Volker Grabsch <vog@xxxxxxxxxxxxxxxxxx> writes: > I'm confused about the absence of a very simple optimization > in PostgreSQL. Suppose we have a VIEW where some columns are > expensive to be calculated: > CREATE VIEW a AS > SELECT > (... expensive calculation ...) as expensive, > count(*) as cheap > FROM > x; > where "x" is a sufficiently large table. I would expect the > following query to be very fast: > SELECT cheap FROM a; > However, it takes the same time as "SELECT * FROM a;". I think your main error is in supposing that count(*) is cheap ... PG will suppress unused columns when a view can be flattened, but typically not otherwise. In particular a view involving aggregates won't get flattened; but given that the aggregates will force a whole-table scan anyway, I can't get that excited about removing a subset of them. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
