Hi
I recently came across a performance problem with a big transaction block, which doesn't make sense to me and hopefully someone more knowledgeable can explain the reasons and point out a direction for a solution.
-- 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.
Why is updating the same row large number of times progressively slower inside a transaction? And is there a way to avoid this performance degradation?
I set up a POC repository to demonstrate the problem:
https://github.com/DeadAlready/pg-test
-- Backstory
Needed to run a large block of operations (a mix of inserts and updates) on a table. It took a considerable amount of time inside a transaction and was about 10x faster without the transaction. Since I need all the operations to run as a single block that can be rolled back this was unsatisfactory. Thus began my quest to locate the problem. Since the actual data structure is complex and involves a bunch of triggers, foreign keys etc it took some time to narrow down, but in the end I found that the structure itself is irrelevant. The issue occurs even if you have a single two column table with a handful of rows. The only requirement seems to be that the NR of UPDATEs per single row is large. While the update performance inside a transaction starts out faster than outside, the performance starts to degrade from the get go. It really isn't noticeable until about 5k UPDATEs on a single row. At around 100k UPDATEs it is about 2.5x slower than the same operation outside the transaction block and about 4x slower than at the beginning of the transaction.
Thanks,
Karl