Take the following table as an example: CREATE TABLE grue ( id SERIAL PRIMARY KEY, size VARCHAR(255) ); CREATE INDEX grue_size ON grue(size);
Now insert approximately eleventy jillion rows, but ensure that there are only about 20 distinct values for size. SELECT DISTINCT size FROM grue; Always does a seq scan on Postgres 9.5.2. (Yes, I know we're a patch behind, the upgrade is on the schedule) on Ubuntu 14. I would expect it to be possible, and significantly more efficient to do an index scan for that query. Is this a bug, an optimization that is simply waiting for someone to take the time to implement, or is there some underlying reason why this isn't possible that I'm not seeing. And, yes, I know that's not a normalized table and that properly normalizing it makes the problem disappear. And yes, this is repeatable (I'm working with about 6 tables with similar structure that all exhibit the same behavior) and yes I've done ANALYZE and VACUUM and the behavior doesn't change. -- Bill Moran -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
