Dirk Lutzebaeck <lutzeb@aeccom.com> writes:
> say I have three tables a,b,c with lots of columns (say 10 for each
> table). Using psql command line, why does
> SELECT a.*,b.*,c.* FROM a,b,c
> takes much more longer (in my specifc case 3 times) than only
> selecting one column like
> SELECT a.oid, b.oid, c.oid FROM a,b,c.
I'd guess you are simply looking at the increased time to transfer
the additional data to the frontend, format it for display, etc.
With a pure cartesian-product join like that (no WHERE), the backend
is basically going to be spitting out tuples as fast as it can read
them from disk...
regards, tom lane