Обсуждение: String literal doesn't autocast to text type

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

Hi

2016-03-04 18:29 GMT+01:00 Melvin Davidson <melvin6925@gmail.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); 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@postgrespro.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.

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

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.

Alex Ignatov <a.ignatov@postgrespro.ru> writes:
> Why string literal like 'Hello world!' doesnt automagicaly cast to text
> type?

Because it's not necessarily a string.  It might be meant to be point,
or json, or any number of other types.

> Sure we can create our cast:
> postgres=# create cast (unknown as text) with inout as implicit;
> CREATE CAST

That's a seriously bad idea; it will have all sorts of corner-case
effects that you aren't expecting.

There has been some talk of forcing unknown to text in the output
columns of a sub-SELECT, which would fix the example you show with
a lot less risk of side-effects elsewhere.  But it's not exactly
trivial because of interactions with INSERT ... SELECT.

            regards, tom lane

It would be nice if you'd follow the list convention of bottom-posting.  Not doing that is much more annoying than the omission of version and o/s - especially the later since PostgreSQL purports to be generally o/s agnostic.

On Fri, Mar 4, 2016 at 12:17 PM, Alex Ignatov <a.ignatov@postgrespro.ru> wrote:

Hello!
Why string literal like 'Hello world!' doesnt automagicaly cast to text type?


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?

On Friday, March 4, 2016, Melvin Davidson <melvin6925@gmail.com> wrote:

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

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:"

While true this doesn't actually answer the question - that being what's the motiviation for not implicitly casting away from unknown.  I suspect that the main reason is that in an extensible system like PostgreSQL it is deemed risky to have too many implicit casts.  Having one from unknown required the system to make decisions that could result in unexpected results that would be hard to catch without careful review of queries and results.  It is an issue of some contention in the community but so far no one has convinced the committees to change how this works.

 

Please in the future, ALWAYS specify your PostgreSQL version and O/S, regardless of whether or not you thinnk it is pertinent.

But don't feel too bad if you forget...

 

David J.

 

> On 04 Mar 2016, at 21:08, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Alex Ignatov <a.ignatov@postgrespro.ru> writes:
>> Why string literal like 'Hello world!' doesnt automagicaly cast to text
>> type?
>
> Because it's not necessarily a string.  It might be meant to be point,
> or json, or any number of other types.
>
>> Sure we can create our cast:
>> postgres=# create cast (unknown as text) with inout as implicit;
>> CREATE CAST
>
> That's a seriously bad idea; it will have all sorts of corner-case
> effects that you aren't expecting.
>
> There has been some talk of forcing unknown to text in the output
> columns of a sub-SELECT, which would fix the example you show with
> a lot less risk of side-effects elsewhere.  But it's not exactly
> trivial because of interactions with INSERT ... SELECT.
>
>            regards, tom lane
Oh! Thank you , Tom!