Re: 'Expensive' column in result set

Tom,
Thank you.
I shall experiment.

> "Harvey, Allan AC" <HarveyA(at)OneSteel(dot)com> writes:
> > select
> > b.block,
> > b.p_code,
> > p.description,
> > p.blk_speed as "set",
> > blk_speed( b.block ) as "actual",
> > blk_speed( b.block ) / ( p.blk_speed + 0.0001 ) * 100
> as "Speed %"
> > from block b, product p
> > where b.p_code = p.p_code;
> >
> > OR is postgres smart enough to know it only needs to get
> blk_speed() once.
>
> No, PG will not stop to notice the common subexpression.
> (Searching for
> such would eat more cycles than it saves, on average.) What
> you can do
> is use a two-level select:
>
> select
> block,
> p_code,
> description,
> col as "set",
> func as "actual",
> func / ( col + 0.0001 ) * 100 as "Speed %"
> from
> (select
> b.block,
> b.p_code,
> p.description,
> p.blk_speed as col,
> blk_speed( b.block ) as func
> from block b, product p
> where b.p_code = p.p_code) as ss;
>
> (The example would've been clearer if you'd not used the same name for
> both a column and a function; but I digress.)
>
> Now as this is written, the optimizer is likely to flatten
> the two-level
> select into one level and thereby copy the blk_speed function
> call into
> two places, which you don't want. The best workaround for that is to
> add "offset 0" to the sub-select. Another possibility (as of
> PG 8.2) is
> to mark the function as volatile --- but that might prevent some
> optimizations that you would like to happen, so it's probably not the
> best answer.
>
> regards, tom lane
>

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