINCT FROM blocked_locks.objsubid
29  AND blocking_locks.pid != blocked_locks.pid
30JOIN
31  pg_catalog.pg_stat_activity blocking_activity
32ON
33  blocking_activity.pid = blocking_locks.pid
34WHERE
35  NOT blocked_locks.granted;

Code Breakdown: The query accomplishes this by joining several system views:

  • pg_locks: Provides information about all current locks held within the database.
  • pg_stat_activity: Tracks current activity of all server processes.

The core logic is:

  1. Identify blocked locks: It finds locks in pg_locks that haven't been granted (NOT blocked_locks.granted).
  2. Link to blocked processes: It joins the blocked locks with their respective processes using pg_stat_activity. This gives us the blocked_pid, blocked_user, blocked_statement, and blocked_application.
  3. Find the blocking locks: It identifies the locks causing the blockage. It does this by looking for other locks in pg_locks that match the blocked lock's characteristics (type, database, relation, etc.), but held by a different process (blocking_locks.pid != blocked_locks.pid).
  4. Link to blocking processes: Similar to step 2, it joins the blocking locks to their processes using pg_stat_activity. This reveals the blocking_pid, blocking_user, current_statement_in_blocking_process, and blocking_application.

Key Points:

  • Lock Conflict Resolution: The query's primary function is to troubleshoot lock conflicts by revealing the blocking and blocked processes.
  • Application Visibility: The inclusion of application_name allows you to tie locks back to specific applications, aiding in problem isolation.
  • Requires PostgreSQL 9.2+: This query leverages features introduced in PostgreSQL 9.2 and later.
  • Detailed Information: Provides the blocked_statement and even the current_statement_in_blocking_process, offering insights into the specific operations causing the conflict.

Insights:

  • Performance Bottlenecks: Lock contention can lead to significant performance degradation. This query helps you identify and address those bottlenecks.
  • Application Troubleshooting: If you suspect a specific application is causing locking issues, this query can confirm your suspicions.
  • Concurrency Management: Understanding lock conflicts is key to designing and managing concurrent database operations effectively.

Explanations:

  • IS NOT DISTINCT FROM: This clause is used instead of = to handle cases where either side might be NULL. It acts like = when both sides are non-NULL and evaluates to true if both sides are NULL.
  • pg_locks fields:
    • locktype: Type of lock (e.g., relation, tuple, virtualxid).
    • DATABASE, relation, page, tuple, virtualxid, transactionid, classid, objid, objsubid: These fields identify the locked object at varying levels of granularity.
    • pid: Process ID holding the lock.
    • granted: Whether the lock has been granted or is still waiting.
  • pg_stat_activity fields:
    • pid: Process ID.
    • usename: Username associated with the process.
    • query: The currently executing query (for the blocking process) or the most recent query (for the blocked process).
    • application_name: Name of the application associated with the process.

References: