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Commit5d8aa8b

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Fix missed optimization in relation_excluded_by_constraints().
In commit3fc6e2d, I (tgl) argued that we only need to check fora constant-FALSE restriction clause when there's exactly onerestriction clause, on the grounds that const-folding would havethrown away anything ANDed with a Const FALSE. That's true just afterconst-folding has been applied, but subsequent processing such asequivalence class expansion could result in cases where a Const FALSEis ANDed with some other stuff. (Compare for instance joinrels.c'srestriction_is_constant_false.) Hence, tweak this logic to check allthe elements of the baserestrictinfo list, not just one; that's cheapenough to not be worth worrying about.There is one existing test case where this visibly improves the plan.There would not be any savings in runtime, but the planner effort andexecutor startup effort will be reduced, and anyway it's odd thatwe can detect related cases but not this one.Richard Guo (independently discovered by David Rowley)Discussion:https://postgr.es/m/CAMbWs4_x3-CnVVrCboS1LkEhB5V+W7sLSCabsRiG+n7+5_kqbg@mail.gmail.com
1 parent16671ba commit5d8aa8b

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+12
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lines changed

2 files changed

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lines changed

‎src/backend/optimizer/util/plancat.c

Lines changed: 9 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -1549,16 +1549,17 @@ relation_excluded_by_constraints(PlannerInfo *root,
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/*
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* Regardless of the setting of constraint_exclusion, detect
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* constant-FALSE-or-NULL restriction clauses. Because const-folding will
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* reduce "anything AND FALSE" to just "FALSE", any such case should
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* result in exactly one baserestrictinfo entry. This doesn't fire very
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* often, but it seems cheap enough to be worth doing anyway. (Without
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* this, we'd miss some optimizations that 9.5 and earlier found via much
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* more roundabout methods.)
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* constant-FALSE-or-NULL restriction clauses. Although const-folding
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* will reduce "anything AND FALSE" to just "FALSE", the baserestrictinfo
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* list can still have other members besides the FALSE constant, due to
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* qual pushdown and other mechanisms; so check them all. This doesn't
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* fire very often, but it seems cheap enough to be worth doing anyway.
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* (Without this, we'd miss some optimizations that 9.5 and earlier found
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* via much more roundabout methods.)
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*/
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if (list_length(rel->baserestrictinfo)==1)
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foreach(lc,rel->baserestrictinfo)
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{
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RestrictInfo*rinfo= (RestrictInfo*)linitial(rel->baserestrictinfo);
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RestrictInfo*rinfo= (RestrictInfo*)lfirst(lc);
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Expr*clause=rinfo->clause;
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if (clause&&IsA(clause,Const)&&

‎src/test/regress/expected/join.out

Lines changed: 3 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -5788,13 +5788,11 @@ explain (costs off)
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select p.* from
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parent p left join child c on (p.k = c.k)
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where p.k = 1 and p.k = 2;
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QUERY PLAN
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------------------------------------------------
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QUERY PLAN
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--------------------------
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Result
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One-Time Filter: false
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-> Index Scan using parent_pkey on parent p
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Index Cond: (k = 1)
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(4 rows)
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(2 rows)
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select p.* from
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(parent p left join child c on (p.k = c.k)) join parent x on p.k = x.k

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