Re: [PATCH] Tracking statements entry timestamp in pg_stat_statements

Hi Andrei,

On Tue, Jan 25, 2022 at 02:58:17PM +0300, Andrei Zubkov wrote:
>
> Of course we can replace old min/max metrics with the new "aux" min/max
> metrics. It seems reasonable to me because they will have the same
> behavior until we touch reset_aux. I think we can assume that min/max
> data is saved somewhere if reset_aux was performed, but how about the
> backward compatibility?
> There may be some monitoring solutions that doesn't expect min/max
> stats reset independently of other statement statistics.
> It seems highly unlikely to me, because the min/max stats for "the last
> 3 years" is obvious unusable but maybe someone uses them as a sign of
> something?

To be honest I don't see how any monitoring solution could make any use of
those fields as-is. For that reason in PoWA we unfortunately have to entirely
ignore them. There was a previous attempt to provide a way to reset those
counters only (see [1]), but it was returned with feedback due to lack of TLC
from the author.

> Are we need to worry about that?

I don't think it's a problem, as once you have a solution on top of
pg_stat_statements, you get information order of magnitude better from that
solution instead of pg_stat_statements. And if that's a problem, well either
don't reset those counters, or don't use the external solution if it does it
automatically and you're not ok with it.

> Also, there is one more dramatic consequence of such decision...
> What min/max values should be returned after the auxiliary reset but
> before the next statement execution?
> The NULL values seems reasonable but there was not any NULLs before and
> backward compatibility seems broken. Another approach is to return the
> old values of min/max stats and the old aux_stats_since value until the
> next statement execution but it seems strange when the reset was
> performed and it doesn't affected any stats instantly.

If you're worried about some external table having a NOT NULL constraint for
those fields, how about returning NaN instead? That's a valid value for a
double precision data type.

[1] https://www.postgresql.org/message-id/1762890.8ARNpCrDLI@peanuts2