Re: UPDATE of partition key

From: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
To: Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>, David Fetter <david(at)fetter(dot)org>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: UPDATE of partition key
Date: 2017-02-16 07:27:39
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On 2017/02/16 15:50, Amit Khandekar wrote:
> On 15 February 2017 at 20:26, David Fetter <david(at)fetter(dot)org> wrote:
>> When an UPDATE can't happen, there are often ways to hint at
>> what went wrong and how to correct it. Violating a uniqueness
>> constraint would be one example.
>> When an UPDATE can't happen and the depth of the subtree is a
>> plausible candidate for what prevents it, there might be a way to say
>> so.
>> Let's imagine a table called log with partitions on "stamp" log_YYYY
>> and subpartitions, also on "stamp", log_YYYYMM. If you do something
>> like
>> UPDATE log_2017 SET "stamp"='2016-11-08 23:03:00' WHERE ...
>> it's possible to know that it might have worked had the UPDATE taken
>> place on log rather than on log_2017.
>> Does that make sense, and if so, is it super invasive to HINT that?
> Yeah, I think it should be possible to find the root partition with

I assume you mean root *partitioned* table.

> the help of pg_partitioned_table,

The pg_partitioned_table catalog does not store parent-child
relationships, just information about the partition key of a table. To
get the root partitioned table, you might want to create a recursive
version of get_partition_parent(), maybe called
get_partition_root_parent(). By the way, get_partition_parent() scans
pg_inherits to find the inheritance parent.

> and then run ExecFindPartition()
> again using the root. Will check. I am not sure right now how involved
> that would turn out to be, but I think that logic would not change the
> existing code, so in that sense it is not invasive.

I couldn't understand why run ExecFindPartition() again on the root
partitioned table, can you clarify? ISTM, we just want to tell the user
in the HINT that trying the same update query with root partitioned table
might work. I'm not sure if it would work instead to find some
intermediate partitioned table (that is, between the root and the one that
update query was tried with) to include in the HINT.


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