session, planner: add a session variable "tidb_opt_force_inline_cte"

executor/set_test.go

@@ -801,6 +801,14 @@ func TestSetVar(t *testing.T) {
 	tk.MustQuery("select @@global.tidb_enable_foreign_key").Check(testkit.Rows("0")) // default value
 	tk.MustExec("set global tidb_enable_foreign_key = 1")
 	tk.MustQuery("select @@global.tidb_enable_foreign_key").Check(testkit.Rows("1"))
+
+	// test variable 'tidb_opt_force_inline_cte'
+	tk.MustQuery("select @@session.tidb_opt_force_inline_cte").Check(testkit.Rows("0")) // default value is 0
+	tk.MustExec("set session tidb_opt_force_inline_cte=1")
+	tk.MustQuery("select @@session.tidb_opt_force_inline_cte").Check(testkit.Rows("1"))
+	tk.MustQuery("select @@global.tidb_opt_force_inline_cte").Check(testkit.Rows("0")) // default value is 0
+	tk.MustExec("set global tidb_opt_force_inline_cte=1")
+	tk.MustQuery("select @@global.tidb_opt_force_inline_cte").Check(testkit.Rows("1"))
 }
 
 func TestGetSetNoopVars(t *testing.T) {

planner/core/logical_plan_builder.go

@@ -3842,20 +3842,35 @@ func (b *PlanBuilder) buildSelect(ctx context.Context, sel *ast.SelectStmt) (p L
 		b.isForUpdateRead = true
 	}
 
-	// Verify Merge hints in the current query, we will update parameters for those that meet the rules, and warn those that do not.
-	//If the current query uses Merge Hint and the query is a CTE, we update the HINT information for the current query.
-	//If the current query is not a CTE query (it may be a subquery within a CTE query or an external non-CTE query), we will give a warning.
-	//In particular, recursive CTE have separate warnings, so they are no longer called.
-	if hints := b.TableHints(); hints != nil && hints.MergeHints.preferMerge {
+	// If the session variable "tidb_opt_force_inline_cte" is true, all of CTEs will be inlined.
+	// Otherwise, whether CTEs are inlined depends on whether the merge() hint is declared.
+	if b.ctx.GetSessionVars().EnableForceInlineCTE() {
+		if b.buildingCTE && b.isCTE {
+			b.outerCTEs[len(b.outerCTEs)-1].isInline = true
+		}
+	} else if hints := b.TableHints(); hints != nil && hints.MergeHints.preferMerge {
+		// Verify Merge hints in the current query,
+		// we will update parameters for those that meet the rules, and warn those that do not.
+		// If the current query uses Merge Hint and the query is a CTE,
+		// we update the HINT information for the current query.
+		// If the current query is not a CTE query (it may be a subquery within a CTE query
+		// or an external non-CTE query), we will give a warning.
+		// In particular, recursive CTE have separate warnings, so they are no longer called.
 		if b.buildingCTE {
 			if b.isCTE {
 				b.outerCTEs[len(b.outerCTEs)-1].isInline = hints.MergeHints.preferMerge
 			} else if !b.buildingRecursivePartForCTE {
 				//If there has subquery which is not CTE and using `MERGE()` hint, we will show this warning;
-				b.ctx.GetSessionVars().StmtCtx.AppendWarning(ErrInternal.GenWithStack("Hint merge() is inapplicable. Please check whether the hint is using in the right place, you should use this hint in CTE inner query."))
+				b.ctx.GetSessionVars().StmtCtx.AppendWarning(
+					ErrInternal.GenWithStack("Hint merge() is inapplicable. " +
+						"Please check whether the hint is used in the right place, " +
+						"you should use this hint inside the CTE."))
 			}
 		} else if !b.buildingCTE && !b.isCTE {
-			b.ctx.GetSessionVars().StmtCtx.AppendWarning(ErrInternal.GenWithStack("Hint merge() is inapplicable. Please check whether the hint is using in the right place, you should use this hint in CTE inner query."))
+			b.ctx.GetSessionVars().StmtCtx.AppendWarning(
+				ErrInternal.GenWithStack("Hint merge() is inapplicable. " +
+					"Please check whether the hint is used in the right place, " +
+					"you should use this hint inside the CTE."))
 		}
 	}
 
@@ -4244,10 +4259,10 @@ func (b *PlanBuilder) tryBuildCTE(ctx context.Context, tn *ast.TableName, asName
 			lp.SetSchema(getResultCTESchema(cte.seedLP.Schema(), b.ctx.GetSessionVars()))
 
 			if cte.recurLP != nil && cte.isInline {
-				b.ctx.GetSessionVars().StmtCtx.AppendWarning(ErrInternal.GenWithStack("Hint merge() is inapplicable. Please check whether the CTE use recursive."))
+				b.ctx.GetSessionVars().StmtCtx.AppendWarning(
+					ErrInternal.GenWithStack("Recursive CTE can not be inlined."))
 			}
 			if cte.recurLP == nil && cte.isInline {
-				lp.MergeHints.preferMerge = cte.isInline
 				saveCte := make([]*cteInfo, len(b.outerCTEs[i:]))
 				copy(saveCte, b.outerCTEs[i:])
 				b.outerCTEs = b.outerCTEs[:i]

planner/core/logical_plans.go

@@ -1916,7 +1916,6 @@ type LogicalCTE struct {
 	cteAsName      model.CIStr
 	seedStat       *property.StatsInfo
 	isOuterMostCTE bool
-	MergeHints     MergeHintInfo
 }
 
 // LogicalCTETable is for CTE table

planner/core/physical_plan_test.go

@@ -1154,6 +1154,61 @@ func TestCTEMergeHint(t *testing.T) {
 	}
 }
 
+func TestForceInlineCTE(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("CREATE TABLE `t` (`a` int(11));")
+	tk.MustExec("insert into t values (1), (5), (10), (15), (20), (30), (50);")
+
+	var (
+		input  []string
+		output []struct {
+			SQL     string
+			Plan    []string
+			Warning []string
+		}
+	)
+	planSuiteData := core.GetPlanSuiteData()
+	planSuiteData.LoadTestCases(t, &input, &output)
+
+	for i, ts := range input {
+		testdata.OnRecord(func() {
+			output[i].SQL = ts
+		})
+		if strings.HasPrefix(ts, "set") {
+			tk.MustExec(ts)
+			continue
+		}
+		testdata.OnRecord(func() {
+			output[i].SQL = ts
+			output[i].Plan = testdata.ConvertRowsToStrings(tk.MustQuery("explain format='brief' " + ts).Rows())
+		})
+		tk.MustQuery("explain format='brief' " + ts).Check(testkit.Rows(output[i].Plan...))
+
+		comment := fmt.Sprintf("case:%v sql:%s", i, ts)
+		warnings := tk.Session().GetSessionVars().StmtCtx.GetWarnings()
+		testdata.OnRecord(func() {
+			if len(warnings) > 0 {
+				output[i].Warning = make([]string, len(warnings))
+				for j, warning := range warnings {
+					output[i].Warning[j] = warning.Err.Error()
+				}
+			}
+		})
+		if len(output[i].Warning) == 0 {
+			require.Len(t, warnings, 0)
+		} else {
+			require.Len(t, warnings, len(output[i].Warning), comment)
+			for j, warning := range warnings {
+				require.Equal(t, stmtctx.WarnLevelWarning, warning.Level, comment)
+				require.Equal(t, output[i].Warning[j], warning.Err.Error(), comment)
+			}
+		}
+	}
+}
+
 func TestPushdownDistinctEnable(t *testing.T) {
 	var (
 		input  []string

planner/core/testdata/plan_suite_in.json

@@ -596,6 +596,27 @@
       "with cte1 as (with cte2 as (with cte3 as (select  /*+ MERGE() */  * from t2) select  /*+ MERGE() */  * from cte3) select * from cte2,(select * from t1) ttt) select * from cte1,(select * from t3) ttw;"
     ]
   },
+  {
+    "name": "TestForceInlineCTE",
+    "cases": [
+      "set tidb_opt_force_inline_cte=1; -- enable force inline CTE",
+      "with cte as (select * from t) select * from cte; -- inline",
+      "with cte as (select /*+ MERGE() */ * from t) select * from cte; -- inline",
+      "with cte as (select * from t) select * from cte cte1, cte cte2; -- inline CTEs is used by multi consumers",
+      "with cte1 as (select * from t), cte2 as (select a from cte1 group by a) select * from cte1, cte2; -- multi inline CTEs",
+      "with recursive cte1(c1) as (select 1 union select c1 + 1 c1 from cte1 where c1 < 100) select * from cte1; -- Recursive CTE can not be inlined",
+      "with cte1 as (with cte2 as (select * from t) select * from cte2) select * from cte1; -- non-recursive 'cte2' definition inside another non-recursive 'cte1'",
+      "with recursive cte2(c1) as (with cte1 as (select * from t) select a c1 from cte1 union select c1+1 c1 from cte2 where c1 < 100) select * from cte2; -- non-recursive 'cte1' inside recursive 'cte2'",
+      "with cte1 as (with recursive cte2(c1) as (select 1 union select c1 + 1 c1 from cte2 where c1 < 100) select * from cte2) select * from cte1; -- recursive 'cte2' inside non-recursive 'cte1'",
+      "set tidb_opt_force_inline_cte=0; -- disable force inline CTE",
+      "with cte as (select * from t) select * from cte; -- cannot be inlined",
+      "with cte as (select /*+ MERGE() */ * from t) select * from cte; -- inline, merge hint override session variable",
+      "with recursive cte1(c1) as (select 1 union select /*+ MERGE() */ c1 + 1 c1 from cte1 where c1 < 100) select * from cte1; -- Recursive CTE can not be inlined",
+      "with cte1 as (with cte2 as (select * from t) select * from cte2) select * from cte1; -- non-recursive 'cte2' definition inside another non-recursive 'cte1'",
+      "with recursive cte2(c1) as (with cte1 as (select * from t) select a c1 from cte1 union select c1+1 c1 from cte2 where c1 < 100) select * from cte2; -- non-recursive 'cte1' inside recursive 'cte2'",
+      "with cte1 as (with recursive cte2(c1) as (select 1 union select c1 + 1 c1 from cte2 where c1 < 100) select * from cte2) select * from cte1; -- recursive 'cte2' inside non-recursive 'cte1'"
+    ]
+  },
   {
     "name": "TestPushdownDistinctEnable",
     "cases": [