From 19fae36e03b6e2b4cd2ea1702ffbe9676c0aca52 Mon Sep 17 00:00:00 2001
From: Tomas Vondra
Date: Tue, 26 Jan 2016 18:14:33 +0100
Subject: [PATCH 8/9] change how we apply selectivity to number of groups
estimate
Instead of simply multiplying the ndistinct estimate with selecticity,
we instead use the formula for the expected number of distinct values
observed in 'k' rows when there are 'd' distinct values in the bin
d * (1 - ((d - 1) / d)^k)
This is 'with replacements' which seems appropriate for the use, and it
mostly assumes uniform distribution of the distinct values. So if the
distribution is not uniform (e.g. there are very frequent groups) this
may be less accurate than the current algorithm in some cases, giving
over-estimates. But that's probably better than OOM.
---
src/backend/utils/adt/selfuncs.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index a84dd2b..ce3ad19 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3465,7 +3465,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
/*
* Multiply by restriction selectivity.
*/
- reldistinct *= rel->rows / rel->tuples;
+ reldistinct = reldistinct * (1 - powl((reldistinct - 1) / reldistinct,rel->rows));
/*
* Update estimate of total distinct groups.
--
2.1.0