Re: Should PQconsumeInput/PQisBusy be expensive to use?

Michael Clark <codingninja(at)gmail(dot)com> writes:
> In doing some experiments I found that using
> PQsendQueryParams/PQconsumeInput/PQisBusy/PQgetResult produces slower
> results than simply calling PQexecParams.

Well, PQconsumeInput involves at least one extra kernel call (to see
whether data is available) so I don't know why this surprises you.
The value of those functions is if your application can do something
else useful while it's waiting. If the data comes back so fast that
you can't afford any extra cycles expended on the client side, then
you don't have any use for those functions.

However, if you do have something useful to do, the problem with
this example code is that it's not doing that, it's just spinning:

> while ( ((consume_result = PQconsumeInput(self.db)) == 1) &&
> ((is_busy_result = PQisBusy(self.db)) == 1) )
> ;

That's a busy-wait loop, so it's no wonder you're eating cycles there.
You want to sleep, or more likely do something else productive, when
there is no data available from the underlying socket. Usually the
idea is to include libpq's socket in the set of files being watched
by select() or poll(), and dispatch off to something that absorbs
the data whenever you see some data is available to read.

regards, tom lane