Re: computing completion tag is expensive for pgbench -S -M prepared

On 7 June 2018 at 16:13, Andres Freund <andres(at)anarazel(dot)de> wrote:
> in PortalRun(). That's actually fairly trivial to optimize - we don't
> need the full blown snprintf machinery here. A quick benchmark
> replacing it with:
>
> memcpy(completionTag, "SELECT ", sizeof("SELECT "));
> pg_lltoa(nprocessed, completionTag + 7);

I'd also noticed something similar with some recent benchmarks I was
doing for INSERTs into partitioned tables. In my case I saw as high as
0.7% of the time spent building the INSERT tag. So I think it's worth
fixing this.

I think it would be better to invent a function that accepts a
CmdType, int64 and Oid that copies the tag into the supplied buffer,
then make a more generic change that also replaces the code in
ProcessQuery() which builds the tag. I'm sure there must be some way
to get the CmdType down to the place you've patched so we can get rid
of the if (strcmp(portal->commandTag, "SELECT") == 0) line too.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services