On Thu, 7 Jul 2022 at 04:07, DAVID ROTH <adaptron@comcast.net> wrote:
> I understand the planner can use multiple indexes to get the best plan.
> Can someone point me a paper that explains how this works.
I don't know of a paper, but if you're talking about using multiple
indexes to scan a single relation in order to satisfy a condition such
as; WHERE a = 5 OR b = 12; then the query planner is able to make use
of "bitmap index scans". A bitmap index scan simply scans an index
type which supports such scans and collects a set of ctids. For this
example, providing there's a suitable index on the "a" column and
another on the "b" column, the planner may choose to perform a bitmap
index scan on the "a" index and another on the "b" index then perform
a bitmap OR operation to obtain the intersecting ctids. The heap of
the table can then be scanned to fetch the intersecting ctids.
A ctid is the physical (more physiological) address of a tuple the
heap of a table.
David
