Add ALTER TABLE MODIFY QUERY support (#123)

Added parsing and explain support for the ALTER TABLE MODIFY QUERY
statement which modifies the SELECT query of a materialized view.

Changes:
- Added AlterModifyQuery command type to ast/ast.go
- Added Query field to AlterCommand struct
- Added parsing logic for MODIFY QUERY in parser.go
- Added explain output handling in statements.go

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

Author: kyleconroy

ast/ast.go

@@ -624,6 +624,7 @@ type AlterCommand struct {
 	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
+	Query             Statement         `json:"query,omitempty"`              // For MODIFY QUERY
 }
 
 // Projection represents a projection definition.
@@ -702,6 +703,7 @@ const (
 	AlterModifyComment         AlterCommandType = "MODIFY_COMMENT"
 	AlterModifyOrderBy         AlterCommandType = "MODIFY_ORDER_BY"
 	AlterModifySampleBy        AlterCommandType = "MODIFY_SAMPLE_BY"
+	AlterModifyQuery           AlterCommandType = "MODIFY_QUERY"
 	AlterRemoveSampleBy        AlterCommandType = "REMOVE_SAMPLE_BY"
 	AlterApplyDeletedMask      AlterCommandType = "APPLY_DELETED_MASK"
 )

internal/explain/statements.go

@@ -1931,6 +1931,11 @@ func explainAlterCommand(sb *strings.Builder, cmd *ast.AlterCommand, indent stri
 		if cmd.SampleByExpr != nil {
 			Node(sb, cmd.SampleByExpr, depth+1)
 		}
+	case ast.AlterModifyQuery:
+		// MODIFY QUERY: output the SELECT statement
+		if cmd.Query != nil {
+			Node(sb, cmd.Query, depth+1)
+		}
 	case ast.AlterResetSetting:
 		// RESET SETTING outputs ExpressionList with Identifier children
 		if len(cmd.ResetSettings) > 0 {
@@ -2192,6 +2197,11 @@ func countAlterCommandChildren(cmd *ast.AlterCommand) int {
 		if cmd.SampleByExpr != nil {
 			children = 1
 		}
+	case ast.AlterModifyQuery:
+		// MODIFY QUERY: SELECT statement (1 child)
+		if cmd.Query != nil {
+			children = 1
+		}
 	case ast.AlterResetSetting:
 		// RESET SETTING: ExpressionList with setting names (1 child)
 		if len(cmd.ResetSettings) > 0 {

parser/parser.go

@@ -5809,6 +5809,11 @@ func (p *Parser) parseAlterCommand() *ast.AlterCommand {
 				p.nextToken() // skip BY
 			}
 			cmd.SampleByExpr = p.parseExpression(LOWEST)
+		} else if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "QUERY" {
+			// MODIFY QUERY SELECT ...
+			cmd.Type = ast.AlterModifyQuery
+			p.nextToken() // skip QUERY
+			cmd.Query = p.parseSelectWithUnion()
 		}
 	case token.RENAME:
 		p.nextToken()

parser/testdata/01019_alter_materialized_view_query/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt9": true
-  }
-}
+{}

parser/testdata/02834_alter_exception/metadata.json

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

parser/testdata/02931_alter_materialized_view_query_inconsistent/metadata.json

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

parser/testdata/03002_modify_query_cte/metadata.json

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

parser/testdata/03022_alter_materialized_view_query_has_inner_table/metadata.json

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

parser/testdata/03243_check_for_nullable_nothing_in_alter/metadata.json

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

parser/testdata/03740_alter_modify_query_dict_name_in_cse/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt9": true
-  }
-}
+{}