Re: Best way to scan on-disk bitmaps

Tom Lane wrote:
> "Victor Y. Yegorov" <viy(at)mits(dot)lv> writes:
> > If I have on-disk bitmap
> > ON (a, b, c)
> > will the planner pick an index scan for
> > WHERE a = 42 AND b = 'foo'
> > (i.e. only part of the index attributes are involved)? Any modifications
> > needed to achieve this functionality?
>
> Hmm. That particular case will work, but the planner believes that only
> consecutive columns in the index are usable --- that is, if you have
> quals for a and c but not for b, it will think that the condition for c
> isn't usable with the index. This is true for btree and gist indexes,
> so I suppose we'd need to introduce a pg_am column that tells what to
> do.

We do have a TODO for this:

* Use index to restrict rows returned by multi-key index when used with
non-consecutive keys to reduce heap accesses

For an index on col1,col2,col3, and a WHERE clause of col1 = 5 and
col3 = 9, spin though the index checking for col1 and col3 matches,
rather than just col1; also called skip-scanning.

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