Re: Strange performance boost with random()

On 02/10/2014 09:52 PM, M Putz wrote:

Hello,

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);

- Heikki

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