Why DDLUtils keeps create temporary tables even when operation is allowed in Driver

[Rasha]

Hi, 

I was searching the mailing list for an explanation on why DDLUtils attempt
to resolve most operations by creating a temporary table, migrating data to
it, dropping and recreating original column. 
According to the post in
http://www.nabble.com/Plataform.getAlterTablesSql-results-ts4864470.html#a4864470
 
This should happen only if the implementation does not support the requested
operation. 

In my case, I am using a Derby database, and I am attempting to do a
ColumnRequiredChange 

Original Table:
<table name="author">
        <column name="author_id" type="INTEGER" primaryKey="true" 
required="true"
/>
        <column name="author_name" type="VARCHAR" size="50" required="true" />
        <column name="organisation" type="VARCHAR" size="50" required="true" />
</table>


Desired Model 

<table name="author">
        <column name="author_id" type="INTEGER" primaryKey="true" 
required="true"
/>
        <column name="author_name" type="VARCHAR" size="50" required="true" />
        <column name="organisation" type="VARCHAR" size="50" required="false" />
</table>

I do not understand why in this case, the table is being dropped and
recreated. 
Although derby supports doing 

alter table author
alter organisation null


Thanks for your help 
Rasha

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