Re: Possible Performance Regression with Transitive Comparisons vs. Constants

On 09/28/2012 03:35 PM, Tom Lane wrote:

> 9.1.what? For me, 8.2.23 and 9.1.6 produce the same plan and just
> about the same runtime for your query 1.

I withdraw that part of my question. I apparently didn't look closely
enough at the actual output. I was basing the version assumption on the
query speed on the new server, when it was probably due to cache effects.

The first part of the question stands, though... Why isn't the optimizer
substituting these values? a.created_date should be exactly equivalent
to '2012-05-05', but it's clearly not being treated that way.

With the full substitutions, I'm seeing things like this:

http://explain.depesz.com/s/3T4

With the column names, it's this:

http://explain.depesz.com/s/Fq7

This is on 8.2, but the behavior is the same on 9.1. From 130s to 23s
simply by substituting the constant wherever the column name is
encountered. For reference, the queries are, slow:

select a.id, f.ezorder_id
from reporting.account a
join ezorder f on f.account_id = a.account_id
where a.process_date = '2012-09-27'
and f.date_created between a.process_date - interval '6 months'
and a.process_date
and a.row_out is null

And fast:

select a.id, f.ezorder_id
from reporting.account a
join ezorder f on f.account_id = a.account_id
where a.process_date = '2012-09-27'
and f.date_created between '2012-09-27'::date - interval '6 months'
and '2012-09-27'
and a.row_out is null

We discovered this during the upgrade, but it seems to equally apply to
both 8.2 and 9.1. I've been telling the devs to replace any of these
they find all day. I can't quite say why we never "noticed" this before,
but it got exposed today pretty plainly. If this were a compiler, I'd
have expected it to treat the values as equivalent, but that's clearly
not what's happening.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas(at)optionshouse(dot)com

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