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Commit19f9376

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Tweak joinlist creation to avoid generating useless one-element subproblems
when collapsing of JOIN trees is stopped by join_collapse_limit. For instancea list of 11 LEFT JOINs with limit 8 now produces something like((1 2 3 4 5 6 7 8) 9 10 11 12)instead of(((1 2 3 4 5 6 7 8) (9)) 10 11 12)The latter structure is really only required for a FULL JOIN.Noted while studying an example from Shane Ambler.
1 parent9a9a143 commit19f9376

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‎src/backend/optimizer/plan/initsplan.c

Lines changed: 27 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.126 2007/01/05 22:19:31 momjian Exp $
11+
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.127 2007/01/08 16:47:30 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -402,13 +402,34 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
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* except at a FULL JOIN or where join_collapse_limit would be
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* exceeded.
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*/
405-
if (j->jointype!=JOIN_FULL&&
406-
(list_length(leftjoinlist)+list_length(rightjoinlist) <=
407-
join_collapse_limit))
405+
if (j->jointype==JOIN_FULL)
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{
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/* force the join order exactly at this node */
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joinlist=list_make1(list_make2(leftjoinlist,rightjoinlist));
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}
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elseif (list_length(leftjoinlist)+list_length(rightjoinlist) <=
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join_collapse_limit)
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{
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/* OK to combine subproblems */
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joinlist=list_concat(leftjoinlist,rightjoinlist);
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}
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else
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/* force the join order at this node */
411-
joinlist=list_make1(list_make2(leftjoinlist,rightjoinlist));
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{
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/* can't combine, but needn't force join order above here */
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Node*leftpart,
420+
*rightpart;
421+
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/* avoid creating useless 1-element sublists */
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if (list_length(leftjoinlist)==1)
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leftpart= (Node*)linitial(leftjoinlist);
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else
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leftpart= (Node*)leftjoinlist;
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if (list_length(rightjoinlist)==1)
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rightpart= (Node*)linitial(rightjoinlist);
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else
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rightpart= (Node*)rightjoinlist;
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joinlist=list_make2(leftpart,rightpart);
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}
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}
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else
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{

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