Speeding up COPY TO for uuids and arrays

While analyzing a customer's performance problem, I noticed that
the performance of pg_dump for large arrays is terrible.

As a test case, I created a table with 10000 rows, each of which
had an array of 10000 uuids. The table resided in shared buffers.

The following took 24.5 seconds:

COPY mytab TO '/dev/null';

Most of the time was spent in array_out and uuid_out.

I tried binary copy, which took 4.4 seconds:

COPY mytab TO '/dev/null' (FORMAT 'binary');

Here, a lot of time was spent in pq_begintypsend.

So I looked for low-hanging fruit, and the result is the attached
patch series.

- Patch 0001 speeds up pq_begintypsend with a custom macro.
That brought the binary copy down to 3.7 seconds, which is a
speed gain of 15%.

- Patch 0001 speeds up uuid_out by avoiding the overhead of
a Stringinfo. This brings text mode COPY to 19.4 seconds,
which is speed gain of 21%.

- Patch 0003 speeds up array_out a bit by avoiding some zero
byte writes. The measured speed gain is under 2%.

Yours,
Laurenz Albe