Commit be86e3d

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Rethink checkpointer's fsync-request table representation.

Instead of having one hash table entry per relation/fork/segment, just have one per relation, and use bitmapsets to represent which specific segments need to be fsync'd. This eliminates the need to scan the whole hash table to implement FORGET_RELATION_FSYNC, which fixes the O(N^2) behavior recently demonstrated by Jeff Janes for cases involving lots of TRUNCATE or DROP TABLE operations during a single checkpoint cycle. Per an idea from Robert Haas. (FORGET_DATABASE_FSYNC still sucks, but since dropping a database is a pretty expensive operation anyway, we'll live with that.) In passing, improve the delayed-unlink code: remove the pass over the list in mdpreckpt, since it wasn't doing anything for us except supporting a useless Assert in mdpostckpt, and fix mdpostckpt so that it will absorb fsync requests every so often when clearing a large backlog of deletion requests.

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src/backend/storage/smgr
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