Subject: Re: unique indexes on partitioned tables
From: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
References: <20180122225559.7pbzomvgp5iwmath@alvherre.pgsql>
 <92ad77b0-2a6f-a96c-4a3c-67ef6ae65742@lab.ntt.co.jp>
Message-ID: <7219558d-c02e-aed4-e169-1dd7c77dfc44@lab.ntt.co.jp>
Date: Mon, 29 Jan 2018 17:29:53 +0900
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To: Alvaro Herrera <alvherre@alvh.no-ip.org>,
        Pg Hackers <pgsql-hackers@postgresql.org>
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On 2018/01/29 16:28, Amit Langote wrote:
> create table p (a int, b int) partition by list (a);
> create table p1 partition of p for values in (1) partition by range (b);
> create table p11 partition of p1 for values from (1) to (10);
> create table p2 partition of p for values in (2);
> 
> create unique index on p (a);
> ERROR:  insufficient columns in UNIQUE constraint definition
> DETAIL:  UNIQUE constraint on table "p1" lacks column "b" which is part of
> the partition key.
> 
> It seems that after recursing to p1 which is itself partitioned,
> DefineIndex() mistakenly looks for column b (which is in the p1's
> partition key) in the unique key.  I think that's unnecessary.
> DefineIndex() should check that only once, that is, before recursing.

Hmm, scratch that...

> Am I missing something?

Yes, I am.

create table p (a int, b int) partition by list (a);
create table p1 partition of p for values in (1) partition by range (b);
create table p11 partition of p1 for values from (1) to (10);
create table p12 partition of p1 for values from (10) to (20);
create table p2 partition of p for values in (2);

-- after applying my delta patch
create unique index on p (a);

insert into p values (1, 1);   -- unique index p11 (a) says all fine
insert into p values (1, 10);  -- unique index p12 (a) says all fine

That can't be right, because p (a) is no longer unique.

So, a unique key on a partitioned table must include the partition key
columns of *all* downstream partitioned tables, as your patch correctly
enforces.  Sorry about the noise.

That said, I think that it might be a good idea to include the above
detail in the documentation of CREATE INDEX and ALTER TABLE ADD UNIQUE.

Thanks,
Amit