Re: Retrying transactions in serializable isolation level

Laurent Birtz wrote:

> loop
>   BEGIN;
>   SELECT hits FROM webpages WHERE url = ...;
>   -- client internally computes $newval = $hits + 1
>   UPDATE webpages SET hits = $newval WHERE url = ..;
>   if (no error)
>      break out of loop;
>   else
>      ROLLBACK;
> end loop
> COMMIT;

I think you should be able to shave some milliseconds off that loop by
using SAVEPOINTs instead of a full-blown transaction for each tuple.
Something like

BEGIN
outer loop
    inner loop
      SAVEPOINT foo;
      SELECT hits ...
      UPDATE webpages SET hits = ...
      if (no error)
          break inner loop
      else
          ROLLBACK TO foo
    end inner loop
end outer loop
COMMIT;

> However, I am having problem implementing this scheme in C with libpq.
> Transactions can be aborted because a deadlock occurred or another
> transaction already made some changes to the database.
>
> My question is how exactly do I detect that this occurred?

IIRC you can apply PQresultStatus to the PGresult returned by the UPDATE.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.