Re: Query Plan choice with timestamps

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Giorgio Valoti <giorgio_v@xxxxxxx> writes:
>   GroupAggregate  (cost=98431.58..119773.92 rows=74226 width=8)
>     ->  Sort  (cost=98431.58..99050.92 rows=247736 width=8)
>           Sort Key: (day_trunc(ts))
>           ->  Seq Scan on blackbox  (cost=0.00..72848.36 rows=247736 width=8)

>   GroupAggregate  (cost=0.00..19109.66 rows=74226 width=8)
>     ->  Index Scan using test_2_idx on blackbox  (cost=0.00..16943.16 rows=247736 width=8)

These numbers seem pretty bogus: there is hardly any scenario in which a
full-table indexscan should be costed as significantly cheaper than a
seqscan.  Have you put in silly values for random_page_cost?

If you haven't mucked with the cost parameters, the only way I can think
of to get this result is to have an enormously bloated table that's
mostly empty.  Maybe you need to review your vacuuming procedures.

			regards, tom lane


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