Add APPLY DELETED MASK alter command support

Parse and explain ALTER TABLE ... APPLY DELETED MASK command,
including optional IN PARTITION clause.

Fixes stmt13 and stmt21 in 02932_apply_deleted_mask and additional
tests in 02932_lwd_and_mutations and 03404_json_tables.

Author: claude

ast/ast.go

@@ -700,6 +700,7 @@ const (
 	AlterModifyOrderBy         AlterCommandType = "MODIFY_ORDER_BY"
 	AlterModifySampleBy        AlterCommandType = "MODIFY_SAMPLE_BY"
 	AlterRemoveSampleBy        AlterCommandType = "REMOVE_SAMPLE_BY"
+	AlterApplyDeletedMask      AlterCommandType = "APPLY_DELETED_MASK"
 )
 
 // TruncateQuery represents a TRUNCATE statement.

internal/explain/statements.go

@@ -1802,7 +1802,7 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 	case ast.AlterModifySetting:
 		fmt.Fprintf(sb, "%s Set\n", indent)
 	case ast.AlterDropPartition, ast.AlterDetachPartition, ast.AlterAttachPartition,
-		ast.AlterReplacePartition, ast.AlterFetchPartition, ast.AlterMovePartition, ast.AlterFreezePartition, ast.AlterApplyPatches:
+		ast.AlterReplacePartition, ast.AlterFetchPartition, ast.AlterMovePartition, ast.AlterFreezePartition, ast.AlterApplyPatches, ast.AlterApplyDeletedMask:
 		if cmd.Partition != nil {
 			// PARTITION ALL is shown as Partition_ID (empty) in EXPLAIN AST
 			if ident, ok := cmd.Partition.(*ast.Identifier); ok && strings.ToUpper(ident.Name()) == "ALL" {
@@ -2085,7 +2085,7 @@ func countAlterCommandChildren(cmd *ast.AlterCommand) int {
 	case ast.AlterModifySetting:
 		children = 1
 	case ast.AlterDropPartition, ast.AlterDetachPartition, ast.AlterAttachPartition,
-		ast.AlterReplacePartition, ast.AlterFetchPartition, ast.AlterMovePartition, ast.AlterFreezePartition, ast.AlterApplyPatches:
+		ast.AlterReplacePartition, ast.AlterFetchPartition, ast.AlterMovePartition, ast.AlterFreezePartition, ast.AlterApplyPatches, ast.AlterApplyDeletedMask:
 		if cmd.Partition != nil {
 			children++
 		}

parser/parser.go

@@ -5879,6 +5879,7 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 		}
 	case token.APPLY:
 		// APPLY PATCHES IN PARTITION expr
+		// APPLY DELETED MASK [IN PARTITION expr]
 		p.nextToken() // skip APPLY
 		if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "PATCHES" {
 			p.nextToken() // skip PATCHES
@@ -5890,6 +5891,19 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 					cmd.Partition = p.parseExpression(LOWEST)
 				}
 			}
+		} else if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "DELETED" {
+			p.nextToken() // skip DELETED
+			if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "MASK" {
+				p.nextToken() // skip MASK
+				cmd.Type = ast.AlterApplyDeletedMask
+				if p.currentIs(token.IN) {
+					p.nextToken() // skip IN
+					if p.currentIs(token.PARTITION) {
+						p.nextToken() // skip PARTITION
+						cmd.Partition = p.parseExpression(LOWEST)
+					}
+				}
+			}
 		}
 	case token.DELETE:
 		// DELETE WHERE condition - mutation to delete rows

parser/testdata/02932_apply_deleted_mask/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt13": true,
-    "stmt21": true
-  }
-}
+{}

parser/testdata/02932_lwd_and_mutations/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt23": true
-  }
-}
+{}

parser/testdata/03404_json_tables/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true
-  }
-}
+{}