Alter Column Statements
During forward engineer script generation, DbWrench will try to script column changes using an ALTER TABLE... ALTER COLUMN... statement. However database servers have limitations on their support for this statement. If DbWrench can not use this statement to implement a column definition change because of these limitations, it will generally resort to rebuilding the entire table.
MySQL Limitations
Generally MySQL has good support for ALTER(CHANGE) COLUMN statements, however errors may occur if the altered column is involved in a foreign key.
Consult MySQL documentation for more details.
PostgreSQL Limitations
PostgreSQL supports ALTER COLUMN only if the column's default value or the column's nullability has changed. All other changes will result in a rebuild of the table.
Consult PostgreSQL documentation for more details.
Microsoft SQL Server Limitations
SQL Server will support almost all column changes except changing the column's default value. However ALTER COLUMN will not work on columns used in primary keys, indexes, foreign keys and some othe restrictions. Also SQL Server versions before and including 6.5 may not support ALTER COLUMN at all.
Consult SQL Server documentation for more details.
See also: