Re: atomic multi-threaded upsert

On 2010-11-24, "Mikhail V. Puzanov" <misha(dot)puzanov(at)gmail(dot)com> wrote:
> Next, I execute the following two queries for that table:
>
> -- increment and get the counter if exists
> UPDATE sequences SET counter = counter + 1
> WHERE section = 'testSection' AND name = 'testKey'
> RETURNING counter;
>
> -- insert new counter if not exists
> INSERT INTO sequences ( section, name, counter )
> SELECT 'testSection', 'testKey', 0
> WHERE NOT EXISTS (
> SELECT * FROM sequences
> WHERE section = 'testSection' AND name = 'testKey'
> );

> ERROR: duplicate key value violates unique constraint "idx_sequences_sn"
>
> 1) That looks weird actually, should the INSERT ... NOT EXISTS (...) be
> executed
> in atomic fashion? Also, AFAIK, UNIQUE INDEX locks rows before
> insert (or not?).
> Can it be related to JDBC?
> Or it's the result of MVCC conflict resolution?

Perhaps -

Thread A UPDATEs, affecting no row.
Thread B UPDATEs, affecting no row.
Thread A INSERTs one row. Autocommit is on, so it commits the INSERT.
Thread B attempts INSERT and fails on the duplicate.

--
KM