Re: [BUGS] Failure to coerce unknown type to specific type - Mailing list pgsql-hackers

On Wed, 2015-04-22 at 20:35 -0700, David G. Johnston wrote:

> But the fact that column "b" has the data type "unknown" is only a
> warning - not an error.
> 
I get an error:

postgres=# SELECT '  '::text = 'a';?column? 
----------f
(1 row)

postgres=# SELECT a=b FROM (SELECT ''::text, '  ') x(a,b);
ERROR:  failed to find conversion function from unknown to text

So that means the column reference "b" is treated differently than the
literal. Here I don't mean a reference to an actual column of a real
table, just an identifier ("b") that parses as a columnref.

Creating the table gives you a warning (not an error), but I think that
was a poor example for me to choose, and not important to my point.
> 
> This seems to be a case of the common problem (or, at least recently
> mentioned) where type conversion only deals with data and not context.
> 
> 
> http://www.postgresql.org/message-id/CADx9qBmVPQvSH3
> +2cH4cwwPmphW1mE18e=WUmLFUC-QZ-t7Q6Q@mail.gmail.com
> 
> 
I think that is a different problem. That's a runtime type conversion
error (execution time), and I'm talking about something happening at
parse analysis time.

> 
> but this too works - which is why the implicit cast concept above
> fails (I'm leaving it since the thought process may help in
> understanding):
> 
> 
> SELECT 1 = '1';
> 
> 
> From which I infer that an unknown literal is allowed to be fed
> directly into a type's input function to facilitate a direct coercion.

Yes, I believe that's what's happening. When we use an unknown literal,
it's acting more like a value constructor and will pass it to the type
input function. When it's a columnref, even if unknown, it tries to cast
it and fails.

But that is very confusing. In the example at the top of this email, it
seems like the second query should be equivalent to the first, or even
that postgres should be able to rewrite the second into the first. But
the second query fails where the first succeeds.


> At this point...backward compatibility?

Backwards compatibility of what queries? I guess the ones that return
unknowns to the client or create tables with unknown columns?

> create table a(u) as select '1';
> 
> 
> WARNING: "column "u" has type "unknown"​
> DETAIL:  Proceeding with relation creation anyway.
> 
> 
> Related question: was there ever a time when the above failed instead
> of just supplying a warning?

Not that I recall.



> ​My gut reaction is if you feel strongly enough to add some additional
> documentation or warnings/hints/details related to this topic they
> probably would get put in; but disallowing "unknown" as first-class
> type is likely to fail to pass a cost-benefit evaluation.

I'm not proposing that we eliminate unknown. I just think columnrefs and
literals should behave consistently. If we really don't want unknown
columnrefs, it seems like we could at least throw a better error.

If we were starting from scratch, I'd also not return unknown to the
client, but we have to worry about the backwards compatibility.

> Distinguishing between "untyped" literals and "unknown type" literals
> seems promising concept to aid in understanding the difference in the
> face of not being able (or wanting) to actually change the behavior.

Not sure I understand that proposal, can you elaborate?

Regards,Jeff Davis