Merge #115304

115304: opt: make floating-point stats calculations precise r=DrewKimball a=DrewKimball

This patch makes two changes to the floating-point operators in the `adjustNullCountsForOuterJoins` statistics method:
1. The results of floating-point subtractions are adjusted to be at least `epsilon`; this avoids precision errors.
2. Calculation is avoided entirely when the result is known to be zero. This ensures an exact result in the common case when no adjustment is made to the null count estimate.

This commit also reverts #115287, since the failing tests are now fixed.

Fixes #115278

Release note: None

Co-authored-by: Drew Kimball <drewk@cockroachlabs.com>

pkg/sql/opt/memo/statistics_builder.go

@@ -1652,24 +1652,25 @@ func (sb *statisticsBuilder) adjustNullCountsForOuterJoins(
 	// extension - such as right columns for non-matching rows in left joins.
 	switch joinType {
 	case opt.LeftJoinOp, opt.LeftJoinApplyOp:
-		if !rightColsAreEmpty {
-			colStat.NullCount += (rowCount - innerJoinRowCount) * leftNullCount / leftRowCount
+		if !rightColsAreEmpty && leftNullCount > 0 && rowCount > innerJoinRowCount {
+			addedRows := max(rowCount-innerJoinRowCount, epsilon)
+			colStat.NullCount += addedRows * leftNullCount / leftRowCount
 		}
 
 	case opt.RightJoinOp:
-		if !leftColsAreEmpty {
-			colStat.NullCount += (rowCount - innerJoinRowCount) * rightNullCount / rightRowCount
+		if !leftColsAreEmpty && rightNullCount > 0 && rowCount > innerJoinRowCount {
+			addedRows := max(rowCount-innerJoinRowCount, epsilon)
+			colStat.NullCount += addedRows * rightNullCount / rightRowCount
 		}
 
 	case opt.FullJoinOp:
-		leftJoinRowCount := max(innerJoinRowCount, leftRowCount)
-		rightJoinRowCount := max(innerJoinRowCount, rightRowCount)
-
-		if !leftColsAreEmpty {
-			colStat.NullCount += (rightJoinRowCount - innerJoinRowCount) * rightNullCount / rightRowCount
+		if !leftColsAreEmpty && rightNullCount > 0 && rightRowCount > innerJoinRowCount {
+			addedRows := max(rightRowCount-innerJoinRowCount, epsilon)
+			colStat.NullCount += addedRows * rightNullCount / rightRowCount
 		}
-		if !rightColsAreEmpty {
-			colStat.NullCount += (leftJoinRowCount - innerJoinRowCount) * leftNullCount / leftRowCount
+		if !rightColsAreEmpty && leftNullCount > 0 && leftRowCount > innerJoinRowCount {
+			addedRows := max(leftRowCount-innerJoinRowCount, epsilon)
+			colStat.NullCount += addedRows * leftNullCount / leftRowCount
 		}
 	}
 }
@@ -3008,15 +3009,6 @@ func (sb *statisticsBuilder) rowsProcessed(e RelExpr) float64 {
 	}
 }
 
-// TODO(#115278): We should be able to replace this with Go's built-in max
-// function, but doing breaks some optimizer tests on ARM64.
-func max(a, b float64) float64 {
-	if a > b {
-		return a
-	}
-	return b
-}
-
 //////////////////////////////////////////////////
 // Helper functions for selectivity calculation //
 //////////////////////////////////////////////////