Re: Wrong query results caused by loss of join quals

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От Richard Guo
Тема Re: Wrong query results caused by loss of join quals
Дата
Msg-id CAMbWs4_WoRbxaoe4Rt1doSEiEewBNFuuF=iwQXpZKv0WZyfm0A@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Wrong query results caused by loss of join quals  (Tom Lane <tgl@sss.pgh.pa.us>)
Ответы Re: Wrong query results caused by loss of join quals  (Tom Lane <tgl@sss.pgh.pa.us>)
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Дерево обсуждения

On Mon, Feb 20, 2023 at 4:56 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
* I suspect the other use of get_common_eclass_indexes, in
have_relevant_eclass_joinclause, is broken as well.
 
It seems have_relevant_joinclause is broken for the presented case.  It
does not get a change to call have_relevant_eclass_joinclause, because
flag 'has_eclass_joins' is not set for t1 due to t1 being not in the
EC's ec_relids.  As a result, have_relevant_joinclause thinks there is
no joinclause that involves t1 and t2, which is not right.
 
* This fix throws away a fair bit of the optimization intended by
3373c7155, since it will result in examining some irrelevant ECs.
I'm not sure if it's worth complicating get_common_eclass_indexes
to try to recover that by adding knowledge about outer joins.
 
Yeah, this is also my concern that we'd lose some optimization about
finding ECs.
 
* I'm now kind of wondering whether there are pre-existing bugs of the
same ilk.  Maybe not, because before 2489d76c4 an EC constraint that was
computable at the join but not earlier would have to have mentioned both
sides of the join ... but I'm not quite sure.
 
I also think there is no problem before, because if a clause was
computable at the join but not earlier and only mentioned one side of
the join, then it was a non-degenerate outer join qual or an
outerjoin_delayed qual, and cannot enter into EC.
 
BTW, while looking at this I saw that generate_join_implied_equalities'
calculation of nominal_join_relids is wrong for child rels, because it
fails to fold the join relid into that if appropriate. 
 
I dug a little into this and it seems this is all right as-is.  Among
all the calls of generate_join_implied_equalities, it seems only
build_joinrel_restrictlist would have outer join's ojrelid in param
'join_relids'.  And build_joinrel_restrictlist does not get called for
child rels.  The restrictlist of a child rel is constructed from that of
its parent rel.

Thanks
Richard

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