Hello,
I am trying to execute a long series of statements within a transaction
in "serializable" isolation level.
I've read Tom Lane's excellent document describing concurrency issues in
Postgres and learned of the general method of doing this:
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;
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? Will Postgres
tell me that the transaction failed when I receive a result for a
particular statement? Can Postgres returns an error when I try to commit,
as well? And which exactly are the error codes returned by Postgres when
I should retry the transaction? I guess that SERIALIZATION FAILURE is one
of these errors, but are there others too? Clearly I don't want to retry
a transaction that will always fail for reasons unrelated to concurrency.
I spent 4 hours trying to find a code snippet that does this. So far I've
been unsuccessful. Any precisions would be greatly appreciated.
Thanks,
Laurent Birtz
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