http://www.postgresql.org/docs/9.3/static/using-explain.html On Sun, Nov 10, 2013 at 4:32 PM, monalee_dba <monalee@xxxxxxxxxxxxxxxxxxxxxx> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
