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Re: Slow alter sequence with PG10.1

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On Mon, Jan 22, 2018 at 8:24 AM, Michael Krüger <michael@kruegers.email> wrote:
Dear community,

I'm using PG10.1 on CentOS Linux release 7.4.1708 (Core) after upgrading it from PG9.6.6. My application heavily uses sequences and requires different increments of sequence numbers, e.g. a range of 100, 1000 or 5000 numbers, so it is not possible to set a fixed increment on a sequence that can be used by my application.

With PG10.1 the performance has dropped seriously so that my application becomes unusable. After investigating different aspects, I was able to isolate the issue to be related to the sequences in Postgres 10.1. 

Below shows a simple test script showing the problem:

​[...]​
 

On my computer I tried this code on PG9.6.6 and it executed in roughly 3 seconds.
When running it on PG10.1 it takes over 7 minutes.

Further investigation showed that the problem is related to ALTER SEQUENCE... 

I can't believe that PG10.1 was changed that dramatically without providing a workaround or a way to switch to the old PG9.6 performance, at least I can't find anything in the documentation. 

Is this a bug? 

​Without testing/confirming I'd be inclined to agree that this is a regression for an unusual usage of sequences.  Work was done to make typical use cases of sequences more feature-full and it is quite possible the added effort involved hurts your specific scenario.  I'd expect a hacker to eventually pick this up, confirm the observation, and provide feedback.  This seems like sufficient amount of detail to get the ball rolling.

David J.


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