Re: document the need to analyze partitioned tables

On Wed, Jan 18, 2023 at 10:15:18AM +0100, Laurenz Albe wrote:
> On Tue, 2023-01-17 at 16:16 -0500, Bruce Momjian wrote:
> > On Tue, Jan 17, 2023 at 03:00:50PM -0600, Justin Pryzby wrote:
> > > Maybe (all?) the clarification the docs need is to say:
> > > "Partitioned tables are not *themselves* processed by autovacuum."
> >
> > Yes, I think the lack of autovacuum needs to be specifically mentioned
> > since most people assume autovacuum handles _all_ statistics updating.

That's what 61fa6ca79 aimed to do. Laurenz is suggesting further
clarification.

> > Can someone summarize how bad it is we have no statistics on partitioned
> > tables?  It sounds bad to me.
>
> Andrey Lepikhov had an example earlier in this thread[1]. It doesn't take
> an exotic query.
>
> Attached is a new version of my patch that tries to improve the wording.

I tweaked this a bit to end up with:

> - Partitioned tables are not processed by autovacuum. Statistics
> - should be collected by running a manual <command>ANALYZE</command> when it is
> + The leaf partitions of a partitioned table are normal tables and are processed
> + by autovacuum; however, autovacuum does not process the partitioned table itself.
> + This is no problem as far as <command>VACUUM</command> is concerned, since
> + there's no need to vacuum the empty, partitioned table. But, as mentioned in
> + <xref linkend="vacuum-for-statistics"/>, it also means that autovacuum won't
> + run <command>ANALYZE</command> on the partitioned table.
> + Although statistics are automatically gathered on its leaf partitions, some queries also need
> + statistics on the partitioned table to run optimally. You should collect statistics by
> + running a manual <command>ANALYZE</command> when the partitioned table is
> first populated, and again whenever the distribution of data in its
> partitions changes significantly.
> </para>

"partitions are normal tables" was techically wrong, as partitions can
also be partitioned.

--
Justin