expression: Add warning info for exprs that can not be pushed to stor…

…age layer (#22713)

executor/show_test.go

@@ -139,6 +139,16 @@ func (s *testSuite5) TestShowErrors(c *C) {
 	tk.MustQuery("show errors").Check(testutil.RowsWithSep("|", "Error|1050|Table 'test.show_errors' already exists"))
 }
 
+func (s *testSuite5) TestShowWarningsForExprPushdown(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	testSQL := `create table if not exists show_warnings_expr_pushdown (a int, value date)`
+	tk.MustExec(testSQL)
+	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"))
+}
+
 func (s *testSuite5) TestShowGrantsPrivilege(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("create user show_grants")

expression/expression.go

@@ -1173,6 +1173,13 @@ func canScalarFuncPushDown(scalarFunc *ScalarFunction, pc PbConverter, storeType
 
 	// Check whether this function can be pushed.
 	if !canFuncBePushed(scalarFunc, storeType) {
+		if pc.sc.InExplainStmt {
+			storageName := storeType.Name()
+			if storeType == kv.UnSpecified {
+				storageName = "storage layer"
+			}
+			pc.sc.AppendWarning(errors.New("Scalar function '" + scalarFunc.FuncName.L + "'(signature: " + scalarFunc.Function.PbCode().String() + ") can not be pushed to " + storageName))
+		}
 		return false
 	}
 
@@ -1196,6 +1203,9 @@ func canScalarFuncPushDown(scalarFunc *ScalarFunction, pc PbConverter, storeType
 
 func canExprPushDown(expr Expression, pc PbConverter, storeType kv.StoreType) bool {
 	if storeType == kv.TiFlash && expr.GetType().Tp == mysql.TypeDuration {
+		if pc.sc.InExplainStmt {
+			pc.sc.AppendWarning(errors.New("Expr '" + expr.String() + "' can not be pushed to TiFlash because it contains Duration type"))
+		}
 		return false
 	}
 	switch x := expr.(type) {

planner/core/physical_plan_test.go

@@ -974,7 +974,7 @@ func (s *testPlanSuite) TestLimitToCopHint(c *C) {
 		output []struct {
 			SQL     string
 			Plan    []string
-			Warning string
+			Warning []string
 		}
 	)
 
@@ -991,15 +991,20 @@ func (s *testPlanSuite) TestLimitToCopHint(c *C) {
 		warnings := tk.Se.GetSessionVars().StmtCtx.GetWarnings()
 		s.testData.OnRecord(func() {
 			if len(warnings) > 0 {
-				output[i].Warning = warnings[0].Err.Error()
+				output[i].Warning = make([]string, len(warnings))
+				for j, warning := range warnings {
+					output[i].Warning[j] = warning.Err.Error()
+				}
 			}
 		})
-		if output[i].Warning == "" {
+		if len(output[i].Warning) == 0 {
 			c.Assert(len(warnings), Equals, 0, comment)
 		} else {
-			c.Assert(len(warnings), Equals, 1, comment)
-			c.Assert(warnings[0].Level, Equals, stmtctx.WarnLevelWarning, comment)
-			c.Assert(warnings[0].Err.Error(), Equals, output[i].Warning, comment)
+			c.Assert(len(warnings), Equals, len(output[i].Warning), comment)
+			for j, warning := range warnings {
+				c.Assert(warning.Level, Equals, stmtctx.WarnLevelWarning, comment)
+				c.Assert(warning.Err.Error(), Equals, output[i].Warning[j], comment)
+			}
 		}
 	}
 }

planner/core/task.go

@@ -16,6 +16,7 @@ package core
 import (
 	"math"
 
+	"github.com/pingcap/errors"
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/charset"
 	"github.com/pingcap/parser/mysql"
@@ -1114,6 +1115,13 @@ func CheckAggCanPushCop(sctx sessionctx.Context, aggFuncs []*aggregation.AggFunc
 			return false
 		}
 		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
 		}
 		if !expression.CanExprsPushDown(sc, aggFunc.Args, client, storeType) {

planner/core/testdata/plan_suite_out.json

@@ -1487,7 +1487,7 @@
           "    └─Selection_13 0.83 cop[tikv]  gt(test.tn.c, 50)",
           "      └─IndexRangeScan_12 2.50 cop[tikv] table:tn, index:a(a, b, c, d) range:(1 10,1 20), keep order:false, stats:pseudo"
         ],
-        "Warning": ""
+        "Warning": null
       },
       {
         "SQL": "select * from tn where a = 1 and b > 10 and b < 20 and c > 50 order by d limit 1",
@@ -1497,7 +1497,7 @@
           "  └─Selection_19 0.83 cop[tikv]  gt(test.tn.c, 50)",
           "    └─IndexRangeScan_18 2.50 cop[tikv] table:tn, index:a(a, b, c, d) range:(1 10,1 20), keep order:false, stats:pseudo"
         ],
-        "Warning": ""
+        "Warning": null
       },
       {
         "SQL": "select /*+ LIMIT_TO_COP() */ a from tn where mod(a, 2) order by a limit 1",
@@ -1507,7 +1507,10 @@
           "  └─IndexReader_21 1.00 root  index:IndexFullScan_20",
           "    └─IndexFullScan_20 1.00 cop[tikv] table:tn, index:a(a, b, c, d) keep order:true, stats:pseudo"
         ],
-        "Warning": "[planner:1815]Optimizer Hint LIMIT_TO_COP is inapplicable"
+        "Warning": [
+          "Scalar function 'mod'(signature: ModInt) can not be pushed to storage layer",
+          "[planner:1815]Optimizer Hint LIMIT_TO_COP is inapplicable"
+        ]
       },
       {
         "SQL": "select /*+ LIMIT_TO_COP() */ a from tn where a > 10 limit 1",
@@ -1517,7 +1520,7 @@
           "  └─Limit_11 1.00 cop[tikv]  offset:0, count:1",
           "    └─IndexRangeScan_10 1.00 cop[tikv] table:tn, index:a(a, b, c, d) range:(10,+inf], keep order:false, stats:pseudo"
         ],
-        "Warning": ""
+        "Warning": null
       }
     ]
   },