On 1/13/15 5:02 AM, David Rowley wrote:
I can't quite get my head around what you mean here, as the idea sounds quite
similar to something that's been discussed already and ruled out.
If we're joining relation a to relation b, say the plan chosen is a merge join.
If we put some special node as the parent of the merge join then how will we
know to skip or not skip any sorts that are there especially for the merge
join, or perhaps the planner choose an index scan as a sorted path, where now
that the join is removed, could become a faster seqscan. The whole plan
switching node discussion came from this exact problem. Nobody seemed to like
the non-optimal plan that was not properly optimised for having the relation
removed.
In my mind this is the same as a root level Alternative Plan, so you'd be free
to do whatever you wanted in the alternate:
-> blah blah
-> Alternate
-> Merge join
...
-> SeqScan
I'm guessing this would be easier to code, but that's just a guess. The other
advantage is if you can't eliminate the join to table A at runtime you could
still eliminate table B, whereas a top-level Alternate node doesn't have that
flexibility.
This does have a disadvantage of creating more plan variations to consider.
With a single top-level Alternate node there's only one other option. I believe
multiple Alternates would create more options to consider.
Ultimately, unless this is easier to code than a top-level alternate, it's
probably not worth pursuing.
It also seems that transitioning through needless nodes comes at a cost. This
is why I quite liked the Alternative Plan node idea, as it allowed me to skip
over the alternative plan node at plan initialisation.
For init I would expect this to result in a smaller number of nodes than a
top-level Alternate, because you wouldn't be duplicating all the stuff above
the joins. That said, I rather doubt it's worth worrying about the cost to
init; won't it be completely swamped by extra planning cost, no matter how we
go about this?
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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