Comment 9 for bug 1505286

The cause can be found in the PG docs: http://www.postgresql.org/docs/9.1/static/sql-createfunction.html

argname
The name of an argument. Some languages (including PL/pgSQL, but currently not SQL) let you use the name in the function body. For other languages the name of an input argument is just extra documentation, so far as the function itself is concerned; but you can use input argument names when calling a function to improve readability (see Section 4.3). In any case, the name of an output argument is significant, because it defines the column name in the result row type. (If you omit the name for an output argument, the system will choose a default column name.)

That changed in 9.2, so we don't have to push for 9.3 just yet, and I'm personally in favor of moving the PG ball forward to 9.2 (at the very least) ASAP.

That said, this can be made to work in 9.1 very easily by replacing ignore_facet_classes with $1 and hits with $2 in the bodies of search.facets_for_record_set() and search.facets_for_metarecord_set().

Dealer's (fixer's) choice, IMO.