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

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)
>>
>>
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