Re: Choosing between seqscan and bitmap scan

On Thu, 29 Apr 2010, Tom Lane wrote:

> Teodor Sigaev <teodor(at)sigaev(dot)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.

hmm, what if we add index scan preference inside 1% tolerance ?

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru)
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83