Make ALTER TABLE foo ALTER COLUMN a SET DATA TYPE text options unrepresentable (#508)

Ensure that SQL statements of the form:

```sql
ALTER TABLE foo ALTER COLUMN a SET DATA TYPE text COLLATE "en_US"
ALTER TABLE foo ALTER COLUMN a SET DATA TYPE text USING 'foo'
```

are converted to raw SQL operations instead of an `OpAlterColumn`
operation. The change of collation is not currently representable by an
`OpAlterColumn` operation and neither is the `USING` clause.

Part of #504

Author: andrew-farries

pkg/sql2pgroll/alter_table.go

@@ -78,6 +78,10 @@ func convertAlterTableAlterColumnType(stmt *pgq.AlterTableStmt, cmd *pgq.AlterTa
 		return nil, fmt.Errorf("expected column definition, got %T", cmd.GetDef().Node)
 	}
 
+	if !canConvertColumnForSetDataType(node.ColumnDef) {
+		return nil, nil
+	}
+
 	return &migrations.OpAlterColumn{
 		Table:  stmt.GetRelation().GetRelname(),
 		Column: cmd.GetName(),
@@ -170,6 +174,19 @@ func canConvertUniqueConstraint(constraint *pgq.Constraint) bool {
 	return true
 }
 
+// canConvertColumnForSetDataType checks if `column` can be faithfully
+// converted as part of an OpAlterColumn operation to set a new type for the
+// column.
+func canConvertColumnForSetDataType(column *pgq.ColumnDef) bool {
+	if column.GetCollClause() != nil {
+		return false
+	}
+	if column.GetRawDefault() != nil {
+		return false
+	}
+	return true
+}
+
 func ptr[T any](x T) *T {
 	return &x
 }

pkg/sql2pgroll/alter_table_test.go

@@ -67,6 +67,11 @@ func TestUnconvertableAlterTableAddConstraintStatements(t *testing.T) {
 		"ALTER TABLE foo ADD CONSTRAINT bar UNIQUE (a) INCLUDE (b)",
 		"ALTER TABLE foo ADD CONSTRAINT bar UNIQUE (a) WITH (fillfactor=70)",
 		"ALTER TABLE foo ADD CONSTRAINT bar UNIQUE (a) USING INDEX TABLESPACE baz",
+
+		// COLLATE and USING clauses are not representable by `OpAlterColumn`
+		// operations when changing data type.
+		`ALTER TABLE foo ALTER COLUMN a SET DATA TYPE text COLLATE "en_US"`,
+		"ALTER TABLE foo ALTER COLUMN a SET DATA TYPE text USING 'foo'",
 	}
 
 	for _, sql := range tests {