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Re: more problems with count(*) on large table

From: Mike Charnoky <noky(at)nextbus(dot)com>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>,"A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>,pgsql-general(at)postgresql(dot)org
Subject: Re: more problems with count(*) on large table
Date: 2007-09-28 20:23:11
Message-ID: 46FD62AF.10204@nextbus.com (view raw or flat)
Thread:
Lists: pgsql-general
With respect to the ALTER TABLE SET STATISTICS... how do I determine a
good value to use?  This wasn't really clear in the pg docs.  Also, do I
need to run ANALYZE on the table after I change the statistics?

Here are the EXPLAINs from the queries:

db=# explain select count(*) from prediction_accuracy where evtime
between '2007-09-25' and '2007-09-26';

QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=475677.40..475677.41 rows=1 width=0)
   ->  Index Scan using pred_acc_evtime_index on prediction_accuracy
(cost=0.00..444451.44 rows=12490383 width=0)
         Index Cond: ((evtime >= '2007-09-25 00:00:00-07'::timestamp
with time zone) AND (evtime <= '2007-09-26 00:00:00-07'::timestamp with
time zone))
(3 rows)

db=# explain select count(*) from prediction_accuracy where evtime
between '2007-09-26' and '2007-09-27';

QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=486615.04..486615.05 rows=1 width=0)
   ->  Index Scan using pred_acc_evtime_index on prediction_accuracy
(cost=0.00..454671.07 rows=12777586 width=0)
         Index Cond: ((evtime >= '2007-09-26 00:00:00-07'::timestamp
with time zone) AND (evtime <= '2007-09-27 00:00:00-07'::timestamp with
time zone))
(3 rows)


Mike

Gregory Stark wrote:
> "Alvaro Herrera" <alvherre(at)commandprompt(dot)com> writes:
> 
>> A. Kretschmer wrote:
>>> am  Fri, dem 28.09.2007, um 11:56:46 -0400 mailte Mike Charnoky folgendes:
>>>> Hi,
>>>>
>>>> I am still having problems performing a count(*) on a large table.  This
>>>>
>>>> Now, certain count(*) queries are failing to complete for certain time
>>>> ranges (I killed the query after about 24 hours).  The table is indexed
>>>> on a timestamp field.  Here is one query that hangs:
>>> Again: an index can't help! Because of MVCC: 'select count(*)' without
>>> WHERE-condition forces an seq. table-scan. 
>> But he does have a WHERE condition.  THe problem is, probably, that the
>> condition is not selective enough so the planner chooses to do a
>> seqscan.
> 
> What does EXPLAIN SELECT ... say?
> 
> 
> 

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