Add support for ALTER TABLE MODIFY SAMPLE BY

- Add AlterModifySampleBy type in ast/ast.go
- Add SampleByExpr field to AlterCommand struct
- Add parsing for MODIFY SAMPLE BY in parseAlterCommand
- Add EXPLAIN output for MODIFY_SAMPLE_BY

Fixes 5 statements in 01430_modify_sample_by_zookeeper_long plus
1 bonus test (02096_sample_by_tuple).

Author: claude

ast/ast.go

@@ -599,6 +599,7 @@ type AlterCommand struct {
 	StatisticsTypes   []*FunctionCall   `json:"statistics_types,omitempty"`   // For ADD/MODIFY STATISTICS TYPE
 	Comment           string            `json:"comment,omitempty"`            // For COMMENT COLUMN
 	OrderByExpr       []Expression      `json:"order_by_expr,omitempty"`      // For MODIFY ORDER BY
+	SampleByExpr      Expression        `json:"sample_by_expr,omitempty"`     // For MODIFY SAMPLE BY
 	ResetSettings     []string          `json:"reset_settings,omitempty"`     // For MODIFY COLUMN ... RESET SETTING
 }
 
@@ -675,6 +676,7 @@ const (
 	AlterMaterializeStatistics AlterCommandType = "MATERIALIZE_STATISTICS"
 	AlterModifyComment         AlterCommandType = "MODIFY_COMMENT"
 	AlterModifyOrderBy         AlterCommandType = "MODIFY_ORDER_BY"
+	AlterModifySampleBy        AlterCommandType = "MODIFY_SAMPLE_BY"
 	AlterRemoveSampleBy        AlterCommandType = "REMOVE_SAMPLE_BY"
 )
 

internal/explain/statements.go

@@ -1516,6 +1516,11 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 				Node(sb, expr, depth+1)
 			}
 		}
+	case ast.AlterModifySampleBy:
+		// Single expression - output directly
+		if cmd.SampleByExpr != nil {
+			Node(sb, cmd.SampleByExpr, depth+1)
+		}
 	default:
 		if cmd.Partition != nil {
 			Node(sb, cmd.Partition, depth+1)
@@ -1747,6 +1752,11 @@ func countAlterCommandChildren(cmd *ast.AlterCommand) int {
 		if len(cmd.OrderByExpr) > 0 {
 			children = 1
 		}
+	case ast.AlterModifySampleBy:
+		// MODIFY SAMPLE BY: single expression (1 child)
+		if cmd.SampleByExpr != nil {
+			children = 1
+		}
 	default:
 		if cmd.Partition != nil {
 			children++

parser/parser.go

@@ -5023,6 +5023,14 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				// Single expression without parentheses
 				cmd.OrderByExpr = []ast.Expression{p.parseExpression(LOWEST)}
 			}
+		} else if p.currentIs(token.SAMPLE) {
+			// MODIFY SAMPLE BY expr
+			cmd.Type = ast.AlterModifySampleBy
+			p.nextToken() // skip SAMPLE
+			if p.currentIs(token.BY) {
+				p.nextToken() // skip BY
+			}
+			cmd.SampleByExpr = p.parseExpression(LOWEST)
 		}
 	case token.RENAME:
 		p.nextToken()

parser/testdata/01430_modify_sample_by_zookeeper_long/metadata.json

@@ -1,9 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true,
-    "stmt17": true,
-    "stmt18": true,
-    "stmt25": true,
-    "stmt7": true
-  }
-}
+{}

parser/testdata/02096_sample_by_tuple/metadata.json

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