Re: String literal doesn't autocast to text type

Hi

2016-03-04 18:29 GMT+01:00 Melvin Davidson <melvin6925(at)gmail(dot)com>:

> Probably because pg_typeof() returns the OID of a COLUMN in a table def..
> Strings literals do not have oid's.
>

no this is not a reason.

String literal has fictive "unknown" type. Real type is derivated from
context - operators, function parameters. pg_typeof has parameter of type
"any", and then no conversions from "unknown" is possible.

I don't known why "text" type is not default for string literal, but I see
some logical relations. If we cast "unknown" to "text" early, then we will
be limited by "text" type available conversions. Now, it is possible, but
years ago, the cast between "text" type and others was disallowed. Still we
require explicit cast, and I see it as benefit. Hidden cast (implicit cast)
are query performance killers. So "unknown" type requires less explicit
casting, because there are implicit casts from this type to any type.

I am not sure if comparation with numbers is valid. Numbers are much more
consistent class than string literals - more implicit casts over this class
is there.

Probably this design can be enhanced, and more consistent - "text" type can
be used as fallback type.

Regards

Pavel

>
> http://www.postgresql.org/docs/9.4/interactive/functions-info.html
>
> "pg_typeof returns the OID of the data type of the value that is passed
> to it. This can be helpful for troubleshooting or dynamically constructing
> SQL queries. The function is declared as returning regtype, which is an
> OID alias type (see Section 8.18
> <http://www.postgresql.org/docs/9.4/interactive/datatype-oid.html>); this
> means that it is the same as an OID for comparison purposes but displays as
> a type name. For example:"
>
> Please in the future, ALWAYS specify your PostgreSQL version and O/S,
> regardless of whether or not you thinnk it is pertinent.
>
> On Fri, Mar 4, 2016 at 12:17 PM, Alex Ignatov <a(dot)ignatov(at)postgrespro(dot)ru>
> wrote:
>
>> Hello!
>> Why string literal like 'Hello world!' doesnt automagicaly cast to text
>> type?
>>
>> postgres=# select pg_typeof('Hello world');
>> pg_typeof
>> -----------
>> unknown
>> (1 row)
>>
>> But for example literal like 1.1 automagically cast to numeric( not
>> float8, float4, whatever)
>> postgres=# select pg_typeof(1.1);
>> pg_typeof
>> -----------
>> numeric
>> (1 row)
>>
>> That why we cant do the following without explicit type casting:
>> postgres=# select t.c||' world' from (select 'Hello' as c) as t;
>> ERROR: failed to find conversion function from unknown to text
>>
>> but that ok:
>> postgres=# select t.c||' world' from (select 'Hello'::text as c) as t;
>> ?column?
>> -------------
>> Hello world
>> (1 row)
>>
>> or this is ok too:
>> postgres=# select t.c::text||' world' from (select 'Hello' as c) as t;
>> ?column?
>> -------------
>> Hello world
>> (1 row)
>>
>> Sure we can create our cast:
>> postgres=# create cast (unknown as text) with inout as implicit;
>> CREATE CAST
>> and after that we have:
>> postgres=# select t.c||' world' from (select 'Hello' as c) as t;
>> ?column?
>> -------------
>> Hello world
>> (1 row)
>>
>> But why we don't have this type cast by default in Postgres? Is there any
>> fundamental restriction on that or there is some reasons for that?
>>
>>
>> --
>> Alex Ignatov
>> Postgres Professional: http://www.postgrespro.com
>> The Russian Postgres Company
>>
>>
>>
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>
>
>
> --
> *Melvin Davidson*
> I reserve the right to fantasize. Whether or not you
> wish to share my fantasy is entirely up to you.
>