Re: Trouble incrementing a column

On 11/23/19 3:28 PM, Blake McBride wrote:
> Greetings,
>
> I am using PostgreSQL 10.10.  I am having trouble incrementing a column
> for reasons I can't see.  It's probably some basic SQL thing.  Your help
> is appreciated.
>
> create table my_table (
>     listid char(36) not null,
>     seq smallint not null,
>     item varchar(4096),
>     primary key (listid, seq)
> );
>
> insert into my_table (listid, seq) values ('abc', 1);
> insert into my_table (listid, seq) values ('abc', 2);
>
> -- the following works some of the time
> update my_table set seq=seq+1;
>
> -- the following doe not work for reasons I do not know
> update my_table set seq=seq+1 where listid='abc';
>
> What I get is a duplicate primary key.  I wouldn't think I'd get that
> because I'd think the whole thing is done in a transaction so that
> duplicate checks wouldn't be done till the end (essentially).
>
> Is there a clean way to do this?

A deferrable constraint might solve the problem.

https://www.commandprompt.com/blog/postgres_deferred_primary_keys/

https://www.postgresql.org/docs/9.6/sql-altertable.html

ALTER TABLE ... ALTER CONSTRAINT ... DEFERRABLE INITIALLY DEFERRED;

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