That is pretty much what I suspected, but I held out hope that there is some functionality I could use to clear the bloat as the transaction progresses and bring the UPDATE time back down again.
"dont do that" is sensible, but much more easily said than done, as the in the actual use case I have, the single row updates are caused by various triggers running on separate operations -
which means I will have to muck about with conditional trigger disabling and/or change a large part of the database logic around these tables. But I guess that is a whole other issue.
Karl Düüna <karl.dyyna@gmail.com> writes: > -- TL; DR; > UPDATE on a row takes relatively constant amount of time outside a > transaction block, but running UPDATE on a single row over and over inside > a transaction gets slower and slower as the number of UPDATE operations > increases.
Yeah, that's unsurprising. Each new update creates a new version of its row. When you do them in separate transactions, then as soon as transaction N+1 commits the system can recognize that the row version created by transaction N is dead (no longer visible to anybody) and recycle it, allowing the number of row versions present on-disk to stay more or less constant. However, there's not equivalently good housekeeping for row versions created by a transaction that's still running. So when you do N updates in one transaction, there are going to be N doomed-but-not-yet-recyclable row versions on disk.
Aside from the disk-space bloat, this is bad because the later updates have to scan through all the row versions created by earlier updates, looking for the version they're supposed to update. So you have an O(N^2) cost associated with that, which no doubt is what you're observing.
There isn't any really good fix for this, other than "don't do that". David's nearby suggestion of using a temp table won't help, because this behavior is the same whether the table is temp or regular.
In principle perhaps we could improve the granularity of dead-row detection, so that if a row version is both created and deleted by the current transaction, and we have no live snapshots that could see it, we could go ahead and mark the row dead. But it's not clear that that'd be worth the extra cost to do. Certainly no existing PG release tries to do it.
