My point was to demonstrate the property of unique indexes which prevent duplication: a safety measure. I never said to use the error for program control. In fact I didn't exhibit any DBI code at all.

In practice, with the index in place:

my $known_dep=$dbi->prepare_cached("select 1 from depends where usedby
+=? and uses=?");
sub known_dep ($$) {#prototype because we have a fixed number of place
+holders
    $known_dep->execute(@_);
    $known_dep->rows; # 1 or 0 for unique index.
}
[download]

For frequent operations, we can improve performance with the sth->bind_params() method.

We search on an index, using a pre-prepared statement handle with placeholders. That will be fast.

I didn't mention before that NULL in multiindexes is only available in MyISAM tables.

After Compline,
Zaxo