On Sat, Dec 23, 2023 at 8:36 AM Phillip Diffley <phillip6402@gmail.com> wrote:
>
> Thank you for all the suggestions! I ended up using pgcrypto's
> pg_random_bytes() to build the random int. I haven't fully tested the
> function yet, but it looks like this works.
>
> CREATE EXTENSION IF NOT EXISTS pgcrypto;
> CREATE OR REPLACE FUNCTION gen_random_int() RETURNS INT8 AS $$
> DECLARE
> bytes bytea;
> BEGIN
> bytes := gen_random_bytes(8);
> RETURN
> (get_byte(bytes,0)::int8 << 8*0) |
> (get_byte(bytes,1)::int8 << 8*1) |
> (get_byte(bytes,2)::int8 << 8*2) |
> (get_byte(bytes,3)::int8 << 8*3) |
> (get_byte(bytes,4)::int8 << 8*4) |
> (get_byte(bytes,5)::int8 << 8*5) |
> (get_byte(bytes,6)::int8 << 8*6) |
> (get_byte(bytes,7)::int8 << 8*7);
> END;
> $$ LANGUAGE plpgsql;
>
postgres=# explain analyze select gen_random_int() from
generate_series(1, 1000000);
Time: 4794.352 ms (00:04.794)
postgres=# explain analyze select ('x'||encode(gen_random_bytes(8),
'hex'))::bit(64)::int8 from generate_series(1, 1000000);
Time: 2816.014 ms (00:02.816)
postgres=# explain analyze select
('x'||encode(pg_read_binary_file('/dev/urandom', 0, 8),
'hex'))::bit(64)::bigint from generate_series(1, 1000000);
Time: 18947.639 ms (00:18.948)
postgres=# explain analyze select (random() * 2147483648)::int8 *
4294967296 + (random() * 4294967296)::int8 from generate_series(1,
1000000);
Time: 728.368 ms
Peter's way has the best performance.
>
> On Thu, Dec 21, 2023 at 6:14 AM Peter J. Holzer <hjp-pgsql@hjp.at> wrote:
> >
> > On 2023-12-21 00:06:39 -0600, Phillip Diffley wrote:
> > > Postgres's random() function generates a random double. That can be converted
> > > to a random int for smaller integers, but a double can't represent all of the
> > > values in a bigint. Is there a recommended way to generate a random bigint in
> > > Postgres?
> >
> > Call random() twice and add the results?
> >
> > Like this:
> >
> > select (random() * 2147483648)::int8 * 4294967296
> > + (random() * 4294967296)::int8;
> >
> > (This assumes that random() actually returns at least 32 random bits.
> > If that's not the case you'll need more calls to random())
> >
> > hp
> >
> > --
> > _ | Peter J. Holzer | Story must make more sense than reality.
> > |_|_) | |
> > | | | hjp@hjp.at | -- Charles Stross, "Creative writing
> > __/ | http://www.hjp.at/ | challenge!"
>
>
--
Regards
Junwang Zhao
