in reply to What are placeholders in DBI, and why would I want to use them?
... you can't use placeholders for variable-length WHERE IN('foo', 'bar', 'baz') constructs.
PostgreSQL offers the following construct:
where array[ column_name ] <@ ( ? )
you can then $sth->execute(), passing an arrayref (i.e.: ['foo', 'bar', 'baz'] ) to the single placeholder.
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Seekers of Perl Wisdom