Re: [PATCH] Change wait_time column of pg_stat_lock to double precision

Hi Bertrand-san, Horiguchi-san,

Thanks for confirming the original intent.

Before we conclude, I would like to share a couple of points that
make me wonder whether changing the type might still be worth
considering.

1. pg_stat_lock is new in v19, so the type can still be changed
before release without any backwards-compatibility cost. This
makes now a relatively low-risk moment to revisit the choice.

2. Looking across the other stats views, the "sub-millisecond
precision is not particularly useful" criterion does not seem to
be the basis for picking a type in general.
pg_stat_database.session_time, for example, can accumulate to
large values for which sub-millisecond precision is also noise,
yet it uses double precision. From a user's point of view,
the common pattern across the stats views seems to be
"measured time columns are double precision", regardless of
expected magnitude or required precision.

3. As a minor point, deadlock_timeout is a GUC and can be lowered,
so under diagnostic configurations sub-millisecond precision
in wait_time is not entirely hypothetical.

So my point is not that the original bigint choice was wrong, but
that pg_stat_lock currently differs from the other stats views in
this respect, and v19 may be a good moment to make it uniform.

If the consensus after considering these points is still that the
existing bigint type is preferable, I am happy to withdraw and send
a docs-only patch making the rationale explicit instead.

Regards,
Tatsuya Kawata