pg_stat_user_tables.n_mod_since_analyze persistence?

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pg_stat_user_tables.n_mod_since_analyze persistence?

Philip Semanchuk-2
Hi all,
I saw some unexpected behavior that I'm trying to understand. I suspect it might be a quirk specific to AWS Aurora and I'd like to confirm that.

When I restart my local Postgres instance (on my Mac), the values in pg_stat_user_tables.n_mod_since_analyze are preserved. In other words, if table foo had n_mod_since_analyze=33 before the reboot, it still has n_mod_since_analyze=33 after the restart.

When I restart an AWS Aurora instance, the values in pg_stat_user_tables.n_mod_since_analyze all seem to be reset to 0.

Can anyone confirm (or refute) that the behavior I see on my Mac (preservation of these values through a restart) is common & expected behavior?

Thanks
Philip

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Re: pg_stat_user_tables.n_mod_since_analyze persistence?

Tom Lane-2
Philip Semanchuk <[hidden email]> writes:
> I saw some unexpected behavior that I'm trying to understand. I suspect it might be a quirk specific to AWS Aurora and I'd like to confirm that.

> When I restart my local Postgres instance (on my Mac), the values in pg_stat_user_tables.n_mod_since_analyze are preserved. In other words, if table foo had n_mod_since_analyze=33 before the reboot, it still has n_mod_since_analyze=33 after the restart.

> When I restart an AWS Aurora instance, the values in pg_stat_user_tables.n_mod_since_analyze all seem to be reset to 0.

> Can anyone confirm (or refute) that the behavior I see on my Mac (preservation of these values through a restart) is common & expected behavior?

Yeah, in PG those stats would be preserved, at least as long as it's
a clean shutdown.

                        regards, tom lane


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Re: pg_stat_user_tables.n_mod_since_analyze persistence?

Philip Semanchuk-2


> On Feb 15, 2021, at 3:55 PM, Tom Lane <[hidden email]> wrote:
>
> Philip Semanchuk <[hidden email]> writes:
>> I saw some unexpected behavior that I'm trying to understand. I suspect it might be a quirk specific to AWS Aurora and I'd like to confirm that.
>
>> When I restart my local Postgres instance (on my Mac), the values in pg_stat_user_tables.n_mod_since_analyze are preserved. In other words, if table foo had n_mod_since_analyze=33 before the reboot, it still has n_mod_since_analyze=33 after the restart.
>
>> When I restart an AWS Aurora instance, the values in pg_stat_user_tables.n_mod_since_analyze all seem to be reset to 0.
>
>> Can anyone confirm (or refute) that the behavior I see on my Mac (preservation of these values through a restart) is common & expected behavior?
>
> Yeah, in PG those stats would be preserved, at least as long as it's
> a clean shutdown.


Thanks, Tom. A colleague pointed me to a blog post by Michael Vitale that confirms this bug on AWS and contains more detail:
https://elephas.io/685-2/


Hope this helps someone else
Philip