Tom,
> (I'm also wondering whether this
> doesn't overlap the use-case for GIN.)
It does not. GIN is strictly for multi-value fields. I can think of
applications where either GIN or Bitmaps would be an option, but for the
majority, they wouldn't.
One particular compelling situation for on-disk bitmaps is for terabyte
tables where a btree index would not fit into memory. Index
performance for an index which is 10x or more the size of RAM really
sucks ... I can come up with some test results if you doubt that.
Also note that "low cardinality" is relative. For a 1 billion row
table, a column with 10,000 values is "low-cardinality", having around
100,000 rows per value ... but that's still 0.01% of the table per
value, making index use still applicable.
--Josh