Add DESCRIBE subquery support for DESCRIBE (SELECT ...) statements (#98)

Author: kyleconroy

ast/ast.go

@@ -623,12 +623,13 @@ func (a *AttachQuery) statementNode()      {}
 
 // DescribeQuery represents a DESCRIBE statement.
 type DescribeQuery struct {
-	Position      token.Position `json:"-"`
-	Database      string         `json:"database,omitempty"`
-	Table         string         `json:"table,omitempty"`
-	TableFunction *FunctionCall  `json:"table_function,omitempty"`
-	Settings      []*SettingExpr `json:"settings,omitempty"`
-	Format        string         `json:"format,omitempty"`
+	Position      token.Position   `json:"-"`
+	Database      string           `json:"database,omitempty"`
+	Table         string           `json:"table,omitempty"`
+	TableFunction *FunctionCall    `json:"table_function,omitempty"`
+	TableExpr     *TableExpression `json:"table_expr,omitempty"` // For DESCRIBE (SELECT ...)
+	Settings      []*SettingExpr   `json:"settings,omitempty"`
+	Format        string           `json:"format,omitempty"`
 }
 
 func (d *DescribeQuery) Pos() token.Position { return d.Position }

internal/explain/explain.go

@@ -134,7 +134,7 @@ func Node(sb *strings.Builder, node interface{}, depth int) {
 	case *ast.UseQuery:
 		explainUseQuery(sb, n, indent)
 	case *ast.DescribeQuery:
-		explainDescribeQuery(sb, n, indent)
+		explainDescribeQuery(sb, n, indent, depth)
 	case *ast.ExistsQuery:
 		explainExistsTableQuery(sb, n, indent)
 	case *ast.DetachQuery:

internal/explain/statements.go

@@ -577,8 +577,19 @@ func explainUseQuery(sb *strings.Builder, n *ast.UseQuery, indent string) {
 	fmt.Fprintf(sb, "%s Identifier %s\n", indent, n.Database)
 }
 
-func explainDescribeQuery(sb *strings.Builder, n *ast.DescribeQuery, indent string) {
-	if n.TableFunction != nil {
+func explainDescribeQuery(sb *strings.Builder, n *ast.DescribeQuery, indent string, depth int) {
+	if n.TableExpr != nil {
+		// DESCRIBE on a subquery - TableExpr contains a TableExpression with a Subquery
+		children := 1
+		if len(n.Settings) > 0 {
+			children++
+		}
+		fmt.Fprintf(sb, "%sDescribeQuery (children %d)\n", indent, children)
+		Node(sb, n.TableExpr, depth+1)
+		if len(n.Settings) > 0 {
+			fmt.Fprintf(sb, "%s Set\n", indent)
+		}
+	} else if n.TableFunction != nil {
 		// DESCRIBE on a table function - wrap in TableExpression
 		children := 1
 		if len(n.Settings) > 0 {

parser/parser.go

@@ -3253,9 +3253,12 @@ func (p *Parser) parseDescribe() *ast.DescribeQuery {
 		p.nextToken()
 	}
 
-	// Parse table name or table function
-	// Table functions look like: format(CSV, '...'), url('...'), s3Cluster(...)
-	if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
+	// Check for subquery: DESCRIBE (SELECT ...)
+	if p.currentIs(token.LPAREN) {
+		desc.TableExpr = p.parseTableExpression()
+	} else if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
+		// Parse table name or table function
+		// Table functions look like: format(CSV, '...'), url('...'), s3Cluster(...)
 		pos := p.current.Pos
 		tableName := p.current.Value
 		p.nextToken()

parser/testdata/00515_shard_desc_table_functions_and_subqueries/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true,
-    "stmt14": true,
-    "stmt16": true
-  }
-}
+{}

parser/testdata/01549_low_cardinality_materialized_view/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt4": true,
-    "stmt7": true
+    "stmt4": true
   }
 }

parser/testdata/01937_nested_chinese/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt3": true,
-    "stmt4": true,
-    "stmt5": true
-  }
-}
+{}

parser/testdata/02096_sql_user_defined_function_alias/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt2": true
-  }
-}
+{}

parser/testdata/02337_analyzer_columns_basic/metadata.json

@@ -1,19 +1,6 @@
 {
   "explain_todo": {
-    "stmt17": true,
-    "stmt20": true,
-    "stmt23": true,
-    "stmt26": true,
-    "stmt29": true,
-    "stmt3": true,
-    "stmt32": true,
-    "stmt35": true,
     "stmt42": true,
-    "stmt43": true,
-    "stmt45": true,
-    "stmt48": true,
-    "stmt51": true,
-    "stmt6": true,
-    "stmt9": true
+    "stmt43": true
   }
 }

parser/testdata/02338_analyzer_constants_basic/metadata.json

@@ -1,13 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true,
-    "stmt14": true,
-    "stmt17": true,
-    "stmt2": true,
-    "stmt20": true,
-    "stmt23": true,
-    "stmt25": true,
-    "stmt5": true,
-    "stmt8": true
-  }
-}
+{}