Heikki Linnakangas <hlinnaka@iki.fi> writes:
>> Here's a patch that implements page at a time index scans discussed at
>> pgsql-hackers earlier. See proposal 1 at:
>> http://archives.postgresql.org/pgsql-hackers/2006-03/msg01237.php
One potential performance lossage from this is that it partially defeats
the keys_are_unique optimization: bt_checkkeys will be run across all
the matching tuples on the index page even if the waiting caller is
going to stop after the first live one. (I don't see any way to avoid
that without breaking the entire concept, since we can't know which of
the index entries the caller will think is live.)
I suspect this is not a deal-breaker, but we have to test to make sure
that case isn't getting markedly worse. The thing to look at would be
unique indexes with expensive comparison functions (eg, text in a
non-C locale).
regards, tom lane