The background on index-only scans:
https://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.2#Index-only_scans
In either case(index, index-only) the index has to be scanned. The
difference is where the data is pulled from. In the index-only scan
the query still needs to consult the visibility map to determine
whether the tuple pointed to by the index entry is visible. I would
say that in the limit 1 case the planner determines it is just as easy
to check and pull the data from the actual tuple as to to check the
visibility map. In the limit 100 case it becomes more cost effective
to use the visibility map and pull data directly from the index data.
Thank you for your answer
Enrico
--
That's one small step for man; one giant leap for mankind
Enrico Pirozzi
Tel. +39 0861 1855771 - Mob.+39 328 4164437 - Fax +39 0861 1850310
http://www.pgtraining.com <http://www.pgtraining.com-> -
info@xxxxxxxxxxxxxx
www.enricopirozzi.info - info@xxxxxxxxxxxxxxxxxx
PEC: enrico.pirozzi@xxxxxx
Skype sscotty71 - Gtalk sscotty71@xxxxxxxxx
--
Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general
