Re: Expression Pruning in postgress

HarmeekSingh Bedi  writes:
>    1. May be I am mistaken Kindly  help me understand a bit more. I do agree
>    that passing datums up the node chain helps - but consider the case when
>    either Sort or Hash joins spills on disk - large columns that get written on
>    to the disk will still cause a lot of performance issues {as sorts spills
>    will detoast}

No, they don't.  What gets sent to disk is normally just the toast
pointer datum (19 bytes or whatever it is these days).

> I did make a fix at least to alleviate this case in the optimizer . But I am
> going to work on a more general approach of expression pruning based on the
> lifetime of an expression. Basically each node will either references or
> generate an expression. Any expression that is generated and is not
> referenced by any top on top will be eliminated.

Sounds like overkill.
		regards, tom lane