Обсуждение: BUG #18390: exponentiation produces float datatype, but nth-root produces integer

The following bug has been logged on the website:

Bug reference:      18390
Logged by:          Robb Allan
Email address:      robb@helical.com
PostgreSQL version: 15.5
Operating system:   Mac OS Sonoma 14.3.1 (23D60)
Description:

An exponentiation from a float produces a floating point result:

funds=# select (1.1^10);
      ?column?      
--------------------
 2.5937424601000000
(1 row)

However, an nth-root calculation from that float result does NOT produce a
new float result:

funds=# select (2.5937424601000000^(1/10));
      ?column?      
--------------------
 1.0000000000000000
(1 row)


Only when the root factors are floats does the result coerce to a float:

funds=# select (2.5937424601000000^(1.0/10));
        ?column?        
------------------------
 1.10000000000000000000
(1 row)

funds=# select (2.5937424601000000^(1/10.0));
        ?column?        
------------------------
 1.10000000000000000000
(1 row)

The following bug has been logged on the website:

Bug reference:      18390
Logged by:          Robb Allan
Email address:      robb@helical.com
PostgreSQL version: 15.5
Operating system:   Mac OS Sonoma 14.3.1 (23D60)
Description:       

However, an nth-root calculation from that float result does NOT produce a
new float result:

funds=# select (2.5937424601000000^(1/10));
      ?column?     
--------------------
 1.0000000000000000
(1 row)

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Tuesday, March 12, 2024, PG Bug reporting form <noreply@postgresql.org>
> wrote:
>> However, an nth-root calculation from that float result does NOT produce a
>> new float result:

> The issue is you expect dividing two integers to produce a float but that
> isn’t how that works.  Dividing two integers produces an integer.  In this
> case zero, or .1 rounded down/truncated.

Possibly adding to the OP's confusion: not one of these examples
contains any float arithmetic whatsoever.  They're of type numeric.

=# select pg_typeof(2.5937424601000000^(1.0/10));
 pg_typeof
-----------
 numeric
(1 row)

See
https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-CONSTANTS-NUMERIC
which says

    A numeric constant that contains neither a decimal point nor an
    exponent is initially presumed to be type integer if its value
    fits in type integer (32 bits); otherwise it is presumed to be
    type bigint if its value fits in type bigint (64 bits); otherwise
    it is taken to be type numeric. Constants that contain decimal
    points and/or exponents are always initially presumed to be type
    numeric.

A cast to float would occur only if the value is fed to an operator
that doesn't come in an exactly matching datatype.

            regards, tom lane