fix explain plan on server engine (#2983)

Co-authored-by: James Cor <[email protected]>

Author: jycor

enginetest/queries/queries.go

@@ -10260,6 +10260,16 @@ from typestable`,
 			{false},
 		},
 	},
+	{
+		Query: "explain plan select count(*) from mytable",
+		Expected: []sql.Row{
+			{"Project"},
+			{" ├─ columns: [count(1)]"},
+			{" └─ Project"},
+			{"     ├─ columns: [mytable.COUNT(1) as COUNT(1)]"},
+			{"     └─ table_count(mytable) as COUNT(1)"},
+		},
+	},
 	{
 		Query:            "explain select 1",
 		SkipServerEngine: true,
@@ -10268,8 +10278,7 @@ from typestable`,
 		},
 	},
 	{
-		Query:            "explain plan select 1",
-		SkipServerEngine: true,
+		Query: "explain plan select 1",
 		Expected: []sql.Row{
 			{"Project"},
 			{" ├─ columns: [1]"},
@@ -10278,8 +10287,7 @@ from typestable`,
 		},
 	},
 	{
-		Query:            "explain format=tree select 1",
-		SkipServerEngine: true,
+		Query: "explain format=tree select 1",
 		Expected: []sql.Row{
 			{"Project"},
 			{" ├─ columns: [1]"},

enginetest/server_engine.go

@@ -216,7 +216,7 @@ func (s *ServerQueryEngine) queryOrExec(ctx *sql.Context, stmt *gosql.Stmt, pars
 	var err error
 	switch parsed.(type) {
 	// TODO: added `FLUSH` stmt here (should be `exec`) because we don't support `FLUSH BINARY LOGS` or `FLUSH ENGINE LOGS`, so nil schema is returned.
-	case *sqlparser.Select, *sqlparser.SetOp, *sqlparser.Show, *sqlparser.Set, *sqlparser.Call, *sqlparser.Begin, *sqlparser.Use, *sqlparser.Load, *sqlparser.Execute, *sqlparser.Analyze, *sqlparser.Flush:
+	case *sqlparser.Select, *sqlparser.SetOp, *sqlparser.Show, *sqlparser.Set, *sqlparser.Call, *sqlparser.Begin, *sqlparser.Use, *sqlparser.Load, *sqlparser.Execute, *sqlparser.Analyze, *sqlparser.Flush, *sqlparser.Explain:
 		var rows *gosql.Rows
 		if stmt != nil {
 			rows, err = stmt.Query(args...)

sql/analyzer/analyzer.go

@@ -477,6 +477,7 @@ func (a *Analyzer) Analyze(ctx *sql.Context, node sql.Node, scope *plan.Scope, q
 	switch n := node.(type) {
 	case *plan.DescribeQuery:
 		child, _, err := a.analyzeWithSelector(ctx, n.Query(), scope, SelectAllBatches, DefaultRuleSelector, qFlags)
+		qFlags.Unset(sql.QFlagMax1Row) // the rule replaceCountStar can set this incorrectly for queries containing count(*).
 		return n.WithQuery(child), err
 	}
 	node, _, err := a.analyzeWithSelector(ctx, node, scope, SelectAllBatches, DefaultRuleSelector, qFlags)