There's a great deal about this in the list archives (probably more in pgsql-hackers than in -performance). Most of the current interest has to do with building in-memory bitmaps on the fly, as a way of decoupling index and heap scan processing. Which is not quite what you're talking about but should be pretty effective for low-cardinality cases. In particular it'd allow AND and OR combination of multiple indexes, which we do poorly or not at all at the moment.
Is this called a star join ?
It would also allow to access the data pages in a more sequential order if the rows are not required to be retrieved in index order, which would potentially be a large speedup for index scans concerning more than the usual very small percentage of rows in a table : if several rows to be retrieved are on the same page, it would visit this page only once.
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