Add ALTER TABLE MODIFY COMMENT support

Implement parsing and explain output for ALTER TABLE ... MODIFY COMMENT
statements:
- Add AlterModifyComment command type to AST
- Add parser handling in MODIFY case for COMMENT token
- Add explain output handling for MODIFY_COMMENT command type
- Add child count handling for comment literal

This fixes 14 statements across 4 tests:
- 02111_modify_table_comment: stmt6, stmt10
- 02155_dictionary_comment: stmt13
- 02792_alter_table_modify_comment: stmt4, stmt9, stmt14, stmt19, stmt24, stmt29, stmt34, stmt39, stmt44, stmt49
- 03142_alter_comment_parameterized_view: stmt3

Author: claude

ast/ast.go

@@ -653,6 +653,7 @@ const (
 	AlterDropStatistics     AlterCommandType = "DROP_STATISTICS"
 	AlterClearStatistics    AlterCommandType = "CLEAR_STATISTICS"
 	AlterMaterializeStatistics AlterCommandType = "MATERIALIZE_STATISTICS"
+	AlterModifyComment         AlterCommandType = "MODIFY_COMMENT"
 )
 
 // TruncateQuery represents a TRUNCATE statement.

internal/explain/statements.go

@@ -1210,6 +1210,10 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 		if cmd.Comment != "" {
 			fmt.Fprintf(sb, "%s Literal \\'%s\\'\n", indent, escapeStringLiteral(cmd.Comment))
 		}
+	case ast.AlterModifyComment:
+		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)
@@ -1392,6 +1396,10 @@ func countAlterCommandChildren(cmd *ast.AlterCommand) int {
 		if cmd.Comment != "" {
 			children++
 		}
+	case ast.AlterModifyComment:
+		if cmd.Comment != "" {
+			children++
+		}
 	case ast.AlterRenameColumn:
 		if cmd.ColumnName != "" {
 			children++

parser/parser.go

@@ -4495,6 +4495,14 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				p.nextToken()
 				cmd.StatisticsTypes = p.parseStatisticsTypeList()
 			}
+		} else if p.currentIs(token.COMMENT) {
+			// MODIFY COMMENT 'comment string'
+			cmd.Type = ast.AlterModifyComment
+			p.nextToken()
+			if p.currentIs(token.STRING) {
+				cmd.Comment = p.current.Value
+				p.nextToken()
+			}
 		}
 	case token.RENAME:
 		p.nextToken()

parser/testdata/02111_modify_table_comment/metadata.json

@@ -1,8 +1,6 @@
 {
   "explain_todo": {
-    "stmt10": true,
     "stmt4": true,
-    "stmt6": true,
     "stmt8": true
   }
 }

parser/testdata/02155_dictionary_comment/metadata.json

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

parser/testdata/02792_alter_table_modify_comment/metadata.json

@@ -1,16 +1,6 @@
 {
   "explain_todo": {
-    "stmt14": true,
-    "stmt19": true,
     "stmt22": true,
-    "stmt24": true,
-    "stmt29": true,
-    "stmt34": true,
-    "stmt39": true,
-    "stmt4": true,
-    "stmt44": true,
-    "stmt49": true,
-    "stmt7": true,
-    "stmt9": true
+    "stmt7": true
   }
 }

parser/testdata/03142_alter_comment_parameterized_view/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt2": true,
-    "stmt3": true
+    "stmt2": true
   }
 }