Re: [GENERAL] Why would log_lock_waits affect a query plan?

Evan Martin <postgresql2@realityexists.net> writes:
> I have an application that imports a lot of data and the does some
> queries on it to build some caches in the database, all in one long
> transaction. One of those cache updates repeatedly calls a plpgsql
> function, which internally does some SQL queries. Sometimes this is
> much, much slower than usual: 3-7 hours instead of 12-15 minutes. It was
> totally reproducible when it happened, though (running on the same
> machine, same input data).

> It turns out that the problem only happens when the "log_lock_waits"
> setting was OFF! Many machines had it ON (to troubleshoot a different
> problem), so they never experienced it.

> I eventually tracked it down to the query plan chosen for one particular
> query in the plpgsql function: using a Nested Loop makes it fast and
> using a Hash Join makes it very slow. Running an ANALYZE on one of the
> tables involved fixes the problem - the fast query plan is chosen all
> the time. This itself is a bit strange, because I was already running
> ANALYZE on all tables after the data import - it seems that I needed to
> run it a second time?

Are you using the problematic function earlier in the process?  I wonder
if it's cached a bad plan that dates from when there was much less data
in the table.  I also wonder if maybe the specific table is being updated
after the ANALYZEs.  In the situation you've got here, you can't expect
any help from auto-analyze; only your own manual ANALYZEs are going to be
able to see the uncommitted data in the tables.

> But what I'd really like to understand is: why did
> setting log_lock_waits to ON always change the query plan to use a
> Nested Loop? It's just not something I'd ever expect to affect a query plan.

TBH, I don't believe it.  There are a lot of moving parts here, but
I don't see how that could be relevant.

            regards, tom lane