Re: Add SPLIT PARTITION/MERGE PARTITIONS commands

Hi, Pavel!

On Fri, May 17, 2024 at 2:02 PM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
> On Fri, 17 May 2024 at 14:05, Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>>
>> On Tue, May 14, 2024 at 5:49 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
>> > On Thu, May 09, 2024 at 12:51:32AM +0300, Alexander Korotkov wrote:
>> > > > > However, parent's table extended statistics already covers all its
>> > > > > children.
>> > > >
>> > > > => That's the wrong explanation. It's not that "stats on the parent
>> > > > table cover its children". It's that there are two types of stats:
>> > > > stats for the "table hierarchy" and stats for the individual table.
>> > > > That's true for single-column stats as well as for extended stats.
>> > > > In both cases, that's indicated by the inh flag in the code and in the
>> > > > catalog.
>> > > >
>> > > > The right explanation is that extended stats on partitioned tables are
>> > > > not similar to indexes. Indexes on parent table are nothing other than
>> > > > a mechanism to create indexes on the child tables. That's not true for
>> > > > stats.
>> > > >
>> > > > See also my prior messages
>> > > > ZiJW1g2nbQs9ekwK(at)pryzbyj2023
>> > > > Zi5Msg74C61DjJKW(at)pryzbyj2023
>> > >
>> > > Yes, I understand that parents pg_statistic entry with stainherit ==
>> > > true includes statistics for the children. I tried to express this by
>> > > word "covers". But you're right, this is the wrong explanation.
>> > >
>> > > Can I, please, ask you to revise the patch?
>> >
>> > I tried to make this clear but it'd be nice if someone (Tomas/Alvaro?)
>> > would check that this says what's wanted.
>>
>> Thank you!
>>
>> I've assembled the patches with the pending fixes.
>> 0001 – The patch by Dmitry Koval for fixing detection of name
>> collision in SPLIT partition operation. Also, I found that name
>> collision detection doesn't work well for MERGE partitions. I've
>> added fix for that to this patch as well.
>> 0002 -– Patch for skipping copy of extended statistics. I would
>> appreciate more feedback about wording, but I'd like to get a correct
>> behavior into the source tree sooner. If the docs and/or comments
>> need further improvements, we can fix that later.
>>
>> I'm going to push both if no objections.
>
> Thank you for working on this patch set!
>
> Some minor things:
> 0001:
> partition_split.sql
> 157 +-- Check that detection, that the new partition has the same name as one of
> 158 +-- the merged partitions, works correctly for temporary partitions
> Test for split with comment for merge. Maybe better something like:
> "Split partition of a temporary table when one of the partitions after split has the same name as the partition being split"

Thank you, fixed as proposed.

> 0002:
> analgous -> analogous (maybe better using "like" instead of "analogous to")
> heirarchy -> hierarchy

Changed "are not analgous to" to "don't behave like".

> alter_table.sgml:
> Maybe in documentation it's better not to provide reasoning, just state how it works:
> for consistency with <command>CREATE TABLE PARTITION OF</command> -> similar to <command>CREATE TABLE PARTITION OF</command>

I'd like to keep this. This is the question, which should naturally
arise when you read: "Why this is not just INCLUDING ALL?"

------
Regards,
Alexander Korotkov
Supabase