Andrus,Hi！ 1.From the query plan,it makes no difference. 2.PostgreSQL 8.0+ will not exclude columns c3..c20 while executing. 3.I just make a test,From query plan or executing time,it makes no difference. I'm from China.I hope you could see what I mean :) ============================================================ >I have lot of autogenerated from projection queries in form > >SELECT source.c1, source.c2, t1.col1, t1.col2, ... >FROM (SELECT c1, c2, c3, ..... , c20 FROM ... WHERE ... ) source >LEFT JOIN t2 USING (somecolumn) > >Main SELECT uses only few columns (source.c1 and source.c2 in this sample) >from source projection. >Inner table may have up to 500000 rows. > >I read 8.4 WITH documentation and this describes somewhat that PostgreSQL >can discard unused columns automatically but this is not clear. I havent >found exact doc about this. I havent way how to determine this using EXPLAIN >or similar command. > >Questions: > >1. Does selecting of unused columns ( c3 ... c20) affect to query prefomance >? >2. Will PostgreSQL 8.0+ automatically exclude columns c3 .. c20 while >executing >query ? >3. Or should I make sql builder much more sophisticated so that it will not >generate columns c3 .. c20 when it creates sql ? > >Andrus. > > >-- >Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) >To make changes to your subscription: >http://www.postgresql.org/mailpref/pgsql-general weinna.chen Beijing China -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)To make changes to your subscription:http://www.postgresql.org/mailpref/pgsql-general
