Re: Why is pq_begintypsend so slow?

Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> I saw that the hotspot was pq_begintypsend at 20%, which was twice the
> percentage as the next place winner (AllocSetAlloc).

Weird.

> Why is this such a bottleneck?

Not sure, but it seems like a pretty dumb way to push the stringinfo's
len forward. We're reading/updating the len word in each line, and
if your perf measurements are to be believed, it's the re-fetches of
the len values that are bottlenecked --- maybe your CPU isn't too
bright about that? The bytes of the string value are getting written
twice too, thanks to uselessly setting up a terminating nul each time.

I'd be inclined to replace the appendStringInfoCharMacro calls with
appendStringInfoSpaces(buf, 4) --- I don't think we care exactly what
is inserted into those bytes at this point. And maybe
appendStringInfoSpaces could stand some micro-optimization, too.
Use a memset and a single len adjustment, perhaps?

regards, tom lane