Thread: Odd numeric->float4/8 casting behaviour

Odd numeric->float4/8 casting behaviour

From
Gregory Stark
Date:
I noticed this odd discrepancy:

postgres=# select -0.999::numeric(3,3)::float4 = -0.999::numeric(3,3);?column? 
----------f
(1 row)


I believe this is happening because the numeric is being cast to float8 and
then the float4-float8 cross-data-type operator is being used. It seems like
it would be preferable to cast it to float4 and use the non-cross-data-type
operator. They're both marked as implicit casts so I'm unclear what decides
which gets used.

Also, as a side note I was surprised to find the above being parsed as
-(0.999::numeric(3,3)) rather than (-0.999)::numeric(3,3) is that expected?



regression=# create or replace view x as select   -0.999::numeric(3,3)::float4 = -0.999::numeric(3,3);
CREATE VIEW
regression=# \d x       View "public.x" Column  |  Type   | Modifiers 
----------+---------+-----------?column? | boolean | 
View definition:SELECT (- 0.999::numeric(3,3)::real) = (- 0.999::numeric(3,3))::double precision;

regression=# create or replace view x as select   (-0.999)::numeric(3,3)::float4 = (-0.999)::numeric(3,3);
CREATE VIEW
regression=# \d x       View "public.x" Column  |  Type   | Modifiers 
----------+---------+-----------?column? | boolean | 
View definition:SELECT -0.999::numeric(3,3)::real = -0.999::numeric(3,3)::double precision;


--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com


Re: Odd numeric->float4/8 casting behaviour

From
Tom Lane
Date:
Gregory Stark <stark@enterprisedb.com> writes:
> I believe this is happening because the numeric is being cast to float8 and
> then the float4-float8 cross-data-type operator is being used. It seems like
> it would be preferable to cast it to float4 and use the non-cross-data-type
> operator. They're both marked as implicit casts so I'm unclear what decides
> which gets used.

Without having traced through the code, I think the fact that float8 is
a "preferred type" is driving it.  It's not clear whether we could
change this without getting into a "can't resolve ambiguous operator"
problem.

> Also, as a side note I was surprised to find the above being parsed as
> -(0.999::numeric(3,3)) rather than (-0.999)::numeric(3,3) is that expected?

Yeah, :: binds VERY tightly.
        regards, tom lane


Re: Odd numeric->float4/8 casting behaviour

From
Gregory Stark
Date:
"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> Gregory Stark <stark@enterprisedb.com> writes:
>> I believe this is happening because the numeric is being cast to float8 and
>> then the float4-float8 cross-data-type operator is being used. It seems like
>> it would be preferable to cast it to float4 and use the non-cross-data-type
>> operator. They're both marked as implicit casts so I'm unclear what decides
>> which gets used.
>
> Without having traced through the code, I think the fact that float8 is
> a "preferred type" is driving it.  It's not clear whether we could
> change this without getting into a "can't resolve ambiguous operator"
> problem.

This is pre-operator-families, I thought "preferred type" was new with them.

Perhaps we should have preferred operators rather than preferred types?

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com


Re: Odd numeric->float4/8 casting behaviour

From
Tom Lane
Date:
Gregory Stark <stark@enterprisedb.com> writes:
> This is pre-operator-families, I thought "preferred type" was new with them.

No, preferred types have been around for a very long time.
        regards, tom lane