While analyzing performance, we encountered the following phenomenon,
SELECT sum(pow(.5*generate_series,.5)) FROM generate_series(1,1000000);
is much much (a hundred times) slower than
SELECT sum(pow(random()*generate_series,.5)) FROM generate_series(1,1000000);
and asymptotic difference is even more astounding. This seems counter-intuitive, considering the cost of an additional random() call instead of a constant factor. What are the reasons for this strange performance boost?
Different data type. The first uses numeric, which is pretty slow for doing calculations. random() returns a double, which makes the pow and sum to also use double, which is a lot faster.
To see the effect, try these variants:
SELECT sum(pow(.5::float8 * generate_series,.5)) FROM generate_series(1,1000000);
SELECT sum(pow(random()::numeric * generate_series,.5)) FROM generate_series(1,1000000);
