adjust fanout factor

angelamayxie · angelamayxie · commit 86cbaca5560d · 2025-07-24T15:32:23.000-07:00
diff --git a/sql/memo/coster.go b/sql/memo/coster.go
@@ -132,7 +132,7 @@ func (c *coster) costRel(ctx *sql.Context, n RelExpr, s sql.StatsProvider) (floa
 					return lBest*seqIOCostFactor + lBest*(seqIOCostFactor+randIOCostFactor), nil
 				}
 				// Fanout is the average number of right-side rows that match each left-side row
-				fanout := 2.0
+				fanout := 1.5
 				// The total expected number of right row lookups
 				expectedRightRows := selfJoinCard * matchRate * fanout
 				// Estimate for reading each left row and each expected right row

Original file line number	Diff line number	Diff line change
`@@ -132,7 +132,7 @@ func (c coster) costRel(ctx sql.Context, n RelExpr, s sql.StatsProvider) (floa`
`132`	`132`	`return lBestseqIOCostFactor + lBest(seqIOCostFactor+randIOCostFactor), nil`
`133`	`133`	`}`
`134`	`134`	`// Fanout is the average number of right-side rows that match each left-side row`
`135`		`- fanout := 2.0`
	`135`	`+ fanout := 1.5`
`136`	`136`	`// The total expected number of right row lookups`
`137`	`137`	`expectedRightRows := selfJoinCard * matchRate * fanout`
`138`	`138`	`// Estimate for reading each left row and each expected right row`