What exceptions does DBI throw? - perl

Question

If I set RaiseError = 1, what exception is raised when connecting or executing?
If I were to surround my execute() method in try catch, what exception should I be catching?

Answer 1

The Perl Error.pm module, which provides try and catch, is deprecated. Exceptions, inasmuch as they exist in Perl, are untyped, and here's how you catch one:
eval {
do_something_which_may_throw_exception();
};
if ($#) {
print "Exception: $#\n";
};
In short, the eval { ... } block acts as a "try", and the if ($#) { ... } acts as a "catch", wherein the exception text is contained in the special variable $#.

Answer 2

The DBI documentation lists and explains a lot of options, many of which relate to error handling.
Perl has two major error handling idioms:
Returning a false value. The reason for the error is in some global variable.
die with some error message (fatal).
By default, DBI uses the first idiom. The error reason is in $DBI::errstr. For this to work, you have to check the return values of each and every call to the DBI API.
When you feel lazy, you can use exceptions. Setting RaiseError in the handle constructor will make DBI methods throw an exception. From the docs:
RaiseError
Type: boolean, inherited
The RaiseError attribute can be used to force errors to raise exceptions rather than simply return error codes in the normal way. It is "off" by default. When set "on", any method which results in an error will cause the DBI to effectively do a die("$class $method failed: $DBI::errstr"), where $class is the driver class and $method is the name of the method that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
[…]
Typically RaiseError is used in conjunction with eval { ... } to catch the exception that's been thrown and followed by an if ($#) { ... } block to handle the caught exception. For example:
eval {
...
$sth->execute();
...
};
if ($#) {
# $sth->err and $DBI::err will be true if error was from DBI
warn $#; # print the error
... # do whatever you need to deal with the error
}
In that eval block the $DBI::lasth variable can be useful for diagnosis and reporting if you can't be sure which handle triggered the error.
As you can see, exceptions in Perl aren't handled with try/catch, but with eval { ... }. After an eval that dies, the $# error variable will be set to that error, and you are free to handle it. Note that DBI does not use exception objects.

Answer 3

If you'd like to get back formal exception objects from DBI, you can use the HandleError attribute and Exception::Class::DBI. I use it myself. From the Synopsis:
use DBI;
use Exception::Class::DBI;
my $dbh = DBI->connect($dsn, $user, $pass, {
PrintError => 0,
RaiseError => 0,
HandleError => Exception::Class::DBI->handler,
});
eval { $dbh->do($sql) };
if (my $ex = $#) {
print STDERR "DBI Exception:\n";
print STDERR " Exception Type: ", ref $ex, "\n";
print STDERR " Error: ", $ex->error, "\n";
print STDERR " Err: ", $ex->err, "\n";
print STDERR " Errstr: ", $ex->errstr, "\n";
print STDERR " State: ", $ex->state, "\n";
print STDERR " Return Value: ", ($ex->retval || 'undef'), "\n";
}