While hacking on the GIN patches, I've come up with a few different ideas for improving performance. It's too late for 9.4, but I'll list them here if someone wants to work on them later:
* Represent ItemPointers as uint64's, to speed up comparisons. ginCompareItemPointers is inlined into only a few instructions, but it's still more expensive than a single-instruction 64-bit comparison. ginCompareItemPointers is called very heavily in a GIN scan, so even a small improvement there would make for a noticeable speedup. It might be an improvement in code clarity, too.
* Keep the entry streams of a GinScanKey in a binary heap, to quickly find the minimum curItem among them.
I did this in various versions of the fast scan patch, but then I realized that the straightforward way of doing it is wrong, because a single GinScanEntry can be part of multiple GinScanKeys. If an entry's curItem is updated as part of advancing one key, and the entry is in a heap of another key, updating the curItem can violate the heap property of the other entry's heap.
* Build a truth table (or cache) of consistent-function's results, and use that instead of calling consistent for every item.
Caching seems more appropriate for me. Intuition tells me that when number of entries is high then far not all consistent combinations will be used. However, intuition might be false :-)
* Deduce AND or OR logic from the consistent function. Or have the opclass provide a tree of AND/OR/NOT nodes directly, instead of a consistent function. For example, if the query is "foo & bar", we could avoid calling consistent function altogether, and only return items that match both.
I also had this idea. But this solution doesn't cover similarity queries. If you have 20 entries and consistent function returns true when at least 10 of 20 are present then representation in AND/OR/NOT nodes would be too enormous, so useless.
* Delay decoding segments during a scan. Currently, we decode all segments of a posting tree page into a single array at once. But with "fast scan", we might be able to skip over all entries in some of the segments. So it would be better to copy the segments into backend-private memory in compressed format, and decode them one segment at a time (or maybe even one item at a time), when needed. That would avoid the unnecessary decoding of segments that can be skipped over, and would also reduce memory usage of a scan.
I would like to add another one as continue of fast-scan:
* Skip scanning of some entries at all forcing recheck instead. Correct decision should be done based on costs. However, I'm not sure about design. Because it's like a planning feature. How correct to do this inside of GIN?
