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## Problem We have some known N^2 behaviors when it comes to large relation counts, due to the monolithic encoding and full rewrites of of RelDirectory each time a relation is added. Ordinarily our backpressure mechanisms give "slow but steady" performance when creating/dropping/truncating relations. However, in the case of a transaction abort, it is possible for a single WAL record to drop an unbounded number of relations. The results in an unavailable compute, as when it sends one of these records, it can stall the pageserver's ingest for many minutes, even though the compute only sent a small amount of WAL. Closes https://github.com/neondatabase/neon/issues/9505 ## Summary of changes - Rewrite relation-dropping code to do one read/modify/write cycle of RelDirectory, instead of doing it separately for each relation in a loop. - Add a test for the bug scenario encountered: `test_tx_abort_with_many_relations` The test has ~40s runtime on my workstation. About 1 second of that is the part where we wait for ingest to catch up after a rollback, the rest is the slowness of creating and truncating a large number of relations. --------- Co-authored-by: Heikki Linnakangas <heikki@neon.tech>