http://www.postgresql.org/docs/9.3/static/using-explain.html
On Sun, Nov 10, 2013 at 4:32 PM, monalee_dba
<monalee@sungraceinfotech.co.in> wrote:
> I would like to know, What is BitMap Heap Scan & BitMap Index Scan? When I
> use EXPLAIN for query, which has LEFT JOIN with 4 different table then some
> time query planner uses Bitmap Heap Scan and some time Bitmap Index Scan?
The way to go here would be to have a look at the documentation first:
http://www.postgresql.org/docs/9.3/static/using-explain.html
Then, AFAIK, Bitmap Heap Scan (upper level) is always coupled with
Bitmap Index Scan (lower level) so there are always in at least 2
nodes, at least because you could have multiple Bitmap Index Scan
nodes. The lower node Bitmap Index Scan creates a bitmap of the pages
of the relation to track pages that might contain tuples satisfying
the index condition (1 bit per page, so a relation with 1 million
pages would have roughly 119kB). Then the bitmap is passed to the
upper node called "Bitmap Index Scan", that reads the pages in a more
sequential fashion.
Regards,
--
Michael
