Fix thinko in estimate_num_groups

The code for the reworked n-distinct estimation on commit 7b504eb282 was
written differently in a previous version of the patch, prior to commit;
on rewriting it, we missed updating an initializer.  This caused the
code to (mistakenly) apply a fudge factor even in the case where a
single value is applied, leading to incorrect results.

This means that the 'relvarcount' variable name is now wrong.  Add a
comment to try and make the situation clearer, and remove an incorrect
comment I added.

Problem noticed, and code patch, by Tomas Vondra.  Additional commentary
by Álvaro.

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index cc24c8aeb56ef087bacb271f6fc0cc7257ad2d76..5c382a2013e2b09905deef9cfaa70bf1ad765e3e 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3404,7 +3404,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
        RelOptInfo *rel = varinfo1->rel;
        double      reldistinct = 1;
        double      relmaxndistinct = reldistinct;
-       int         relvarcount = 1;
+       int         relvarcount = 0;
        List       *newvarinfos = NIL;
        List       *relvarinfos = NIL;
 
@@ -3436,6 +3436,10 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
         * we multiply them together.  Any remaining relvarinfos after
         * no more multivariate matches are found are assumed independent too,
         * so their individual ndistinct estimates are multiplied also.
+        *
+        * While iterating, count how many separate numdistinct values we
+        * apply.  We apply a fudge factor below, but only if we multiplied
+        * more than one such values.
         */
        while (relvarinfos)
        {
@@ -3447,7 +3451,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
                reldistinct *= mvndistinct;
                if (relmaxndistinct < mvndistinct)
                    relmaxndistinct = mvndistinct;
-               relvarcount++;  /* inaccurate, but doesn't matter */
+               relvarcount++;
            }
            else
            {