Re: Choosing between seqscan and bitmap scan

Teodor Sigaev <teodor@sigaev.ru> writes:
> [ planner prefers ]
>     ->  Seq Scan on foo  (cost=0.00..5805.00 rows=4907 width=0)
> to
>     ->  Bitmap Heap Scan on foo  (cost=942.46..5755.08 rows=4907 width=0)

> Why does pgsql choose seqscan (5817.28) instead of bitmap one (5767.36)?

There's a fuzz factor of (IIRC) 1% in path cost comparisons.  It's
deciding that the seqscan and bitmapscan total costs are not
meaningfully different; then since the startup costs *are* meaningfully
different, it's making the choice on the basis of cheaper startup cost.
        regards, tom lane