Programming the Perl DBI

Error Diagnostics

The ability to trap errors within the DBI is very useful, with either manual or automatic error checking, but this information is only marginally useful on its own. To be truly useful, it is necessary to discern exactly what the error was in order to track it down and debug it.

To this end, DBI defines several error diagnostic methods that can be invoked against any valid handle, driver, database, or statement. These methods will inform the programmer of the error code and report the verbose information from the last DBI method called. These are:

$rv  = $h->err();
$str = $h->errstr();
$str = $h->state();

These various methods return the following items of information that can be used for more accurate debugging of errors:

• $h- >err() returns the error number that is associated with the current error flagged against the handle $h. The values returned will be completely dependent on the values produced by the underlying database system. Some systems may not support particularly meaningful information; for example, mSQL errors always have the error number of -1. Oracle is slightly more helpful: a connection failure may flag an ORA-12154 error message upon connection failure, which would return the value of 12154 by invoking $h->err(). Although this value is usually a number, you should not rely on that.

• $h- >errstr() is a slightly more useful method, in that it returns a string containing a description of the error, as provided by the underlying database. This string ...