[m-dev.] ODBC interface & nested transactions

[Simon Taylor]

> 
> Hi,
> 
> Does the Mercury ODBC interface handle nested transactions?
> What happens if you try one?
> 

The interface in odbc.m disallows nested transactions because 
odbc__transaction requires an io__state and you can't get one 
from within odbc__transaction.

ODBC itself only supports flat transactions. In fact ODBC does not 
have any way to explicitly begin a transaction - the next transaction is 
implicitly started after the last one commits. Implementing nested 
transactions would require using the SQL transaction operations. 

Simon.