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

On 7 June 2018 at 06:01, David Rowley <david.rowley@2ndquadrant.com> wrote:
> On 7 June 2018 at 16:13, Andres Freund <andres@anarazel.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.

Sounds better

Do we actually need the completion tag at all? In most cases??

Perhaps we should add a parameter to make it optional and turn it off
by default, except for psql.

--
Simon Riggs                http://www.2ndQuadrant.com/
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