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Thread: float4 problem

float4 problem

From
Oleg Bartunov
Date:
May be I miss something, but seems there is a problem with float4
in 7.2.3 and 7.3RC1 (6.53 works fine):

test=# create table t ( a float4);
CREATE TABLE
test=# insert into t values (0.1);
INSERT 32789 1
test=# select * from t where a=0.1;a
---
(0 rows)

test=# select * from t where a=0.1::float4; a
-----0.1
(1 row)

No problem with float8

test=# create table t8 ( a float8);
CREATE TABLE
test=# insert into t8 values (0.1);
INSERT 32792 1
test=# select * from t8 where a=0.1; a
-----0.1
(1 row)

Regards,    Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83

Re: float4 problem

From
Doug McNaught
Date:
Oleg Bartunov <oleg@sai.msu.su> writes:

> May be I miss something, but seems there is a problem with float4
> in 7.2.3 and 7.3RC1 (6.53 works fine):
> 
> test=# create table t ( a float4);
> CREATE TABLE
> test=# insert into t values (0.1);
> INSERT 32789 1
> test=# select * from t where a=0.1;
>  a
> ---
> (0 rows)


I'm guessing this is because 0.1 is not directly representable as a
binary floating point number, and literal floating constants are
float8 not float4, and 0.1::float4 != 0.1::float8.  Same problem that
causes queries on int2 fields not to use an index unless you cast the
constants in the query...

-Doug

Re: float4 problem

From
Tom Lane
Date:
Doug McNaught <doug@mcnaught.org> writes:
> Oleg Bartunov <oleg@sai.msu.su> writes:
>> May be I miss something, but seems there is a problem with float4
>> in 7.2.3 and 7.3RC1 (6.53 works fine):
>> 
>> test=# create table t ( a float4);
>> CREATE TABLE
>> test=# insert into t values (0.1);
>> INSERT 32789 1
>> test=# select * from t where a=0.1;
>> a
>> ---
>> (0 rows)


> I'm guessing this is because 0.1 is not directly representable as a
> binary floating point number, and literal floating constants are
> float8 not float4, and 0.1::float4 != 0.1::float8.

Right.

I think that this particular form of the problem will go away in 7.4.
Currently, "a = 0.1" is resolved as float4=float8, and there's no way
for the float4 approximation of 0.1 to exactly equal the float8
approximation of it.  However, if we eliminate cross-datatype
comparison operators as I've proposed, the comparison should be resolved
as float4 = float4 and it would work.

Nonetheless, expecting exact equality tests to succeed with float values
is generally folly.  I really do not believe the claim that this worked
in 6.5.3.
        regards, tom lane