Re: [9.2devel] why it doesn't do index scan only?

Hello

2011/10/8 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> hubert depesz lubaczewski <depesz(at)depesz(dot)com> writes:
>> it is selecting 20 rows out of 30 million. why is it:
>> 1. not using index only scan
>> 2. not using even normal index scan?
>
> It thinks the bitmap scan is cheaper.  Whether that's true or not is not
> very clear, but nobody is claiming that the costing of index-only scans
> is accurate yet.
>

I did a few tests and bitmap scan is faster. Maybe there is a some
issue. In very simple test (and very syntetic test)

create table omega(a int);
insert into omega select (random()*10000)::int from generate_series(1,400000);

select count(*) from omega where a = 100;

and index scan is faster than index only scan. There is lot of
duplicates. When I used a bigger range, a speed of bitmap index, index
only scan and index scan is similar - but index scan was faster
everywhere.

Regards

Pavel Stehule

>                        regards, tom lane
>
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