If you'll read Neophytos' post which started this thread, you'll see that PG is used to store the indexing data used by the full-text search, and that it uses a sophisticated statistics-based indexing methodology known as GiST, which is available in PG but AFAIK no other
popular RDBMS.
The text can come from anywhere, but given that the guts require Postgres one can safely presume it will be used universally by those using the PG version of OpenACS and not at all by those using another RDBMS.
