Re: Never Ending query in PostgreSQL

On 2/27/22 18:20, Jeff Janes wrote:
>
> On Sun, Feb 27, 2022 at 7:09 AM Kumar, Mukesh <MKumar(at)peabodyenergy(dot)com
> <mailto:MKumar(at)peabodyenergy(dot)com>> wrote:
>
> Hi Team, 
>
> Can you please help in tunning the attached query as , i am trying
> to run this query and it runs for several hours and it did not give
> any output.
>
>
> Several hours is not all that long.  Without an EXPLAIN ANALYZE, we
> could easily spend several hours scratching our heads and still get
> nowhere.  So unless having this running cripples the rest of your
> system, please queue up another one and let it go longer.  But first, do
> an ANALYZE (and preferably a VACUUM ANALYZE) on all the tables.  If you
> have a test db which is a recent clone of production, you could do it
> there so as not to slow down production.  The problem is that the row
> estimates must be way off (otherwise, it shouldn't take long) and if
> that is the case, we can't use the plan to decide much of anything,
> since we don't trust it.
>

I'd bet Jeff is right and poor estimates are the root cause. The pattern
with a cascade of "nested loop" in the explain is fairly typical. This
is likely due to the complex join conditions and correlation.

> In parallel you could start evicting table joins from the query to
> simplify it until it gets to the point where it will run, so you can
> then see the actual row counts.  To do that it does help if you know
> what the intent of the query is (or for that matter, the text of the
> query--you attached the plan twice).
>

Right, simplify the query. Or maybe do it the other way around - start
with the simplest query (the inner-most part of the explain) and add
joins one by one (by following the explains) until it suddenly starts
being much slower.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company