chore: import upstream snapshot with attribution

server/ast/explain.go

@@ -0,0 +1,73 @@
+// Copyright 2023 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ast
+
+import (
+	"github.com/cockroachdb/errors"
+
+	vitess "github.com/dolthub/vitess/go/vt/sqlparser"
+
+	"github.com/dolthub/doltgresql/postgres/parser/sem/tree"
+)
+
+// nodeExplain handles *tree.Explain nodes.
+func nodeExplain(ctx *Context, node *tree.Explain) (vitess.Statement, error) {
+	if node == nil {
+		return nil, nil
+	}
+
+	if node.TableName != nil {
+		tableName, err := nodeUnresolvedObjectName(ctx, node.TableName)
+		if err != nil {
+			return nil, err
+		}
+
+		var showTableOpts *vitess.ShowTablesOpt
+		if node.AsOf != nil {
+			asOf, err := nodeExpr(ctx, node.AsOf.Expr)
+			if err != nil {
+				return nil, err
+			}
+			showTableOpts = &vitess.ShowTablesOpt{
+				AsOf:       asOf,
+				SchemaName: tableName.SchemaQualifier.String(),
+				DbName:     tableName.DbQualifier.String(),
+			}
+		}
+
+		show := &vitess.Show{
+			Type:          "columns",
+			Table:         tableName,
+			ShowTablesOpt: showTableOpts,
+		}
+
+		return show, nil
+	}
+
+	if stmt, ok := node.Statement.(*tree.Select); ok {
+		// TODO: read tree.ExplainOptions
+		selectStmt, err := nodeSelect(ctx, stmt)
+		if err != nil {
+			return nil, err
+		}
+		explain := &vitess.Explain{
+			ExplainFormat: vitess.TreeStr,
+			Statement:     selectStmt,
+		}
+		return explain, nil
+	}
+
+	return nil, errors.Errorf("This EXPLAIN syntax is not yet supported")
+}