Re: Ryu floating point output patch

>>>>> "Andres" == Andres Freund <andres@anarazel.de> writes:

 >> Is this a path we really want to go down? I'm not convinced the
 >> cost/benefit ratio is attractive.

 Andres> float->text conversion is one of the major bottlenecks when
 Andres> backing up postgres, it's definitely a pain-point in practice.

Also an issue with queries. I got into this whole area after diagnosing
a problem for a user on IRC who was seeing a 4x slowdown for PG as
compared to MSSQL, from 30 seconds to 120 seconds. We determined that
the _entire_ difference was accounted for by float conversion.

Now that was an unusual case, downloading a large dataset of floats at
once into a statistics program, but it's very easy to show
proportionally large overheads from float output:

(using this table:
  create table flttst4 as
    select i a, i b, i c, i d, i::float8 e, random() f
      from generate_series(1::bigint, 10000000::bigint) i;
)

postgres=# copy flttst4(d) to '/dev/null';  -- bigint column
COPY 10000000
Time: 2166.001 ms (00:02.166)

postgres=# copy flttst4(e) to '/dev/null';  -- float8 col, integer values
COPY 10000000
Time: 4233.005 ms (00:04.233)

postgres=# copy flttst4(f) to '/dev/null';  -- float8 col, random()
COPY 10000000
Time: 7261.421 ms (00:07.261)

-- vs. timings with Ryu:

postgres=# copy flttst4(e) to '/dev/null';  -- float8 col, integer values
COPY 10000000
Time: 2391.725 ms (00:02.392)

postgres=# copy flttst4(f) to '/dev/null';  -- float8 col, random()
COPY 10000000
Time: 2245.699 ms (00:02.246)

-- 
Andrew (irc:RhodiumToad)