Re: Zero throughput on a query on a very large table.

OK... I think we may have cracked this.

First, do you think that 128MB work_mem is ok? We have a 64GB machine and expecting fewer than 100 connections. This is really an ETL workload environment at this time.

Second, here is what i found and what messed us up.

select current_setting('random_page_cost'); --> 4

alter database "CMS_TMP" set random_page_cost=0.00000001;
select current_setting('random_page_cost'); --> 4 ????

I also tried:
select current_setting('random_page_cost'); --> 4
select set_config('random_page_cost', '0.000001', true);
select current_setting('random_page_cost'); --> 4 ????

Is there something that is happening that is causing those settings to not stick? I then tried:

select current_setting('random_page_cost'); --> 4
select set_config('random_page_cost', '0.000001', false); -- false now, i.e., global
select current_setting('random_page_cost'); --> 0.000001 !!!!

So i think we just spent 4 days on that issue. I then did

select set_config('enable_seqscan', 'off', false);
And the plan is now using an index scan, and we are getting 12K rows/s in throughput immediately!!! 😊

So i guess my final question is that i really want to only affect that one query executing, and i seem to not be able to change the settings used by the planner just for that one transaction. I have to change it globally which i would prefer not to do. Any help here?

Thanks,

Laurent.

________________________________
From: ldh(at)laurent-hasson(dot)com <ldh(at)laurent-hasson(dot)com>
Sent: Friday, January 25, 2019 1:36:21 PM
To: Tom Lane
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Zero throughput on a query on a very large table.

Sorry :) When i look at the "SQL" tab in PGAdmin when i select the index in the schema browser. But you are right that /d doesn't show that.

________________________________
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Sent: Friday, January 25, 2019 1:34:01 PM
To: ldh(at)laurent-hasson(dot)com
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Zero throughput on a query on a very large table.

"ldh(at)laurent-hasson(dot)com" <ldh(at)laurent-hasson(dot)com> writes:
> Also, the original statement i implemented did not have all of that. This is the normalized SQL that Postgres now gives when looking at the indices.

[ squint... ] What do you mean exactly by "Postgres gives that"?
I don't see any redundant COLLATE clauses in e.g. psql \d.

regards, tom lane