Re: Bitmap index scans use of filters on available columns

On 4 November 2015 at 15:54, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Nicolas Barbier <nicolas(dot)barbier(at)gmail(dot)com> writes:
> > 2015-11-04 Antonin Houska <ah(at)cybertec(dot)at>:
> >> (see expand_indexqual_opclause()), I'm not sure any kind of expansion is
> >> possible for '%abc%' which would result in a b-tree searchable
> condition.
>
> > I think the question is not about using the b-tree for checking the
> > condition, but about just retrieving the value for y from the index,
> > and just using that to check the condition before fetching the
> > corresponding tuple from the heap.
>
> Bitmap index scans only return TID bitmaps, not index tuples; indeed
> the underlying index may not store anything recognizable as tuples.
>

Agreed, though the OP's question was asking why a Filter condition can't be
added to a BitmapIndexScan, so that non-qualifying rows can be excluded
from the TID bitmap it generates.

We generate this plan

postgres=# explain select * from f where x=5 and y like '%abc%';

QUERY PLAN

--------------------------------------------------------------------------

Index Scan using f_x_y_idx on f (cost=0.42..26075.71 rows=209 width=37)
Index Cond: (x = 5)
Filter: (y ~~ '%abc%'::text)

So it should be possible to do the Filter condition on the BitmapIndexScan.

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