Lock tree: All PID's waiting on a lock held by/blocked by single blocker PID. Similar to what you see in the output of this script: https://github.com/dataegret/pg-utils/blob/master/sql/locktree.sql . It uses the dot connotation to draw a tree.
Waiters: The PID (first column) returned by this query, for example
SELECT
activity.pid,
activity.usename,
activity.query,
blocking.pid AS blocking_id,
blocking.query AS blocking_query
FROM pg_stat_activity AS activity
JOIN pg_stat_activity AS blocking ON blocking.pid = ANY(pg_blocking_pids(activity.pid));
activity.pid,
activity.usename,
activity.query,
blocking.pid AS blocking_id,
blocking.query AS blocking_query
FROM pg_stat_activity AS activity
JOIN pg_stat_activity AS blocking ON blocking.pid = ANY(pg_blocking_pids(activity.pid));
DDL example: An 'alter table ... alter column ...' would cause all DML and SELECT statements to wait/block.
Hope this answers your question. Thanks for your interest.
On Fri, Mar 22, 2024 at 12:32 PM Adrian Klaver <adrian.klaver@xxxxxxxxxxx> wrote:
On 3/22/24 09:25, Fred Habash wrote:
> Facing an issue where sometimes humans login to a database and run DDL
> statements causing a long locking tree of over 1000 waiters. As a
The above needs more explanation:
1) Define locking tree.
2) Define waiters.
3) Provide examples of the DDL.
> workaround, we asked developers to always start their DDL sessions
> with 'SET lock_timeout = 'Xs'.
>
> I reviewed the native lock timeout parameter in Postgres and found 7.
> None seem to be related to blocker timeouts directly.
>
> idle_in_transaction_session_timeout
> idle_session_timeout
> lock_timeout: How long a session waits for a lock
> statement_timeout
> authentication_timeout
> deadlock_timeout
> log_lock_waits
>
> Instead, I put together a quick procedure that counts waiter sessions
> for a given blocker and terminates it if waiter count exceeds a threshold.
>
> Is there not a native way to ...
> 1. Automatically time out a blocker
> 2. A metric that shows how many waiters for a blocker?
>
> Thanks
> --
>
> ----------------------------------------
> Thank you
>
>
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
----------------------------------------
Thank you
Thank you
