Re: index usage for min() vs. "order by asc limit 1"

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..0.095rows=1 loops=1) 
>         Index Cond: ((strand)::text = 'sis_batch:account:15'::text)
> Total runtime: 0.129 ms
> (4 rows)
>
>
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