On Thu, Sep 27, 2018 at 3:55 PM Andrew Gierth <and...@tao11.riddles.org.uk> wrote: > >>>>> "Andres" == Andres Freund <and...@anarazel.de> writes: > Andres> Hm, stb's results just for floating point isn't bad. The above > Andres> numbers were for %f %f. But as the minimal usage would be about > Andres> the internal usage of dopr(), here's comparing %.*f: > > Andres> snprintf time = 1324.87 ms total, 0.000264975 ms per iteration > Andres> pg time = 1434.57 ms total, 0.000286915 ms per iteration > Andres> stbsp time = 552.14 ms total, 0.000110428 ms per iteration > > Hmm. We had a case recently on IRC where the performance of float8out > turned out to be the major bottleneck: a table of about 2.7 million rows > and ~70 float columns showed an overhead of ~66 seconds for doing COPY > as opposed to COPY BINARY (the actual problem report was that doing > "select * from table" from R was taking a minute+ longer than expected, > we got comparative timings for COPY just to narrow down causes). > > That translates to approx. 0.00035 ms overhead (i.e. time(float8out) - > time(float8send)) per conversion (Linux server, hardware unknown). > > That 66 seconds was the difference between 18s and 1m24s, so it wasn't a > small factor but totally dominated the query time.
For perfect and cheap round trip to ASCII, not for human consumption, I wonder about the hexadecimal binary float literal format from C99 (and showing up in other places too). -- Thomas Munro http://www.enterprisedb.com
