Re: BUG #15812: Select statement of a very big number, with a division operator seems to round up.

Hi,

On 2019-05-17 12:02:11 -0400, Alvaro Herrera wrote:
> On 2019-May-17, PG Bug reporting form wrote:
>
> > create table test_table
> > (
> > REQUEST_UUID varchar(50) not null,
> > BIG_NUM numeric(20,0) not null
> > );
> >
> > INSERT INTO test_table (REQUEST_UUID, BIG_NUM) values ('TEST',
> > 3691635539999999999);
> > INSERT INTO test_table (REQUEST_UUID, BIG_NUM) values('TEST',
> > 3691635530099999999);
> > INSERT INTO test_table (REQUEST_UUID, BIG_NUM) values('TEST',
> > 3691635530999999999);
> >
> > SELECT BIG_NUM, FLOOR(BIG_NUM/10000000000), BIG_NUM/10000000000 from
> > test_table;
>
> Well, your column definition has room for zero decimal places, so I'm
> not sure this result is all that surprising. Maybe you should cast the
> column to one that has a few decimal places, say
> select bit_num::numeric(30,10) / 10000000000 from test_table;
> and see whether that helps your case.

Arguably it's less the column's and more the divisor's precision that's
the problem. Note that even if big_num were numeric (i.e. without an
implied precision) you'd get the OP's results - the precision is not
"widened" to the appropriate width for the max precision needed for the
division.

Greetings,

Andres Freund