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Since 9.0, removing lots of large objects in a single transaction risks exceeding max_locks_per_transaction, because we merged large object removal into the generic object-drop mechanism, which takes out an exclusive lock on each object to be dropped. This creates a hazard for contrib/vacuumlo, which has historically tried to drop all unreferenced large objects in one transaction. There doesn't seem to be any correctness requirement to do it that way, though; we only need to drop enough large objects per transaction to amortize the commit costs. To prevent a regression from pre-9.0 releases wherein vacuumlo worked just fine, back-patch commits b69f2e36402aaa222ed03c1769b3de6d5be5f302 and 64c604898e812aa93c124c666e8709fff1b8dd26, which break vacuumlo's deletions into multiple transactions with a user-controllable upper limit on the number of objects dropped per transaction. Tim Lewis, Robert Haas, Tom Lane