Re: Regarding query minimizer (simplifier)

Hello Tom,

Sorry for the misleading. Could you try these two queries? I made the query even slower in latest version of postgres. These are information about how we set up evaluation environment and query result.

Thanks,

Jinho Jung

Install Multiple version of DBs in one machine
======================================
# Install 10.5
$ wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
$ sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ xenial-pgdg main" > /etc/apt/sources.list.d/pgdg_xenial.list'
$ sudo apt update
$ sudo apt-get install postgresql-10

# Install 9.6
$ sudo apt-get install postgresql-9.6

# Install 9.5
$ sudo apt-get install postgresql-9.5

# Install 9.4
$ sudo apt-get install postgresql-9.4 postgresql-contrib-9.4 libpq-dev postgresql-server-dev-9.4

# check
$ pg_lsclusters

Original regression query
==========================
explain analyze
select
1
from
information_schema.role_usage_grants as ref_2,
lateral (
select
max((null)) over (partition by ref_3.amopfamily) as c8
from
pg_catalog.pg_amop as ref_3
) as subq_0
;

ORIGINAL querying time
on old version(9.4/9.5): 5.7ms
on latest version(10): 91.76ms

CORRELATED query to maximize error
===================================
explain analyze
select *
from information_schema.role_usage_grants f1
where grantor =
( select max(ref_2.grantor)
from
information_schema.role_usage_grants as ref_2,
lateral (
select
max((null)) over (partition by ref_3.amopfamily) as c8
from
pg_catalog.pg_amop as ref_3
) as subq_0
where ref_2.object_catalog = f1.object_catalog
)
;

CORRELATED querying time
on old version(9.4/9.5): 0.6s
on latest version(10): 113s
188 times slower

________________________________
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Sent: Saturday, October 13, 2018 5:59:06 PM
To: Jung, Jinho
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Regarding query minimizer (simplifier)

"Jung, Jinho" <jinho(dot)jung(at)gatech(dot)edu> writes:
> Hello, I am Jinho Jung, Phd Student from GeorgiaTech, and try to find any SQL queries that cause performance regression. While conducting evaluation, I found an interesting query which makes x80 times slower execution in version 10.5 than version 9.4. Please see the attached files, if you are interested.

Hm, testing this in the regression database, it seems pretty speedy
across all supported branches, and indeed slower in 9.4 than later
branches (~25 ms vs ~10 ms).

It seems likely that you're testing in a very different database,
perhaps one with many more tables ... but if you don't explain the
test scenario, we aren't going to have much luck investigating.

regards, tom lane