How can I identify what is blocking a query on a DB instance that is running Amazon RDS PostgreSQL or Aurora PostgreSQL?
Last updated: 2019-11-07
I tried to run a query on a DB instance that is running Amazon Relational Database Service (Amazon RDS) PostgreSQL or Amazon Aurora PostgreSQL. But the query was blocked, even though no other queries were executing at the same time. Why was the query blocked, and how do I resolve this issue?
Most often, blocked queries are caused by uncommitted transactions. Uncommitted transactions can cause new queries to be blocked, to sleep, and to eventually fail when they exceed the lock wait timeout or the statement timeout. To resolve this issue, first identify the blocking transaction, and then stop the blocking transaction.
1. Identify the current state of the blocked transaction by running the following query against the pg_stat_activity table:
SELECT * FROM pg_stat_activity WHERE query iLIKE '%TABLE NAME%' ORDER BY state;
Note: Replace TABLE NAME with your own table name or condition.
If the value of the wait_event_type column is Lock, the query is blocked by other transactions or queries. If the wait_event_type column has any other value, there is a performance bottleneck with resources such as CPU, storage, or network capacity. To resolve performance bottlenecks, tune the performance of your database, for example, by adding indexes, rewriting queries, or executing vacuum and analyze. For more information, see Best Practices for Working with PostgreSQL.
If you enabled Performance Insights on your DB instance, you can also identify blocked transactions by viewing the DB load that is grouped by wait event, hosts, SQL queries, or users. For more information, see Using Amazon RDS Performance Insights.
2. If the value of the wait_event_type column is Lock, then you can identify the cause of the blocked transaction by running the following:
SELECT blocked_locks.pid AS blocked_pid, blocked_activity.usename AS blocked_user, blocked_activity.client_addr as blocked_client_addr, blocked_activity.client_hostname as blocked_client_hostname, blocked_activity.client_port as blocked_client_port, blocked_activity.application_name as blocked_application_name, blocked_activity.wait_event_type as blocked_wait_event_type, blocked_activity.wait_event as blocked_wait_event, blocked_activity.query AS blocked_statement, blocking_locks.pid AS blocking_pid, blocking_activity.usename AS blocking_user, blocking_activity.client_addr as blocking_user_addr, blocking_activity.client_hostname as blocking_client_hostname, blocking_activity.client_port as blocking_client_port, blocking_activity.application_name as blocking_application_name, blocking_activity.wait_event_type as blocking_wait_event_type, blocking_activity.wait_event as blocking_wait_event, blocking_activity.query AS current_statement_in_blocking_process FROM pg_catalog.pg_locks blocked_locks JOIN pg_catalog.pg_stat_activity blocked_activity ON blocked_activity.pid = blocked_locks.pid JOIN pg_catalog.pg_locks blocking_locks ON blocking_locks.locktype = blocked_locks.locktype AND blocking_locks.DATABASE IS NOT DISTINCT FROM blocked_locks.DATABASE AND blocking_locks.relation IS NOT DISTINCT FROM blocked_locks.relation AND blocking_locks.page IS NOT DISTINCT FROM blocked_locks.page AND blocking_locks.tuple IS NOT DISTINCT FROM blocked_locks.tuple AND blocking_locks.virtualxid IS NOT DISTINCT FROM blocked_locks.virtualxid AND blocking_locks.transactionid IS NOT DISTINCT FROM blocked_locks.transactionid AND blocking_locks.classid IS NOT DISTINCT FROM blocked_locks.classid AND blocking_locks.objid IS NOT DISTINCT FROM blocked_locks.objid AND blocking_locks.objsubid IS NOT DISTINCT FROM blocked_locks.objsubid AND blocking_locks.pid != blocked_locks.pid JOIN pg_catalog.pg_stat_activity blocking_activity ON blocking_activity.pid = blocking_locks.pid WHERE NOT blocked_locks.granted ORDER BY blocked_activity.pid;
3. Review the columns that have a blocking prefix. In the following example table that was generated by this query, you can see that the blocked transaction is running on the 184.108.40.206 host and using psql:
blocked_pid | 9069 blocked_user | master blocked_client_addr | 220.127.116.11 blocked_client_hostname | blocked_client_port | 50035 blocked_application_name | psql blocked_wait_event_type | Lock blocked_wait_event | transactionid blocked_statement | UPDATE test_tbl SET name = 'Jane Doe' WHERE id = 1; blocking_pid | 8740 blocking_user | master blocking_user_addr | 18.104.22.168 blocking_client_hostname | blocking_client_port | 26259 blocking_application_name | psql blocking_wait_event_type | Client blocking_wait_event | ClientRead current_statement_in_blocking_process | UPDATE tset_tbl SET name = 'John Doe' WHERE id = 1;
Tip: Use blocking_user, blocking_user_addr, and blocking_client_port to help identify which sessions are blocking transactions.
Important: Before terminating transactions, evaluate the potential impact that each transaction has on the state of your database and your application.
4. After reviewing the potential impact of each transaction, stop the transactions by running the following query:
Note: Replace PID with blocking_pid of the process that you identified in step 3.