planner: disable tidb_prefer_broadcast_join_by_exchange_data_size by default; set scale factor to optimize estimating broadcast join;

planner/core/exhaust_physical_plans.go

@@ -2129,23 +2129,28 @@ func calcHashExchangeSizeByChild(p1 Plan, p2 Plan, mppStoreCnt int) (float64, fl
 	return row1 + row2, 0, false
 }
 
+// The size of `Build` hash table when using broadcast join is `X`.
+// The size of `Build` hash table when using shuffle join is `X / (mppStoreCnt)`.
+// It will cost more time to search `Probe` data in hash table.
+// Set a scale factor (`mppStoreCnt^*`) when estimating broadcast join in `isJoinFitMPPBCJ` and `isJoinChildFitMPPBCJ` (based on TPCH benchmark, it has been verified in Q9).
+
 func isJoinFitMPPBCJ(p *LogicalJoin, mppStoreCnt int) bool {
 	rowBC, szBC, hasSizeBC := calcBroadcastExchangeSizeByChild(p.children[0], p.children[1], mppStoreCnt)
 	rowHash, szHash, hasSizeHash := calcHashExchangeSizeByChild(p.children[0], p.children[1], mppStoreCnt)
 	if hasSizeBC && hasSizeHash {
-		return szBC <= szHash
+		return szBC*float64(mppStoreCnt) <= szHash
 	}
-	return rowBC <= rowHash
+	return rowBC*float64(mppStoreCnt) <= rowHash
 }
 
 func isJoinChildFitMPPBCJ(p *LogicalJoin, childIndexToBC int, mppStoreCnt int) bool {
 	rowBC, szBC, hasSizeBC := calcBroadcastExchangeSize(p.children[childIndexToBC], mppStoreCnt)
 	rowHash, szHash, hasSizeHash := calcHashExchangeSizeByChild(p.children[0], p.children[1], mppStoreCnt)
 
 	if hasSizeBC && hasSizeHash {
-		return szBC <= szHash
+		return szBC*float64(mppStoreCnt) <= szHash
 	}
-	return rowBC <= rowHash
+	return rowBC*float64(mppStoreCnt) <= rowHash
 }
 
 // If we can use mpp broadcast join, that's our first choice.

sessionctx/variable/tidb_vars.go

@@ -1045,7 +1045,7 @@ const (
 	DefTiDBProjectionConcurrency                   = ConcurrencyUnset
 	DefBroadcastJoinThresholdSize                  = 100 * 1024 * 1024
 	DefBroadcastJoinThresholdCount                 = 10 * 1024
-	DefPreferBCJByExchangeDataSize                 = true
+	DefPreferBCJByExchangeDataSize                 = false
 	DefTiDBOptimizerSelectivityLevel               = 0
 	DefTiDBOptimizerEnableNewOFGB                  = false
 	DefTiDBEnableOuterJoinReorder                  = true