Andrew Gierth <and...@tao11.riddles.org.uk> writes:
> "Tom" == Tom Lane <t...@sss.pgh.pa.us> writes:
> Tom> After further reflection I really don't like Andrew's suggestion
> Tom> that we not document the rule that multiply-referenced CTEs won't
> Tom> be inlined by default. That would be giving up the principle that
> Tom> WITH calculations are not done multiple times by default, and I
> Tom> draw the line at that. It's an often-useful behavior as well as
> Tom> one that's been documented from day one, so I do not accept the
> Tom> argument that we might someday override it on the basis of nothing
> Tom> but planner cost estimates.
> The case that springs to mind is when a CTE with grouping is then joined
> multiple times in the main query with different conditions. If the
> planner is able to deduce (e.g. via ECs) that restrictions on grouped
> columns can be pushed into the CTE, then inlining the CTE multiple times
> might be a significant win. But if that isn't possible, then inlining
> multiple times might be a significant loss.
Sure, but this is exactly the sort of situation where we should offer
a way for the user to force either decision to be made. I think it's
very unlikely that we'll ever be in a position to make a realistic
cost-based decision for that. Actually planning it out both ways would
be horrendously expensive (and probably none too reliable anyway, given
how shaky ndistinct estimates tend to be); and we certainly don't have
enough info to make a smart choice without doing that.
regards, tom lane
