Re: BUG #19101: Ceil on BIGINT could lost precision in decil function

PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> I try to store a large number in `BIGINT` and run `ceil(c1)` command.
> However, the result lost some precision due to calling `decil` function.
> ```sql
> CREATE TABLE t1 (c1 BIGINT);
> INSERT INTO t1 VALUES (4854233034440979799);
> -- dceil
> SELECT ceil(c1) FROM t1; -- {4.854233034440979e+18}
> ```

This is not a bug. There are two versions of ceil() and you're
invoking the wrong one. You'd need to explicitly cast the
argument to numeric if you want ceil(numeric) to be used.

In this context it's a bit unfortunate that the parser's type
preference rules [1] prefer float8 to numeric. But we're pretty
much stuck with that behavior because (a) the SQL standard
says so [2], and (b) even if it didn't, we have a couple of
decades of history to be backwards compatible with.

regards, tom lane

[1] https://www.postgresql.org/docs/current/typeconv.html

[2] Well, what it really says is that expressions that mix
exact and inexact numeric types produce inexact results.
We interpret that as meaning that float8 is the preferred
type in the numeric category, so it wins ambiguous cases.