On Wed, Jan 6, 2016, at 08:41 PM, Tom Lane wrote: > Seamus Abshere <sea...@abshere.net> writes: > > -> Can a function like `LEFT()` use an index? > Since the question makes little sense as stated, I'm going to assume > you mean "can a query like SELECT ... WHERE left(foo, 3) = 'bar' > use an index on column foo?" > > The answer to that is no, there is no such optimization built into > Postgres. (In principle there could be, but I've not heard enough > requests to make me think we'd ever pursue it.) > > The equivalent optimization that *is* built in, and has been for > a long time, is for LIKE: "SELECT ... WHERE foo LIKE 'bar%'" can > use an index on foo, at least if it's an index sorted according to > C collation.
hi Tom, I should have been more general. In layman's/narrative terms, what's the deal with functions vs. operators for postgres indexes? For example, `exist(hstore,text)` vs. `hstore ? text` ? Thank you! Seamus PS. If I have understood correctly over the years, in order for the query planner to use indexes, it needs to see operators - functions are opaque to it. I'm looking for a bit more narrative on this to round out my understanding. -- Seamus Abshere, SCEA +598 99 54 99 54 https://github.com/seamusabshere -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
