planner: Log warnings when agg function can not be pushdown in explain statement (#25553)

executor/show_test.go

@@ -149,6 +149,16 @@ func (s *testSuite5) TestShowWarningsForExprPushdown(c *C) {
 	tk.MustExec("explain select * from show_warnings_expr_pushdown where date_add(value, interval 1 day) = '2020-01-01'")
 	c.Assert(tk.Se.GetSessionVars().StmtCtx.WarningCount(), Equals, uint16(1))
 	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1105|Scalar function 'date_add'(signature: AddDateDatetimeInt) can not be pushed to tikv"))
+	tk.MustExec("explain select max(date_add(value, interval 1 day)) from show_warnings_expr_pushdown group by a")
+	c.Assert(tk.Se.GetSessionVars().StmtCtx.WarningCount(), Equals, uint16(2))
+	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1105|Scalar function 'date_add'(signature: AddDateDatetimeInt) can not be pushed to tikv", "Warning|1105|Aggregation can not be pushed to tikv because arguments of AggFunc `max` contains unsupported exprs"))
+	tk.MustExec("explain select max(a) from show_warnings_expr_pushdown group by date_add(value, interval 1 day)")
+	c.Assert(tk.Se.GetSessionVars().StmtCtx.WarningCount(), Equals, uint16(2))
+	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1105|Scalar function 'date_add'(signature: AddDateDatetimeInt) can not be pushed to tikv", "Warning|1105|Aggregation can not be pushed to tikv because groupByItems contain unsupported exprs"))
+	tk.MustExec("set tidb_opt_distinct_agg_push_down=0")
+	tk.MustExec("explain select max(distinct a) from show_warnings_expr_pushdown group by value")
+	c.Assert(tk.Se.GetSessionVars().StmtCtx.WarningCount(), Equals, uint16(1))
+	tk.MustQuery("show warnings").Check(testutil.RowsWithSep("|", "Warning|1105|Aggregation can not be pushed to storage layer in non-mpp mode because it contains agg function with distinct"))
 }
 
 func (s *testSuite5) TestShowGrantsPrivilege(c *C) {

planner/core/exhaust_physical_plans.go

@@ -19,6 +19,7 @@ import (
 	"math"
 	"sort"
 
+	"github.com/pingcap/errors"
 	"github.com/pingcap/failpoint"
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/model"
@@ -2349,17 +2350,24 @@ func (la *LogicalAggregation) getStreamAggs(prop *property.PhysicalProperty) []P
 
 // TODO: support more operators and distinct later
 func (la *LogicalAggregation) checkCanPushDownToMPP() bool {
+	hasUnsupportedDistinct := false
 	for _, agg := range la.AggFuncs {
 		// MPP does not support distinct except count distinct now
 		if agg.HasDistinct {
 			if agg.Name != ast.AggFuncCount {
-				return false
+				hasUnsupportedDistinct = true
 			}
 		}
 		// MPP does not support AggFuncApproxCountDistinct now
 		if agg.Name == ast.AggFuncApproxCountDistinct {
-			return false
+			hasUnsupportedDistinct = true
+		}
+	}
+	if hasUnsupportedDistinct {
+		if la.ctx.GetSessionVars().StmtCtx.InExplainStmt {
+			la.ctx.GetSessionVars().StmtCtx.AppendWarning(errors.New("Aggregation can not be pushed to storage layer in mpp mode because it contains agg function with distinct"))
 		}
+		return false
 	}
 	return CheckAggCanPushCop(la.ctx, la.AggFuncs, la.GroupByItems, kv.TiFlash)
 }
@@ -2441,10 +2449,13 @@ func (la *LogicalAggregation) getHashAggs(prop *property.PhysicalProperty) []Phy
 		taskTypes = append(taskTypes, property.CopTiFlashLocalReadTaskType)
 	}
 	canPushDownToTiFlash := la.canPushToCop(kv.TiFlash)
-	canPushDownToMPP := la.ctx.GetSessionVars().AllowMPPExecution && la.checkCanPushDownToMPP() && canPushDownToTiFlash
+	canPushDownToMPP := canPushDownToTiFlash && la.ctx.GetSessionVars().AllowMPPExecution && la.checkCanPushDownToMPP()
 	if la.HasDistinct() {
 		// TODO: remove after the cost estimation of distinct pushdown is implemented.
 		if !la.ctx.GetSessionVars().AllowDistinctAggPushDown {
+			if la.ctx.GetSessionVars().StmtCtx.InExplainStmt {
+				la.ctx.GetSessionVars().StmtCtx.AppendWarning(errors.New("Aggregation can not be pushed to storage layer in non-mpp mode because it contains agg function with distinct"))
+			}
 			taskTypes = []property.TaskType{property.RootTaskType}
 		}
 	} else if !la.aggHints.preferAggToCop {

planner/core/task.go

@@ -1358,33 +1358,49 @@ func (sel *PhysicalSelection) attach2Task(tasks ...task) task {
 func CheckAggCanPushCop(sctx sessionctx.Context, aggFuncs []*aggregation.AggFuncDesc, groupByItems []expression.Expression, storeType kv.StoreType) bool {
 	sc := sctx.GetSessionVars().StmtCtx
 	client := sctx.GetClient()
+	ret := true
+	reason := ""
 	for _, aggFunc := range aggFuncs {
 		// if the aggFunc contain VirtualColumn or CorrelatedColumn, it can not be pushed down.
 		if expression.ContainVirtualColumn(aggFunc.Args) || expression.ContainCorrelatedColumn(aggFunc.Args) {
-			return false
-		}
-		pb := aggregation.AggFuncToPBExpr(sc, client, aggFunc)
-		if pb == nil {
-			return false
+			reason = "expressions of AggFunc `" + aggFunc.Name + "` contain virtual column or correlated column, which is not supported now"
+			ret = false
+			break
 		}
 		if !aggregation.CheckAggPushDown(aggFunc, storeType) {
-			if sc.InExplainStmt {
-				storageName := storeType.Name()
-				if storeType == kv.UnSpecified {
-					storageName = "storage layer"
-				}
-				sc.AppendWarning(errors.New("Agg function '" + aggFunc.Name + "' can not be pushed to " + storageName))
-			}
-			return false
+			reason = "AggFunc `" + aggFunc.Name + "` is not supported now"
+			ret = false
+			break
 		}
 		if !expression.CanExprsPushDown(sc, aggFunc.Args, client, storeType) {
-			return false
+			reason = "arguments of AggFunc `" + aggFunc.Name + "` contains unsupported exprs"
+			ret = false
+			break
+		}
+		pb := aggregation.AggFuncToPBExpr(sc, client, aggFunc)
+		if pb == nil {
+			reason = "AggFunc `" + aggFunc.Name + "` can not be converted to pb expr"
+			ret = false
+			break
 		}
 	}
-	if expression.ContainVirtualColumn(groupByItems) {
-		return false
+	if ret && expression.ContainVirtualColumn(groupByItems) {
+		reason = "groupByItems contain virtual columns, which is not supported now"
+		ret = false
+	}
+	if ret && !expression.CanExprsPushDown(sc, groupByItems, client, storeType) {
+		reason = "groupByItems contain unsupported exprs"
+		ret = false
+	}
+
+	if !ret && sc.InExplainStmt {
+		storageName := storeType.Name()
+		if storeType == kv.UnSpecified {
+			storageName = "storage layer"
+		}
+		sc.AppendWarning(errors.New("Aggregation can not be pushed to " + storageName + " because " + reason))
 	}
-	return expression.CanExprsPushDown(sc, groupByItems, client, storeType)
+	return ret
 }
 
 // AggInfo stores the information of an Aggregation.