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Fix thinko in estimate_num_groups

The code for the reworked n-distinct estimation on commit7b504eb waswritten differently in a previous version of the patch, prior to commit;on rewriting it, we missed updating an initializer. This caused thecode to (mistakenly) apply a fudge factor even in the case where asingle value is applied, leading to incorrect results.This means that the 'relvarcount' variable name is now wrong. Add acomment to try and make the situation clearer, and remove an incorrectcomment I added.Problem noticed, and code patch, by Tomas Vondra. Additional commentaryby Álvaro.

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src/backend/utils/adt
- selfuncs.c

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`‎src/backend/utils/adt/selfuncs.c`

Lines changed: 6 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -3404,7 +3404,7 @@ estimate_num_groups(PlannerInfo root, List groupExprs, double input_rows,`
`3404`	`3404`	`RelOptInfo*rel=varinfo1->rel;`
`3405`	`3405`	`doublereldistinct=1;`
`3406`	`3406`	`doublerelmaxndistinct=reldistinct;`
`3407`		`-intrelvarcount=1;`
	`3407`	`+intrelvarcount=0;`
`3408`	`3408`	`List*newvarinfos=NIL;`
`3409`	`3409`	`List*relvarinfos=NIL;`
`3410`	`3410`
`@@ -3436,6 +3436,10 @@ estimate_num_groups(PlannerInfo root, List groupExprs, double input_rows,`
`3436`	`3436`	`* we multiply them together. Any remaining relvarinfos after`
`3437`	`3437`	`* no more multivariate matches are found are assumed independent too,`
`3438`	`3438`	`* so their individual ndistinct estimates are multiplied also.`
	`3439`	`+ *`
	`3440`	`+ * While iterating, count how many separate numdistinct values we`
	`3441`	`+ * apply. We apply a fudge factor below, but only if we multiplied`
	`3442`	`+ * more than one such values.`
`3439`	`3443`	`*/`
`3440`	`3444`	`while (relvarinfos)`
`3441`	`3445`	`{`
`@@ -3447,7 +3451,7 @@ estimate_num_groups(PlannerInfo root, List groupExprs, double input_rows,`
`3447`	`3451`	`reldistinct *=mvndistinct;`
`3448`	`3452`	`if (relmaxndistinct<mvndistinct)`
`3449`	`3453`	`relmaxndistinct=mvndistinct;`
`3450`		`-relvarcount++;/* inaccurate, but doesn't matter */`
	`3454`	`+relvarcount++;`
`3451`	`3455`	`}`
`3452`	`3456`	`else`
`3453`	`3457`	`{`

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Commit1f171a1

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`‎src/backend/utils/adt/selfuncs.c`

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