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Locking and updating the same tuple repeatedly led to some strange multixacts being created which had several subtransactions of the same parent transaction holding locks of the same strength. However, once a subxact of the current transaction holds a lock of a given strength, it's not necessary to acquire the same lock again. This made some coding patterns much slower than required. The fix is twofold. First we change HeapTupleSatisfiesUpdate to return HeapTupleBeingUpdated for the case where the current transaction is already a single-xid locker for the given tuple; it used to return HeapTupleMayBeUpdated for that case. The new logic is simpler, and the change to pgrowlocks is a testament to that: previously we needed to check for the single-xid locker separately in a very ugly way. That test is simpler now. As fallout from the HTSU change, some of its callers need to be amended so that tuple-locked-by-own-transaction is taken into account in the BeingUpdated case rather than the MayBeUpdated case. For many of them there is no difference; but heap_delete() and heap_update now check explicitely and do not grab tuple lock in that case. The HTSU change also means that routine MultiXactHasRunningRemoteMembers introduced in commit 11ac4c73cb895 is no longer necessary and can be removed; the case that used to require it is now handled naturally as result of the changes to heap_delete and heap_update. The second part of the fix to the performance issue is to adjust heap_lock_tuple to avoid the slowness: 1. Previously we checked for the case that our own transaction already held a strong enough lock and returned MayBeUpdated, but only in the multixact case. Now we do it for the plain Xid case as well, which saves having to LockTuple. 2. If the current transaction is the only locker of the tuple (but with a lock not as strong as what we need; otherwise it would have been caught in the check mentioned above), we can skip sleeping on the multixact, and instead go straight to create an updated multixact with the additional lock strength. 3. Most importantly, make sure that both the single-xid-locker case and the multixact-locker case optimization are applied always. We do this by checking both in a single place, rather than them appearing in two separate portions of the routine -- something that is made possible by the HeapTupleSatisfiesUpdate API change. Previously we would only check for the single-xid case when HTSU returned MayBeUpdated, and only checked for the multixact case when HTSU returned BeingUpdated. This was at odds with what HTSU actually returned in one case: if our own transaction was locker in a multixact, it returned MayBeUpdated, so the optimization never applied. This is what led to the large multixacts in the first place. Per bug report #8470 by Oskari Saarenmaa.
The PostgreSQL contrib tree
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This subtree contains porting tools, analysis utilities, and plug-in
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