Opptimizing projections containing unused columns
От | Andrus |
---|---|
Тема | Opptimizing projections containing unused columns |
Дата | |
Msg-id | 1B595E8C7B5E4DFC97C08BF681C4B257@andrusnotebook обсуждение исходный текст |
Ответы |
Re: Opptimizing projections containing unused columns
|
Список | pgsql-general |
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.
В списке pgsql-general по дате отправления: