From 1b88044ad562a71c4e043e7028b66038dd4b9e99 Mon Sep 17 00:00:00 2001
From: Angela Xu <angelax@cockroachlabs.com>
Date: Mon, 8 Feb 2021 12:32:48 -0800
Subject: [PATCH] opt: move fraction function from statistics code to
 selectivity

The fraction function in the statistics builder code was only being used to
construct Selectivity, used inside the MakeSelectivity method.

This change moves the logic from the fraction function into the code for
Selectivity. It is an additional clean-up to #59413, to get rid of direct
numerical operations on Selectivity.

Fixes: #53860

Release note: None
---
 pkg/sql/opt/memo/statistics_builder.go | 20 ++++++--------------
 pkg/sql/opt/props/selectivity.go       |  9 +++++++++
 pkg/sql/opt/props/selectivity_test.go  |  7 +++++++
 3 files changed, 22 insertions(+), 14 deletions(-)

diff --git a/pkg/sql/opt/memo/statistics_builder.go b/pkg/sql/opt/memo/statistics_builder.go
index db3406b0f243..a1d965b25708 100644
--- a/pkg/sql/opt/memo/statistics_builder.go
+++ b/pkg/sql/opt/memo/statistics_builder.go
@@ -2726,14 +2726,6 @@ func max(a, b float64) float64 {
 	return b
 }
 
-// fraction returns a/b if a is less than b. Otherwise returns 1.
-func fraction(a, b float64) float64 {
-	if a < b {
-		return a / b
-	}
-	return 1
-}
-
 //////////////////////////////////////////////////
 // Helper functions for selectivity calculation //
 //////////////////////////////////////////////////
@@ -3624,8 +3616,8 @@ func (sb *statisticsBuilder) selectivityFromDistinctCount(
 	}
 
 	// Calculate the selectivity of the predicate.
-	nonNullSelectivity := props.MakeSelectivity(fraction(newDistinct, oldDistinct))
-	nullSelectivity := props.MakeSelectivity(fraction(colStat.NullCount, inputColStat.NullCount))
+	nonNullSelectivity := props.MakeSelectivityFromFraction(newDistinct, oldDistinct)
+	nullSelectivity := props.MakeSelectivityFromFraction(colStat.NullCount, inputColStat.NullCount)
 	return sb.predicateSelectivity(
 		nonNullSelectivity, nullSelectivity, inputColStat.NullCount, inputRowCount,
 	)
@@ -3658,8 +3650,8 @@ func (sb *statisticsBuilder) selectivityFromHistograms(
 		oldCount := oldHist.ValuesCount()
 
 		// Calculate the selectivity of the predicate.
-		nonNullSelectivity := props.MakeSelectivity(fraction(newCount, oldCount))
-		nullSelectivity := props.MakeSelectivity(fraction(colStat.NullCount, inputColStat.NullCount))
+		nonNullSelectivity := props.MakeSelectivityFromFraction(newCount, oldCount)
+		nullSelectivity := props.MakeSelectivityFromFraction(colStat.NullCount, inputColStat.NullCount)
 		selectivity.Multiply(sb.predicateSelectivity(
 			nonNullSelectivity, nullSelectivity, inputColStat.NullCount, inputStats.RowCount,
 		))
@@ -3749,7 +3741,7 @@ func (sb *statisticsBuilder) selectivityFromEquivalency(
 
 	// The selectivity of an equality condition var1=var2 is
 	// 1/max(distinct(var1), distinct(var2)).
-	return props.MakeSelectivity(fraction(1, maxDistinctCount))
+	return props.MakeSelectivityFromFraction(1, maxDistinctCount)
 }
 
 // selectivityFromEquivalenciesSemiJoin determines the selectivity of equality
@@ -3800,7 +3792,7 @@ func (sb *statisticsBuilder) selectivityFromEquivalencySemiJoin(
 		maxDistinctCountLeft = s.RowCount
 	}
 
-	return props.MakeSelectivity(fraction(minDistinctCountRight, maxDistinctCountLeft))
+	return props.MakeSelectivityFromFraction(minDistinctCountRight, maxDistinctCountLeft)
 }
 
 func (sb *statisticsBuilder) selectivityFromInvertedJoinCondition(
diff --git a/pkg/sql/opt/props/selectivity.go b/pkg/sql/opt/props/selectivity.go
index 5046485c454d..0ae630d4159b 100644
--- a/pkg/sql/opt/props/selectivity.go
+++ b/pkg/sql/opt/props/selectivity.go
@@ -34,6 +34,15 @@ func MakeSelectivity(sel float64) Selectivity {
 	return Selectivity{selectivityInRange(sel)}
 }
 
+// MakeSelectivityFromFraction calculates selectivity as a fraction of a and b
+// if a is less than b and returns OneSelectivity otherwise..
+func MakeSelectivityFromFraction(a, b float64) Selectivity {
+	if a < b {
+		return MakeSelectivity(a / b)
+	}
+	return OneSelectivity
+}
+
 // AsFloat returns the private selectivity field, allowing it to be accessed
 // outside of this package.
 func (s *Selectivity) AsFloat() float64 {
diff --git a/pkg/sql/opt/props/selectivity_test.go b/pkg/sql/opt/props/selectivity_test.go
index 3e0b9b183ae7..dde5889eb705 100644
--- a/pkg/sql/opt/props/selectivity_test.go
+++ b/pkg/sql/opt/props/selectivity_test.go
@@ -59,6 +59,13 @@ func TestSelectivity(t *testing.T) {
 	}
 	inf := math.MaxFloat64
 
+	// MakeSelectivityFromFraction variations.
+	test(MakeSelectivityFromFraction(1, 2), s(0.5))
+	test(MakeSelectivityFromFraction(1, 1), OneSelectivity)
+	test(MakeSelectivityFromFraction(1.5, 1), OneSelectivity)
+	test(MakeSelectivityFromFraction(1, 0), OneSelectivity)
+	test(MakeSelectivityFromFraction(0, 0), OneSelectivity)
+
 	// MinSelectivity variations.
 	test(MinSelectivity(s(0.4), s(0.5)), s(0.4))
 	test(MinSelectivity(s(0.5), s(0.4)), s(0.4))