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MS SQL has a notion of a "filtered index" that matches the notion of a PostgreSQL partial index: the index only applies to the rows matching the index WHERE clause, or filter. The WHERE clause in both case are limited to simple expressions over a base table's row at a time, so we implement a limited WHERE clause parser for MS SQL filters and a transformation routine to rewrite the clause in PostgreSQL slang. In passing, we transform the filter constants using the same transformation functions as in the CAST rules, so that e.g. a MS SQL bit(1) value that got transformed into a PostgreSQL boolean is properly translated, as in the following example: MS SQL: "([deleted]=(0))" (that's from the catalogs) PostgreSQL: deleted = 'f' Of course the parser is still very badly tested, let's see what happens in the wild now. (Should) Fix #365.