Re: String literal doesn't autocast to text type

[Alex Ignatov <a.ignatov@xxxxxxxxxxxxxx>]

Oh! Great answer! Thats what i want to know!!! 

Thank you Pavel about explanation!!!

Alex Ignatov

Postgres Professional: http://www.postgrespro.com
Russian Postgres Company

On 04 Mar 2016, at 20:45, Pavel Stehule <pavel.stehule@xxxxxxxxx> wrote:

Hi

2016-03-04 18:29 GMT+01:00 Melvin Davidson <melvin6925@xxxxxxxxx>:

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); 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.ignatov@xxxxxxxxxxxxxx> 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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