Programming the Perl DBI

Passing Attributes to DBI Methods

Handles carry with them their set of current attribute values that methods often use to control how they behave. Many methods are defined to also accept an optional reference to a hash of attribute values.

This is primarily an escape mechanism for driver developers and their users, and so does not always work in the way you might think. For example, you might expect this code:

$dbh->{RaiseError} = 1;
...
$dbh->do( $sql_statement, undef, { RaiseError => 0 } ); # WRONG

to turn off RaiseError for the do() method call. But it doesn’t! Attribute parameters are ignored by the DBI on all database handle and statement handle method calls. You don’t even get a warning that the attribute has been ignored.

If they’re ignored, then what’s the point in having them? Well, the DBI itself ignores them, but the DBD driver that processed the method call may not. Or then again, it may! Attribute hash parameters to methods are hints to the driver and typically only usefully hold driver-specific attributes.^[54]

That doesn’t apply to the DBI->connect() method call because it’s not a driver method, it’s a DBI method. Its attribute hash parameter, \%attr , is used to set the attributes of the newly created database handle. We gave some examples using RaiseError in Chapter 4, and we give more in the following section.

^[54] It’s possible that a future version of the DBI may look for certain non-driver-specific attributes, such as RaiseError.