Re: Reducing expression evaluation overhead

>>>>> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

Tom> I'm not sure that this would let us catch up to what Arjen
Tom> reports as MySQL's expression evaluation speed, but it should
Tom> at least speed things up a bit with only fairly localized
Tom> changes.

I like the idea of memoizing the switch with function pointers as I
don't think branch prediction helps much with varying switch arms
selected with different exprs. Also I agree that the delta of indirect
function invocation is probably small.

I've forgotten the syntax of case, but for the simple form isn't
expr=const going to be the same expr for each case arm ? If that's the
case, couldn't we actually save the value of expr in a Datum and then
reuse that (through a Const) in each of the other arms to evaluate the
actual exprs ? That should reduce the number of times ExecEvalVar (and
through it heapgetattr) are called.

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Sailesh
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