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We have now grown enough registerable syscache-invalidation callback functions that the original assumption that there would be few of them is causing performance problems. In particular, let's fix things so that CallSyscacheCallbacks doesn't have to search the whole array to find which callback(s) to invoke for a given cache ID. Preserve the original behavior that callbacks are called in order of registration, just in case there's someplace that depends on that (which I doubt). In support of this, export the number of syscaches from syscache.h. People could have found that out anyway from the enum, but adding a #define makes that much safer. This provides a useful additional speedup in Mathieu Fenniak's logical-decoding test case, although we're reaching the point of diminishing returns there. I think any further improvement will have to come from reducing the number of cache invalidations that are triggered in the first place. Still, we can hope that this change gives some incremental benefit for all invalidation scenarios. Back-patch to 9.4 where logical decoding was introduced. Discussion: https://postgr.es/m/CAHoiPjzea6N0zuCi=+f9v_j94nfsy6y8SU7-=bp4=7qw6_i=Rg@mail.gmail.com
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