Re: Opptimizing projections containing unused columns

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 :)

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>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.
>
>
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weinna.chen

Beijing China