Identify & Resolve Blocking Application Locks in PostgreSQL

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 when the process is active (the blocked process is still running the statement that waits), or the most recent query when it is not active (for example, a blocking session sitting idle in a transaction).
    • application_name: Name of the application associated with the process.

References:

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