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Tweak joinlist creation to avoid generating useless one-element subproblems

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when collapsing of JOIN trees is stopped by join_collapse_limit.  For instance
a 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.
This commit is contained in:
Tom Lane 2007-01-08 16:47:30 +00:00
parent 9a9a143a98
commit 19f9376bf4
1 changed files with 28 additions and 7 deletions
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35
src/backend/optimizer/plan/initsplan.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.126 2007/01/05 22:19:31 momjian Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.127 2007/01/08 16:47:30 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -402,13 +402,34 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
* except at a FULL JOIN or where join_collapse_limit would be
* exceeded.
*/
if (j->jointype != JOIN_FULL &&
(list_length(leftjoinlist) + list_length(rightjoinlist) <=
join_collapse_limit))
joinlist = list_concat(leftjoinlist, rightjoinlist);
else
/* force the join order at this node */
if (j->jointype == JOIN_FULL)
{
/* force the join order exactly at this node */
joinlist = list_make1(list_make2(leftjoinlist, rightjoinlist));
}
else if (list_length(leftjoinlist) + list_length(rightjoinlist) <=
join_collapse_limit)
{
/* OK to combine subproblems */
joinlist = list_concat(leftjoinlist, rightjoinlist);
}
else
{
/* can't combine, but needn't force join order above here */
Node *leftpart,
*rightpart;
/* avoid creating useless 1-element sublists */
if (list_length(leftjoinlist) == 1)
leftpart = (Node *) linitial(leftjoinlist);
else
leftpart = (Node *) leftjoinlist;
if (list_length(rightjoinlist) == 1)
rightpart = (Node *) linitial(rightjoinlist);
else
rightpart = (Node *) rightjoinlist;
joinlist = list_make2(leftpart, rightpart);
}
}
else
{