Re: shared-memory based stats collector - v70

Hi,

On 8/18/22 9:51 PM, Andres Freund wrote:
> Hi,
>
> On 2022-08-18 15:26:31 -0400, Greg Stark wrote:
>> And indexes of course. It's a bit frustrating since without the
>> catalog you won't know what table the index actually is for... But
>> they're pretty important stats.
> FWIW, I think we should split relation stats into table and index
> stats. Historically it'd have added a lot of complexity to separate the two,
> but I don't think that's the case anymore. And we waste space for index stats
> by having lots of table specific fields.

It seems to me that we should work on that first then, what do you
think? (If so I can try to have a look at it).

And once done then resume the work to provide the APIs to get all
tables/indexes from all the databases.

That way we'll be able to provide one API for the tables and one for the
indexes (instead of one API for both like my current POC is doing).

>> On that note though... What do you think about having the capability
>> to add other stats kinds to the stats infrastructure?

I think that's a good idea and that would be great to have.

> Getting closer to that was one of my goals working on the shared memory stats
> stuff.
>
>
>> It would make a lot of sense for pg_stat_statements to add its entries here
>> instead of having to reimplement a lot of the same magic.
> Yes, we should move pg_stat_statements over.
>
> It's pretty easy to get massive contention on stats entries with
> pg_stat_statements, because it doesn't have support for "batching" updates to
> shared stats. And reimplementing the same logic in pg_stat_statements.c
> doesn't make sense.
>
> And the set of normalized queries could probably stored in DSA as well - the
> file based thing we have right now is problematic.
>
>
>> To do that I guess more of the code needs to be moved to be table
>> driven from the kind structs either with callbacks or with other meta
>> data.
> Pretty much all of it already is. The only substantial missing bit is
> reading/writing of stats files, but that should be pretty easy. And of course
> making the callback array extensible.
>
>
>> So the kind record could contain tupledesc and the code to construct the
>> returned tuple so that these functions could return any custom entry as well
>> as the standard entries.
> I don't see how this would work well - we don't have functions returning
> variable kinds of tuples. And what would convert a struct to a tuple?
>
> Nor do I think it's needed - if you have an extension providing a new stats
> kind it can also provide accessors.

I think the same (the extension should be able to do that).

I really like the idea of being able to provide new stats kind.

Regards,

--
Bertrand Drouvot
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