Обсуждение: Asynchronous Queries/libpq

Me again.

I am trying to understand the process for asynchronous queries in libpq, and
I want to make sure I understand the process correctly.

Is it that PQgetResult will return a full query result from a single
command, and will only return NULL if there are no more commands that have
not completed, therefore only needing to be called once per command in the
dispatched query; or is it that PQgetResult  must be repeatedly called for
even a single command query, and that it will only return NULL after each
command in the dispatched query has completed fully?

Further, can we *always* assume that query results will be returned in the
order they appear in the query dispatch?

Thanks, 

Keary Suska
Esoteritech, Inc.
"Demystifying technology for your home or business"

Keary Suska <hierophant@pcisys.net> writes:
> Is it that PQgetResult will return a full query result from a single
> command, and will only return NULL if there are no more commands that have
> not completed, therefore only needing to be called once per command in the
> dispatched query; or is it that PQgetResult  must be repeatedly called for
> even a single command query, and that it will only return NULL after each
> command in the dispatched query has completed fully?

You need to loop until PQgetResult returns null, if you want to handle
multi-command strings (you'll get one PGresult per command).  If you
know there's only going to be one result you might be able to get away
without a second PQgetResult call, but I wouldn't recommend it.

> Further, can we *always* assume that query results will be returned in the
> order they appear in the query dispatch?

Yes.  The backend isn't going to execute commands in a query string
out-of-order.
        regards, tom lane