Exclusive lock question

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Exclusive lock question

Emil Briggs

I have been trying to diagnose a performance problem we have been seeing with
a postgres application. The performance of the database server is usually
quite good but every now and then it slows to a crawl. The output of vmstat
does not show excessive CPU usage or disk IO. The output of ps does show that
the number of postgres process's that appear to be stuck in some query spikes
and in some cases restarting the postgres server is the only way to clear
them. While trying to diagnose this problem I ran

select * from pg_locks

I could understand most of the output but I was wondering what a result like
the following indicates

 relation | database | transaction |  pid  |     mode      | granted
----------+----------+-------------+-------+---------------+---------
          |          |    26052434 | 29411 | ExclusiveLock | t
          |          |    26051641 | 29345 | ExclusiveLock | t
          |          |    26052415 | 29519 | ExclusiveLock | t
          |          |    26052407 | 29381 | ExclusiveLock | t
          |          |    26052432 | 29658 | ExclusiveLock | t

When I see the slowdowns there are hundreds of these with no entry for
relation or database. Any ideas what is being locked in this case?

Emil

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Re: Exclusive lock question

Tom Lane-2
Emil Briggs <[hidden email]> writes:
> When I see the slowdowns there are hundreds of these with no entry for
> relation or database. Any ideas what is being locked in this case?

Per the pg_locks documentation:

Every transaction holds an exclusive lock on its transaction ID for its
entire duration. If one transaction finds it necessary to wait
specifically for another transaction, it does so by attempting to
acquire share lock on the other transaction ID. That will succeed only
when the other transaction terminates and releases its locks.

                        regards, tom lane

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