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* index usage for min() vs. "order by asc limit 1"
@ 2011-11-18 01:12 Ben Chobot <bench@silentmedia.com>
0 siblings, 1 reply; 4+ messages in thread
From: Ben Chobot @ 2011-11-18 01:12 UTC (permalink / raw)
To: pgsql-performance
I have two queries in PG 9.1. One uses an index like I would like, the other does not. Is this expected behavior? If so, is there any way around it?
postgres=# explain analyze select min(id) from delayed_jobs where strand='sis_batch:account:15' group by strand;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=0.00..8918.59 rows=66 width=29) (actual time=226.759..226.760 rows=1 loops=1)
-> Seq Scan on delayed_jobs (cost=0.00..8553.30 rows=72927 width=29) (actual time=0.014..169.941 rows=72268 loops=1)
Filter: ((strand)::text = 'sis_batch:account:15'::text)
Total runtime: 226.817 ms
(4 rows)
postgres=# explain analyze select id from delayed_jobs where strand='sis_batch:account:15' order by id limit 1;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..0.33 rows=1 width=8) (actual time=0.097..0.098 rows=1 loops=1)
-> Index Scan using index_delayed_jobs_on_strand on delayed_jobs (cost=0.00..24181.74 rows=72927 width=8) (actual time=0.095..0.095 rows=1 loops=1)
Index Cond: ((strand)::text = 'sis_batch:account:15'::text)
Total runtime: 0.129 ms
(4 rows)
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: index usage for min() vs. "order by asc limit 1"
@ 2011-11-18 01:20 Steve Atkins <steve@blighty.com>
parent: Ben Chobot <bench@silentmedia.com>
0 siblings, 1 reply; 4+ messages in thread
From: Steve Atkins @ 2011-11-18 01:20 UTC (permalink / raw)
To: pgsql-performance
On Nov 17, 2011, at 5:12 PM, Ben Chobot wrote:
> I have two queries in PG 9.1. One uses an index like I would like, the other does not. Is this expected behavior? If so, is there any way around it?
I don't think you want the group by in that first query.
Cheers,
Steve
>
>
> postgres=# explain analyze select min(id) from delayed_jobs where strand='sis_batch:account:15' group by strand;
> QUERY PLAN
> --------------------------------------------------------------------------------------------------------------------------
> GroupAggregate (cost=0.00..8918.59 rows=66 width=29) (actual time=226.759..226.760 rows=1 loops=1)
> -> Seq Scan on delayed_jobs (cost=0.00..8553.30 rows=72927 width=29) (actual time=0.014..169.941 rows=72268 loops=1)
> Filter: ((strand)::text = 'sis_batch:account:15'::text)
> Total runtime: 226.817 ms
> (4 rows)
>
> postgres=# explain analyze select id from delayed_jobs where strand='sis_batch:account:15' order by id limit 1;
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.00..0.33 rows=1 width=8) (actual time=0.097..0.098 rows=1 loops=1)
> -> Index Scan using index_delayed_jobs_on_strand on delayed_jobs (cost=0.00..24181.74 rows=72927 width=8) (actual time=0.095..0.095 rows=1 loops=1)
> Index Cond: ((strand)::text = 'sis_batch:account:15'::text)
> Total runtime: 0.129 ms
> (4 rows)
>
>
> --
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> To make changes to your subscription:
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^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: index usage for min() vs. "order by asc limit 1"
@ 2011-11-18 01:23 Ben Chobot <bench@silentmedia.com>
parent: Steve Atkins <steve@blighty.com>
0 siblings, 1 reply; 4+ messages in thread
From: Ben Chobot @ 2011-11-18 01:23 UTC (permalink / raw)
To: Steve Atkins <steve@blighty.com>; +Cc: pgsql-performance
On Nov 17, 2011, at 5:20 PM, Steve Atkins wrote:
>
> I don't think you want the group by in that first query.
Heh, I tried to simply the example, but in reality that = becomes an in clause of multiple values. So the group by is needed.
>>
>>
>> postgres=# explain analyze select min(id) from delayed_jobs where strand='sis_batch:account:15' group by strand;
>> QUERY PLAN
>> --------------------------------------------------------------------------------------------------------------------------
>> GroupAggregate (cost=0.00..8918.59 rows=66 width=29) (actual time=226.759..226.760 rows=1 loops=1)
>> -> Seq Scan on delayed_jobs (cost=0.00..8553.30 rows=72927 width=29) (actual time=0.014..169.941 rows=72268 loops=1)
>> Filter: ((strand)::text = 'sis_batch:account:15'::text)
>> Total runtime: 226.817 ms
>> (4 rows)
>>
>> postgres=# explain analyze select id from delayed_jobs where strand='sis_batch:account:15' order by id limit 1;
>> QUERY PLAN
>> ---------------------------------------------------------------------------------------------------------------------------------------------------------
>> Limit (cost=0.00..0.33 rows=1 width=8) (actual time=0.097..0.098 rows=1 loops=1)
>> -> Index Scan using index_delayed_jobs_on_strand on delayed_jobs (cost=0.00..24181.74 rows=72927 width=8) (actual time=0.095..0.095 rows=1 loops=1)
>> Index Cond: ((strand)::text = 'sis_batch:account:15'::text)
>> Total runtime: 0.129 ms
>> (4 rows)
>>
>>
>> --
>> Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
>> To make changes to your subscription:
>> http://www.postgresql.org/mailpref/pgsql-performance
>
>
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^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: index usage for min() vs. "order by asc limit 1"
@ 2011-11-18 14:16 MirrorX <mirrorx@gmail.com>
parent: Ben Chobot <bench@silentmedia.com>
0 siblings, 0 replies; 4+ messages in thread
From: MirrorX @ 2011-11-18 14:16 UTC (permalink / raw)
To: pgsql-performance
can you run an analyze command first and then post here the results of:
select * FROM pg_stats WHERE tablename = 'delayed_jobs';
?
--
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2011-11-18 01:12 index usage for min() vs. "order by asc limit 1" Ben Chobot <bench@silentmedia.com>
2011-11-18 01:20 ` Steve Atkins <steve@blighty.com>
2011-11-18 01:23 ` Ben Chobot <bench@silentmedia.com>
2011-11-18 14:16 ` MirrorX <mirrorx@gmail.com>
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