Add ALTER TABLE COMMENT COLUMN support

- Add COMMENT case to parseAlterCommand for COMMENT COLUMN syntax
- Add Comment field to AlterCommand struct for storing column comments
- Update explainAlterQuery to output database and table as separate
  identifiers when database is present
- Update AlterCommentColumn explain output to include comment literal
- Fix various tests that now pass with COMMENT COLUMN support

Author: claude

ast/ast.go

@@ -531,6 +531,7 @@ type AlterCommand struct {
 	ProjectionName string               `json:"projection_name,omitempty"` // For DROP/MATERIALIZE/CLEAR PROJECTION
 	StatisticsColumns []string          `json:"statistics_columns,omitempty"` // For ADD/DROP/CLEAR/MATERIALIZE STATISTICS
 	StatisticsTypes   []*FunctionCall   `json:"statistics_types,omitempty"`   // For ADD/MODIFY STATISTICS TYPE
+	Comment           string            `json:"comment,omitempty"`            // For COMMENT COLUMN
 }
 
 // Projection represents a projection definition.

internal/explain/statements.go

@@ -955,18 +955,22 @@ func explainAlterQuery(sb *strings.Builder, n *ast.AlterQuery, indent string, de
 		return
 	}
 
-	name := n.Table
+	children := 2 // ExpressionList + Identifier for table
 	if n.Database != "" {
-		name = n.Database + "." + n.Table
+		children = 3 // ExpressionList + Identifier for database + Identifier for table
+		fmt.Fprintf(sb, "%sAlterQuery %s %s (children %d)\n", indent, n.Database, n.Table, children)
+	} else {
+		fmt.Fprintf(sb, "%sAlterQuery  %s (children %d)\n", indent, n.Table, children)
 	}
 
-	children := 2
-	fmt.Fprintf(sb, "%sAlterQuery  %s (children %d)\n", indent, name, children)
 	fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, len(n.Commands))
 	for _, cmd := range n.Commands {
 		explainAlterCommand(sb, cmd, indent+"  ", depth+2)
 	}
-	fmt.Fprintf(sb, "%s Identifier %s\n", indent, name)
+	if n.Database != "" {
+		fmt.Fprintf(sb, "%s Identifier %s\n", indent, n.Database)
+	}
+	fmt.Fprintf(sb, "%s Identifier %s\n", indent, n.Table)
 }
 
 func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent string, depth int) {
@@ -1015,6 +1019,9 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 		if cmd.ColumnName != "" {
 			fmt.Fprintf(sb, "%s Identifier %s\n", indent, cmd.ColumnName)
 		}
+		if cmd.Comment != "" {
+			fmt.Fprintf(sb, "%s Literal \\'%s\\'\n", indent, escapeStringLiteral(cmd.Comment))
+		}
 	case ast.AlterAddIndex, ast.AlterDropIndex, ast.AlterClearIndex, ast.AlterMaterializeIndex:
 		if cmd.Index != "" {
 			fmt.Fprintf(sb, "%s Identifier %s\n", indent, cmd.Index)
@@ -1178,10 +1185,17 @@ func countAlterCommandChildren(cmd *ast.AlterCommand) int {
 		if cmd.AfterColumn != "" {
 			children++
 		}
-	case ast.AlterDropColumn, ast.AlterCommentColumn:
+	case ast.AlterDropColumn:
+		if cmd.ColumnName != "" {
+			children++
+		}
+	case ast.AlterCommentColumn:
 		if cmd.ColumnName != "" {
 			children++
 		}
+		if cmd.Comment != "" {
+			children++
+		}
 	case ast.AlterRenameColumn:
 		if cmd.ColumnName != "" {
 			children++

parser/parser.go

@@ -3860,6 +3860,28 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				}
 			}
 		}
+	case token.COMMENT:
+		p.nextToken()
+		if p.currentIs(token.COLUMN) {
+			cmd.Type = ast.AlterCommentColumn
+			p.nextToken()
+			// Handle IF EXISTS
+			if p.currentIs(token.IF) {
+				p.nextToken()
+				p.expect(token.EXISTS)
+				cmd.IfExists = true
+			}
+			// Parse column name
+			if p.currentIs(token.IDENT) || p.current.Token.IsKeyword() {
+				cmd.ColumnName = p.current.Value
+				p.nextToken()
+			}
+			// Parse comment string
+			if p.currentIs(token.STRING) {
+				cmd.Comment = p.current.Value
+				p.nextToken()
+			}
+		}
 	case token.DETACH:
 		p.nextToken()
 		if p.currentIs(token.PARTITION) {

parser/testdata/00030_alter_table/metadata.json

@@ -5,7 +5,6 @@
     "stmt16": true,
     "stmt19": true,
     "stmt20": true,
-    "stmt22": true,
     "stmt9": true
   }
 }

parser/testdata/00725_comment_columns_long/metadata.json

@@ -1,11 +1,9 @@
 {
   "explain_todo": {
     "stmt13": true,
-    "stmt17": true,
     "stmt19": true,
     "stmt21": true,
     "stmt3": true,
-    "stmt7": true,
     "stmt9": true
   }
 }

parser/testdata/00753_comment_columns_zookeeper/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt2": true,
-    "stmt5": true
+    "stmt2": true
   }
 }

parser/testdata/01164_alter_memory_database/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt5": true,
     "stmt6": true,
     "stmt7": true
   }

parser/testdata/01702_system_query_log/metadata.json

@@ -3,14 +3,8 @@
     "stmt15": true,
     "stmt19": true,
     "stmt24": true,
-    "stmt26": true,
-    "stmt27": true,
     "stmt28": true,
-    "stmt29": true,
     "stmt30": true,
-    "stmt31": true,
-    "stmt32": true,
-    "stmt33": true,
     "stmt34": true,
     "stmt37": true,
     "stmt38": true,

parser/testdata/02155_dictionary_comment/metadata.json

@@ -1,7 +1,5 @@
 {
   "explain_todo": {
-    "stmt13": true,
-    "stmt19": true,
-    "stmt7": true
+    "stmt13": true
   }
 }

parser/testdata/02918_alter_temporary_table/metadata.json

@@ -5,7 +5,6 @@
     "stmt15": true,
     "stmt18": true,
     "stmt19": true,
-    "stmt21": true,
     "stmt5": true,
     "stmt8": true
   }