Re: atomic multi-threaded upsert

On 2010-11-24, "Mikhail V. Puzanov" <misha.puzanov@gmail.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.