Add support for altering table comments

enginetest/queries/alter_table_queries.go

@@ -1029,12 +1029,58 @@ var AlterTableScripts = []ScriptTest{
 			},
 		},
 	},
+	{
+		Name: "alter table comment",
+		SetUpScript: []string{
+			"create table t (i int)",
+			"create table tableWithComment (i int) comment = 'comment'",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "show create table t",
+				Expected: []sql.Row{{
+					"t",
+					"CREATE TABLE `t` (\n  `i` int\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin",
+				}},
+			},
+			{
+				Query: "show create table tableWithComment",
+				Expected: []sql.Row{{
+					"tableWithComment",
+					"CREATE TABLE `tableWithComment` (\n  `i` int\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin COMMENT='comment'",
+				}},
+			},
+			{
+				Query:    "alter table t comment = 'new comment'",
+				Expected: []sql.Row{{types.NewOkResult(0)}},
+			},
+			{
+				Query:    "alter table tableWithComment comment = 'new comment'",
+				Expected: []sql.Row{{types.NewOkResult(0)}},
+			},
+			{
+				Query: "show create table t",
+				Expected: []sql.Row{{
+					"t",
+					"CREATE TABLE `t` (\n  `i` int\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin COMMENT='new comment'",
+				}},
+			},
+			{
+				Query: "show create table tableWithComment",
+				Expected: []sql.Row{{
+					"tableWithComment",
+					"CREATE TABLE `tableWithComment` (\n  `i` int\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin COMMENT='new comment'",
+				}},
+			},
+		},
+	},
 	{
 		Name: "alter table comments are escaped",
 		SetUpScript: []string{
 			"create table t (i int);",
 			`alter table t modify column i int comment "newline \n | return \r | backslash \\ | NUL \0 \x00 | ctrlz \Z \x1A"`,
 			`alter table t add column j int comment "newline \n | return \r | backslash \\ | NUL \0 \x00 | ctrlz \Z \x1A"`,
+			`alter table t comment = "newline \n | return \r | backslash \\ | NUL \0 \x00 | ctrlz \Z \x1A"`,
 		},
 		Assertions: []ScriptTestAssertion{
 			{
@@ -1043,7 +1089,7 @@ var AlterTableScripts = []ScriptTest{
 					"t",
 					"CREATE TABLE `t` (\n  `i` int COMMENT 'newline \\n | return \\r | backslash \\\\ | NUL \\0 x00 | ctrlz \x1A x1A'," +
 						"\n  `j` int COMMENT 'newline \\n | return \\r | backslash \\\\ | NUL \\0 x00 | ctrlz \x1A x1A'\n" +
-						") ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin"}},
+						") ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin COMMENT='newline \\n | return \\r | backslash \\\\ | NUL \\0 x00 | ctrlz \u001A x1A'"}},
 			},
 		},
 	},

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/dolthub/go-icu-regex v0.0.0-20250327004329-6799764f2dad
 	github.com/dolthub/jsonpath v0.0.2-0.20240227200619-19675ab05c71
 	github.com/dolthub/sqllogictest/go v0.0.0-20201107003712-816f3ae12d81
-	github.com/dolthub/vitess v0.0.0-20250730174048-497aebb8cea7
+	github.com/dolthub/vitess v0.0.0-20250813175212-45844169a751
 	github.com/go-sql-driver/mysql v1.7.2-0.20231213112541-0004702b931d
 	github.com/gocraft/dbr/v2 v2.7.2
 	github.com/google/uuid v1.3.0

go.sum

@@ -20,6 +20,8 @@ github.com/dolthub/sqllogictest/go v0.0.0-20201107003712-816f3ae12d81 h1:7/v8q9X
 github.com/dolthub/sqllogictest/go v0.0.0-20201107003712-816f3ae12d81/go.mod h1:siLfyv2c92W1eN/R4QqG/+RjjX5W2+gCTRjZxBjI3TY=
 github.com/dolthub/vitess v0.0.0-20250730174048-497aebb8cea7 h1:l+mWO0xoh4eG1J9gMS87opL6N6WGAQitF36R/Lg4bWs=
 github.com/dolthub/vitess v0.0.0-20250730174048-497aebb8cea7/go.mod h1:1gQZs/byeHLMSul3Lvl3MzioMtOW1je79QYGyi2fd70=
+github.com/dolthub/vitess v0.0.0-20250813175212-45844169a751 h1:BBQKyvyODewdQxS+ICklMn1d/fFj2pVlkmMN1QFY4ms=
+github.com/dolthub/vitess v0.0.0-20250813175212-45844169a751/go.mod h1:1gQZs/byeHLMSul3Lvl3MzioMtOW1je79QYGyi2fd70=
 github.com/go-sql-driver/mysql v1.4.1/go.mod h1:zAC/RDZ24gD3HViQzih4MyKcchzm+sOG5ZlKdlhCg5w=
 github.com/go-sql-driver/mysql v1.5.0/go.mod h1:DCzpHaOWr8IXmIStZouvnhqoel9Qv2LBy8hT2VhHyBg=
 github.com/go-sql-driver/mysql v1.6.0/go.mod h1:DCzpHaOWr8IXmIStZouvnhqoel9Qv2LBy8hT2VhHyBg=

memory/table.go

@@ -75,6 +75,7 @@ var _ sql.TruncateableTable = (*Table)(nil)
 var _ sql.AlterableTable = (*Table)(nil)
 var _ sql.IndexAlterableTable = (*Table)(nil)
 var _ sql.CollationAlterableTable = (*Table)(nil)
+var _ sql.CommentAlterableTable = (*Table)(nil)
 var _ sql.ForeignKeyTable = (*Table)(nil)
 var _ sql.CheckAlterableTable = (*Table)(nil)
 var _ sql.RewritableTable = (*Table)(nil)
@@ -2188,6 +2189,13 @@ func (t *Table) ModifyDefaultCollation(ctx *sql.Context, collation sql.Collation
 	return nil
 }
 
+// ModifyComment implements sql.CommentAlterableTable
+func (t *Table) ModifyComment(ctx *sql.Context, comment string) error {
+	data := t.sessionTableData(ctx)
+	data.comment = comment
+	return nil
+}
+
 // Filters implements the sql.FilteredTable interface.
 func (t *Table) Filters() []sql.Expression {
 	return t.filters

sql/errors.go

@@ -409,6 +409,9 @@ var (
 	// ErrAlterTableCollationNotSupported is thrown when the table doesn't support ALTER TABLE COLLATE statements
 	ErrAlterTableCollationNotSupported = errors.NewKind("table %s cannot have its collation altered")
 
+	// ErrAlterTableCommentNotSupported is thrown when the table doesn't support ALTER TABLE COMMENT statements
+	ErrAlterTableCommentNotSupported = errors.NewKind("table %s cannot have its comment altered")
+
 	// ErrCollationNotSupportedOnUniqueTextIndex is thrown when a unique index is created on a TEXT column, with no
 	// prefix length specified, and the collation is case-insensitive or accent-insensitive, meaning we can't
 	// reliably use a content-hashed field to detect uniqueness.

sql/plan/alter_table.go

@@ -968,3 +968,67 @@ func (atc *AlterTableCollation) WithChildren(children ...sql.Node) (sql.Node, er
 	natc.Table = children[0]
 	return &natc, nil
 }
+
+type AlterTableComment struct {
+	ddlNode
+	Table   sql.Node
+	Comment string
+}
+
+var _ sql.Node = (*AlterTableComment)(nil)
+var _ sql.Databaser = (*AlterTableComment)(nil)
+
+func NewAlterTableComment(table *ResolvedTable, comment string) *AlterTableComment {
+	return &AlterTableComment{
+		ddlNode: ddlNode{Db: table.SqlDatabase},
+		Table:   table,
+		Comment: comment,
+	}
+}
+
+// WithDatabase implements the interface sql.Databaser
+func (atc *AlterTableComment) WithDatabase(db sql.Database) (sql.Node, error) {
+	natc := *atc
+	natc.Db = db
+	return &natc, nil
+}
+
+// IsReadOnly implements the interface sql.Node
+func (atc *AlterTableComment) IsReadOnly() bool {
+	return false
+}
+
+// String implements the interface sql.Node
+func (atc *AlterTableComment) String() string {
+	return fmt.Sprintf("alter table %s comment %s", atc.Table.String(), atc.Comment)
+}
+
+// DebugString implements the interface sql.Node
+func (atc *AlterTableComment) DebugString() string {
+	return atc.String()
+}
+
+// Resolved implements the interface sql.Node
+func (atc *AlterTableComment) Resolved() bool {
+	return atc.Table.Resolved() && atc.ddlNode.Resolved()
+}
+
+// Schema implements the interface sql.Node
+func (atc *AlterTableComment) Schema() sql.Schema {
+	return atc.Table.Schema()
+}
+
+// Children implements the interface sql.Node
+func (atc *AlterTableComment) Children() []sql.Node {
+	return []sql.Node{atc.Table}
+}
+
+// WithChildren implements the interface sql.Node
+func (atc *AlterTableComment) WithChildren(children ...sql.Node) (sql.Node, error) {
+	if len(children) != 1 {
+		return nil, sql.ErrInvalidChildrenNumber.New(atc, len(children), 1)
+	}
+	natc := *atc
+	natc.Table = children[0]
+	return &natc, nil
+}

sql/planbuilder/ddl.go

@@ -650,6 +650,10 @@ func (b *Builder) buildAlterTableClause(inScope *scope, ddl *ast.DDL) []*scope {
 			outScopes = append(outScopes, b.buildAlterCollationSpec(tableScope, ddl, rt))
 		}
 
+		if ddl.AlterCommentSpec != nil {
+			outScopes = append(outScopes, b.buildAlterCommentSpec(tableScope, ddl, rt))
+		}
+
 		if ddl.NotNullSpec != nil {
 			outScopes = append(outScopes, b.buildAlterNotNull(tableScope, ddl, rt))
 		}
@@ -1176,6 +1180,12 @@ func (b *Builder) buildAlterCollationSpec(inScope *scope, ddl *ast.DDL, table *p
 	return
 }
 
+func (b *Builder) buildAlterCommentSpec(inScope *scope, ddl *ast.DDL, table *plan.ResolvedTable) (outScope *scope) {
+	outScope = inScope
+	outScope.node = plan.NewAlterTableComment(table, ddl.AlterCommentSpec.Comment)
+	return
+}
+
 func (b *Builder) buildDefaultExpression(inScope *scope, defaultExpr ast.Expr) *sql.ColumnDefaultValue {
 	if defaultExpr == nil {
 		return nil

sql/rowexec/ddl.go

@@ -1196,6 +1196,18 @@ func (b *BaseBuilder) buildAlterTableCollation(ctx *sql.Context, n *plan.AlterTa
 	return rowIterWithOkResultWithZeroRowsAffected(), alterable.ModifyDefaultCollation(ctx, n.Collation)
 }
 
+func (b *BaseBuilder) buildAlterTableComment(ctx *sql.Context, n *plan.AlterTableComment, row sql.Row) (sql.RowIter, error) {
+	tbl, err := getTableFromDatabase(ctx, n.Database(), n.Table)
+	if err != nil {
+		return nil, err
+	}
+	alterable, ok := tbl.(sql.CommentAlterableTable)
+	if !ok {
+		return nil, sql.ErrAlterTableCommentNotSupported.New(tbl.Name())
+	}
+	return rowIterWithOkResultWithZeroRowsAffected(), alterable.ModifyComment(ctx, n.Comment)
+}
+
 func (b *BaseBuilder) buildCreateForeignKey(ctx *sql.Context, n *plan.CreateForeignKey, row sql.Row) (sql.RowIter, error) {
 	db, err := n.DbProvider.Database(ctx, n.FkDef.Database)
 	if err != nil {

sql/rowexec/node_builder.gen.go

@@ -50,6 +50,8 @@ func (b *BaseBuilder) buildNodeExecNoAnalyze(ctx *sql.Context, n sql.Node, row s
 		return b.buildCreateForeignKey(ctx, n, row)
 	case *plan.AlterTableCollation:
 		return b.buildAlterTableCollation(ctx, n, row)
+	case *plan.AlterTableComment:
+		return b.buildAlterTableComment(ctx, n, row)
 	case *plan.CreateRole:
 		return b.buildCreateRole(ctx, n, row)
 	case *plan.Loop:

sql/tables.go

@@ -249,6 +249,13 @@ type CollationAlterableTable interface {
 	ModifyDefaultCollation(ctx *Context, collation CollationID) error
 }
 
+// CommentAlterableTable represents a table that supports altering its comment.
+type CommentAlterableTable interface {
+	Table
+	// ModifyComment modifies the table's comment
+	ModifyComment(ctx *Context, comment string) error
+}
+
 // PrimaryKeyTable is a table with a primary key.
 type PrimaryKeyTable interface {
 	// PrimaryKeySchema returns this table's PrimaryKeySchema