Re: Possibly slow query

Richard,
I tried a left join, which has to be a little weird, because there may or
may not be a corresponding row in Assignment_Settings for each Assignment,
and they may or may not have Setting='Status', so I came up with:

SELECT User_ID
FROM Assignments A NATURAL LEFT JOIN (SELECT * FROM Assignment_Settings
WHERE Setting='Status') ASet
WHERE A.User_ID IS NOT NULL
AND ASet.Assignment_ID IS NULL
GROUP BY User_ID;

Which explain analyze is saying takes 0.816 ms as compared to 0.163 ms for
my other query. So, I'm not sure that I'm writing the best LEFT JOIN that I
can. Also, I suspect that these ratios wouldn't hold as the data got bigger
and started using indexes, etc. I'll mock up a couple of tables with a
bunch of data and see how things go. It would be nice to understand WHY I
get the results I get, which I'm not sure I will.

I'm not sure what you mean by selecting a distinct User_ID first. Since
I'm joining the tables on Assignment_ID, I'm not sure how I'd do a distinct
before the join (because I'd lose Assignment_ID). I was also under the
impression that group by was likely to be faster than a distinct, tho I
can't really recall where I got that idea from.

Thanks for your suggestions!
Peter Darley

-----Original Message-----
From: Richard Huxton [mailto:dev(at)archonet(dot)com]
Sent: Wednesday, January 26, 2005 1:36 AM
To: Peter Darley
Cc: Pgsql-Performance
Subject: Re: [PERFORM] Possibly slow query

Peter Darley wrote:
> Folks,
>
> I'm using PostgreSQL 7.4.1 on Linux, and I'm trying to figure out weather
a
> query I have is going to be slow when I have more information in my
tables.
> both tables involved will likely have ~500K rows within a year or so.
>
> Specifically I can't tell if I'm causing myself future problems with the
> subquery, and should maybe re-write the query to use a join. The reason I
> went with the subquery is that I don't know weather a row in Assignments
> will have a corresponding row in Assignment_Settings
>
> The query is:
> SELECT User_ID
> FROM Assignments A
> WHERE A.User_ID IS NOT NULL
> AND (SELECT Value FROM Assignment_Settings WHERE Setting='Status' AND
> Assignment_ID=A.Assignment_ID) IS NULL
> GROUP BY User_ID;

You could always use a LEFT JOIN instead, like you say. I'd personally
be tempted to select distinct user_id's then join, but it depends on how
many of each.

You're not going to know for sure whether you'll have problems without
testing. Generate 500k rows of plausible looking test-data and give it a
try.

--
Richard Huxton
Archonet Ltd