Handle PARTITION ID syntax in UPDATE mutation commands

- Parse PARTITION ID 'value' syntax in ALTER UPDATE
- Handle both InExpr mis-parse fix path and direct IN PARTITION path
- Add Partition_ID output handling for AlterUpdate explain

Fixes 02399_merge_tree_mutate_in_partition/stmt8

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

internal/explain/statements.go

@@ -1885,6 +1885,15 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 			// PARTITION ALL is shown as Partition_ID (empty) in EXPLAIN AST
 			if ident, ok := cmd.Partition.(*ast.Identifier); ok && strings.ToUpper(ident.Name()) == "ALL" {
 				fmt.Fprintf(sb, "%s Partition_ID \n", indent)
+			} else if cmd.PartitionIsID {
+				// PARTITION ID 'value' is shown as Partition_ID Literal_'value' (children 1)
+				if lit, ok := cmd.Partition.(*ast.Literal); ok {
+					fmt.Fprintf(sb, "%s Partition_ID Literal_\\'%s\\' (children 1)\n", indent, lit.Value)
+					Node(sb, cmd.Partition, depth+2)
+				} else {
+					fmt.Fprintf(sb, "%s Partition_ID (children 1)\n", indent)
+					Node(sb, cmd.Partition, depth+2)
+				}
 			} else {
 				fmt.Fprintf(sb, "%s Partition (children 1)\n", indent)
 				Node(sb, cmd.Partition, depth+2)

parser/parser.go

@@ -6053,7 +6053,12 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				if ident, ok := inExpr.List[0].(*ast.Identifier); ok && strings.ToUpper(ident.Name()) == "PARTITION" {
 					// Fix the mis-parse: the actual assignment value is the left side of IN
 					lastAssign.Value = inExpr.Expr
-					// Current token should be the partition expression (e.g., ALL)
+					// Check for PARTITION ID 'value' syntax
+					if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "ID" {
+						p.nextToken()
+						cmd.PartitionIsID = true
+					}
+					// Current token should be the partition expression (e.g., ALL or '1')
 					cmd.Partition = p.parseExpression(LOWEST)
 				}
 			}
@@ -6062,6 +6067,11 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 			p.nextToken() // skip IN
 			if p.currentIs(token.PARTITION) {
 				p.nextToken() // skip PARTITION
+				// Check for PARTITION ID 'value' syntax
+				if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "ID" {
+					p.nextToken()
+					cmd.PartitionIsID = true
+				}
 				cmd.Partition = p.parseExpression(LOWEST)
 			}
 		}

parser/testdata/02399_merge_tree_mutate_in_partition/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt8": true
-  }
-}
+{}