[Patch] Add a reset_computed_values function in pg_stat_statements

Em dom, 1 de set de 2019 às 06:04, Pierre Ducroquet
<p.psql@pinaraf.info> escreveu:

Hello

pg_stat_statements is a great tool to track performance issue in live
databases, especially when adding interfaces like PoWA on top of it.
But so far, tools like PoWA can not track the min_time, max_time, mean_time
and sum_var_time of queries : these statistics are cumulated over time,
fetching points in time would be of little to no use, especially when looking
at the impact of a DDL change.
This patch thus introduces a simple pg_stat_statements_reset_computed_values
function that reset the computed statistics, leaving all other information
alive, thus allowing the aforementioned scenario.

Pierre, I don't understand why you want to reset part of the statement
statistics. If you want the minimal query time every week, reset all
statistics of that statement (v12 or later). Postgres provides
histogram metrics (min, max, mean, stddev). AFAICS you want a metric
type called timer (combination of histogram and meter). For example,
calls, sum, min, max, mean, standard deviation will be calculated each
hour. If I were to add a new metric to pg_stat_statements, it would be
last_time. Compare histogram metrics with the last statement time is
useful to check if a certain optimization was effective.

Talking about your patch, math is wrong. If you don't reset
total_time, all computed values will be incorrect. You are changing
the actual meaning of those metrics and I think it is unacceptable
because it will break applications. Instead, you should provide (i)
new counters or (ii) add GUC to control this behavior. It is a
trade-off between incompatibility and too many metrics. Though I
wouldn't like to break compatibility (as I said you can always reset
all statement statistics). Why don't you provide a
pg_stat_statements_reset_computed_values(userid Oid, dbid Oid, queryid
bigint)? You forgot to provide documentation for the new function. You
should revoke pg_stat_statements_reset_computed_values from PUBLIC.

Regards,

--
Euler Taveira Timbira -
http://www.timbira.com.br/
PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento

On Monday, September 2, 2019 3:25:00 AM CEST Euler Taveira wrote:

Em dom, 1 de set de 2019 às 06:04, Pierre Ducroquet

<p.psql@pinaraf.info> escreveu:

Hello

pg_stat_statements is a great tool to track performance issue in live
databases, especially when adding interfaces like PoWA on top of it.
But so far, tools like PoWA can not track the min_time, max_time,
mean_time
and sum_var_time of queries : these statistics are cumulated over time,
fetching points in time would be of little to no use, especially when
looking at the impact of a DDL change.
This patch thus introduces a simple
pg_stat_statements_reset_computed_values function that reset the computed
statistics, leaving all other information alive, thus allowing the
aforementioned scenario.

Hello

Thank you for reviewing this patch.

Pierre, I don't understand why you want to reset part of the statement
statistics. If you want the minimal query time every week, reset all
statistics of that statement (v12 or later). Postgres provides
histogram metrics (min, max, mean, stddev). AFAICS you want a metric
type called timer (combination of histogram and meter). For example,
calls, sum, min, max, mean, standard deviation will be calculated each
hour. If I were to add a new metric to pg_stat_statements, it would be
last_time. Compare histogram metrics with the last statement time is
useful to check if a certain optimization was effective.

min/max/mean timing in pg_stat_statements are *computed* fields, not simple
cumulative ones, hence the distinction I do here. For a tool like PoWA, that
(to simplify) copies all the statistics every few minutes to do its own
computation, it makes it impossible to get any sense out of them. The facility
to reset all statistics is an option, but it feels like an heavy solution when
only some statistics are different from the others.

Talking about your patch, math is wrong. If you don't reset
total_time, all computed values will be incorrect. You are changing
the actual meaning of those metrics and I think it is unacceptable
because it will break applications.

I think you misunderstood the way this patch is written.
total_time, in this function, is not a global statistics: pgss_store receive
the total time of the current query. When the computed statistics are reset,
we simply drop them and start them fresh from the first query. There is
absolutely no compatibility impact here.

Instead, you should provide (i) new counters or (ii) add GUC to control this
behavior. It is a trade-off between incompatibility and too many metrics.
Though I wouldn't like to break compatibility (as I said you can always
reset all statement statistics).

As explained: it does not break compatibility, I don't see a need for a new
GUC.

Why don't you provide a pg_stat_statements_reset_computed_values(userid Oid,
dbid Oid, queryid bigint)?

Because I did not need one, and with pg_stat_statements_reset(userid Oid, dbid
Oid, queryid bigint) being newly added, I did not think about it.
I can add one in a new version of the patch of course.

You forgot to provide documentation for the new function.

Indeed, I am not very used to SGML and first wanted a discussion about the
feature.

You should revoke pg_stat_statements_reset_computed_values from PUBLIC.

Indeed, I missed that one.

Attached to this mail is a new version fixing the revoke issue, and adding
documentation.

Regards

Pierre

This patch seems to be failing the contrib build. Please fix.

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Thu, Sep 26, 2019 at 8:05 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

This patch seems to be failing the contrib build. Please fix.

Hi Pierre,

In contrib/pg_stat_statements/pg_stat_statements.c, you need to
declare or define entry_reset_computed() before you use it. I suppose
your compiler must be warning about that. I personally put
"COPT=-Wall -Werror" into src/Makefile.custom to make sure that I
don't miss any warnings. That's also what http://cfbot.cputube.org/
does (a thing that does a full check-world on all registered patches
to look for such problems).

On Tue, Nov 05, 2019 at 11:03:58AM +1300, Thomas Munro wrote:

In contrib/pg_stat_statements/pg_stat_statements.c, you need to
declare or define entry_reset_computed() before you use it. I suppose
your compiler must be warning about that. I personally put
"COPT=-Wall -Werror" into src/Makefile.custom to make sure that I
don't miss any warnings. That's also what http://cfbot.cputube.org/
does (a thing that does a full check-world on all registered patches
to look for such problems).

Still the same problem which has not been addressed after a couple of
weeks, so I am marking the entry as returned with feedback.
--
Michael