primary_key_info()
$dbh->primary_key_info($catalog, $database, $table)
This function is meant to return a statement handle for fetching
information about primary key columns for a table. The values are part
of a hash for the statement handle: TABLE_CAT,
TABLE_SCHEM, TABLE_NAME, and
KEY_SEQ. If there is no primary key for the table
given, it returns no rows. This method does not yet seem to be
supported in MySQL.