Eugene <[EMAIL PROTECTED]> writes:
> Can I force HashAggregation before index scan? 

No.  But look into why the planner's rows estimate is so bad here:

>    ->  Index Scan using refseq_sequence_key2 on refseq_sequence s  
> (cost=0.00..1516.06 rows=389
> width=24) (actual time=0.352..491.107 rows=27391 loops=1)
>          Index Cond: ((species)::text = 'Homo sapiens'::text)

Have you ANALYZEd this table recently? If so, maybe you need a larger
statistics target for the species column.  The estimated row count
shouldn't be off by a factor of seventy...

                        regards, tom lane

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TIP 8: explain analyze is your friend

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