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How to generate random bigint

From
Phillip Diffley
Date:
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?

Thanks,
Phillip

Re: How to generate random bigint

From
Tom Lane
Date:
Phillip Diffley <phillip6402@gmail.com> writes:
> 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?

Doesn't look like there's anything directly exposed for that.
Since PG v13 you could use gen_random_uuid, if you're careful
to extract only bits that aren't fixed by the v4 UUID spec.
pgcrypto's pg_random_bytes() function offers another
some-assembly-required solution that'd work considerably
further back.  Or you could make a custom C function that
leverages pg_strong_random().

            regards, tom lane

Re: How to generate random bigint

From
Thomas Munro
Date:
On Thu, Dec 21, 2023 at 7:21 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Phillip Diffley <phillip6402@gmail.com> writes:
> > 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?
>
> Doesn't look like there's anything directly exposed for that.
> Since PG v13 you could use gen_random_uuid, if you're careful
> to extract only bits that aren't fixed by the v4 UUID spec.
> pgcrypto's pg_random_bytes() function offers another
> some-assembly-required solution that'd work considerably
> further back.  Or you could make a custom C function that
> leverages pg_strong_random().

Also pg_read_binary_file('/dev/urandom', 0, 8) could be useful
(assuming you're on Unix) if you can figure out how to cast it...

Re: How to generate random bigint

From
"Peter J. Holzer"
Date:
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!"
Attachment

Re: How to generate random bigint

From
Phillip Diffley
Date:
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;


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!"

Re: How to generate random bigint

From
Junwang Zhao
Date:
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