Re: [HACKERS] index-only count(*) for indexes supporting bitmap scans

I would like to propose a patch that speeds up the queries of the form 'select
count(*) ... where ...',  where the restriction clauses can be satisfied by some
indexes. At the moment, such queries use index-only scans followed by
aggregation. Index-only scans are only possible for indexes that are capable of
returning indexed tuples, that is, support the 'amgettuple' access method. They
are not applicable to indexes such as GIN and RUM. However, it is possible to
improve count(*) performance for indexes that support bitmap scans. Having
performed a bitmap index scan or a combination of such, the bits in bitmap can
be counted to obtain the final result. Of course, the bitmap pages that are
lossy or not visible to all existing transactions will still require heap
access.

This patch has some important limitations:
* It only supports targetlist consisting of a single expression that can be
projected from count(*).
* count(expr) is not supported. We could support it for cases where the
"expr is not null" restriction can be satisfied with an index.
* The current implementation does not support parallel execution. It could be
implemented during the PostgreSQL 11 release cycle.
* For some indexes, the bitmap index scan will always require rechecking all
the tuples. Bitmap count plans should not be used in such cases. For now, this
check is not implemented.