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Re: Simple but slow

From: "Chad Thompson" <chad(at)weblinkservices(dot)com>
To: <josh(at)agliodbs(dot)com>, "pgsql-novice" <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Simple but slow
Date: 2002-08-22 01:11:42
Message-ID: 009101c24978$e2f78290$32021aac@chad (view raw, whole thread or download thread mbox)
Lists: pgsql-novice
Thanks for your reply Josh, as usual i learn from you whenever you write.
Ive been having a hard time understanding what explain is telling me.
I was able to get the query down to 19 secs w/o the distinct.  I think i'll
move the distinct to one of my faster queries.

If its not too much trouble id like you to look at another.  This is really
being a beast.

select, l.full_phone into "8_21_2002"
from "temp_list_tier" l LEFT JOIN
(select phonenum
from call_results_fixed
where client_id = 8) as cr
ON l.full_phone = cr.phonenum
where cr.phonenum Is Null

I have indexes on cr.phonenum and l.full_phone but it doesnt seem to use
them with the subselect.
cr is about 1 million records, l varies, usually never more than about
30-40K of records.

Merge Join  (cost=265368.44..8176071.25 rows=3161144 width=44) (actual
time=121889.56..126948.26 rows=11384 loops=1)
  ->  Index Scan using temp_list_tier_idx on temp_list_tier l
(cost=0.00..4431.97 rows=99997 width=25) (actual time=0.38..808.34
rows=99997 loops=1)
  ->  Sort  (cost=256113.44..256113.44 rows=1264495 width=14) (actual
time=121887.71..122732.11 rows=422624 loops=1)
        ->  Subquery Scan cr  (cost=0.00..41331.00 rows=1264495 width=14)
(actual time=0.10..12941.66 rows=1274987 loops=1)
              ->  Seq Scan on call_results_fixed  (cost=0.00..41331.00
rows=1264495 width=14) (actual time=0.09..9047.89 rows=1274987 loops=1)
Total runtime: 127273.03 msec

Thanks for your help.
I have also enjoyed your "The Joy of Index".  I look forward to the next

----- Original Message -----
From: "Josh Berkus" <josh(at)agliodbs(dot)com>
To: "Chad Thompson" <chad(at)weblinkservices(dot)com>; "pgsql-novice"
Sent: Wednesday, August 21, 2002 6:48 PM
Subject: Re: [NOVICE] Simple but slow


If you take a look at the Explain content, you'll see where the slow-down

> Limit  (cost=96929.63..97822.45 rows=35713 width=36) (actual
time=42477.82..43491.69 rows=100000 loops=1)
>   ->  Unique  (cost=96929.63..97822.46 rows=35713 width=36) (actual
time=42477.82..43325.87 rows=100001 loops=1)
>         ->  Sort  (cost=96929.63..96929.63 rows=357132 width=36) (actual
time=42477.80..42735.18 rows=102151 loops=1)

These three lines are the selecting unique values and sorting and
the result set.   This is most of your computing time; see the "actual time

>               ->  Hash Join  (cost=9.59..41831.14 rows=357132 width=36)
(actual time=25.29..16456.26 rows=352194 loops=1)
>                     ->  Seq Scan on lists l  (cost=0.00..32881.18
rows=895244 width=29) (actual time=0.10..11342.50 rows=878098 loops=1)
>                     ->  Hash  (cost=9.22..9.22 rows=148 width=7) (actual
time=23.80..23.80 rows=0 loops=1)
>                           ->  Seq Scan on timezone tz  (cost=0.00..9.22
rows=148 width=7) (actual time=21.72..23.45 rows=148 loops=1)

This is you join to the area codes.  It's ignoring the indexes, because the
number of records in timezone is so small compared to the number in lists.
this isn't a problem, though, because as you can see the join operation
only a few milliseconds at a minimum.

> Total runtime: 46247.79 msec

The way I read this, 95% of the time is being spent on the DISTINCT.   Tom,
I reading this right?
1) Indexing lists.full_phone.
2) Check the speed without the DISTINCT as  a benckmark.
3) Increasing the amount of memory available to your queries by altering the
postgresql.conf settings and possibly adding more RAM or improving your disk
access speed.

-Josh Berkus
 Aglio Database Solutions
 San Francisco

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