From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com> |
Cc: | Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Improve OR conditions on joined columns (common star schema problem) |
Date: | 2017-02-14 21:03:24 |
Message-ID: | 26006.1487106204@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com> writes:
> On 2/14/17 1:18 PM, Tom Lane wrote:
>> One point that could use further review is whether the de-duplication
>> algorithm is actually correct. I'm only about 95% convinced by the
>> argument I wrote in planunionor.c's header comment.
> I'll put some thought into it and see if I can find any holes. Are you
> only worried about the removal of "useless" rels or is there more?
Well, the key point is whether it's really OK to de-dup on the basis
of only the CTIDs that are not eliminated in any UNION arm. I was
feeling fairly good about that until I thought of the full-join-to-
left-join-to-no-join conversion issue mentioned in the comment.
Now I'm wondering if there are other holes; or maybe I'm wrong about
that one and it's not necessary to be afraid of full joins.
regards, tom lane
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