Thread: ERROR: failed to find conversion function from "unknown" to text

Hello

2009/1/6 Gurjeet Singh <singh.gurjeet@gmail.com>:
> Q1: select '' union all select ''
> Q2: select '' union all select * from (select '' ) as s
>
> version: PostgreSQL 8.3.1, compiled by Visual C++ build 1400
>
> Hi All,
>
>     Q1 works just fine, but Q2 fails with:
>
> ERROR:  failed to find conversion function from "unknown" to text
>
>     Q2 is a generalization of a huge query we are facing, which we cannot
> modify. I don't think this is a 'removed-casts' problem generally faced in
> 8.3, but I may be wrong. Will adding some cast resolve this?

yes

postgres=#  select '' union all select * from (select ''::text ) as s;
 ?column?
----------


(2 rows)

regards
Pavel Stehule

>
> Best regards,
> --
> gurjeet[.singh]@EnterpriseDB.com
> singh.gurjeet@{ gmail | hotmail | indiatimes | yahoo }.com
>
> EnterpriseDB      http://www.enterprisedb.com
>
> Mail sent from my BlackLaptop device
>

2009/1/6 Gurjeet Singh <singh.gurjeet@gmail.com>:
> As I mentioned, we cannot change the query, so adding casts to the query is
> not an option. I was looking for something external to the query, like a
> CREATE CAST command that'd resolve the issue.

I am sorry, I blind - I tested casting on 8.3.0 and it doesn't work
(but I am have old 8.3)
postgres=# create function unknown2text(unknown) returns text as
$$select $1::text$$ language sql;
CREATE FUNCTION
postgres=# create cast(unknown as text) with function
unknown2text(unknown) as implicit;
CREATE CAST
postgres=# select '' union all select * from (select '' ) as s;
ERROR:  stack depth limit exceeded
HINT:  Increase the configuration parameter "max_stack_depth", after
ensuring the platform's stack depth limit is adequate.
CONTEXT:  SQL function "unknown2text" during startup
SQL function "unknown2text" statement 1
SQL function "unknown2text" statement 1
SQL function "unknown2text" statement 1
SQL function "unknown2text" statement 1
SQL function "unknown2text" statement 1

It working on 8.4

postgres=# create cast (unknown as text) with inout as implicit;
CREATE CAST
postgres=# select '' union all select * from (select '' ) as s;
 ?column?
----------


(2 rows)

regards
Pavel Stehule


>
> Best regards,


>
> On Tue, Jan 6, 2009 at 12:00 PM, Pavel Stehule <pavel.stehule@gmail.com>
> wrote:
>>
>> Hello
>>
>> 2009/1/6 Gurjeet Singh <singh.gurjeet@gmail.com>:
>> > Q1: select '' union all select ''
>> > Q2: select '' union all select * from (select '' ) as s
>> >
>> > version: PostgreSQL 8.3.1, compiled by Visual C++ build 1400
>> >
>> > Hi All,
>> >
>> >     Q1 works just fine, but Q2 fails with:
>> >
>> > ERROR:  failed to find conversion function from "unknown" to text
>> >
>> >     Q2 is a generalization of a huge query we are facing, which we
>> > cannot
>> > modify. I don't think this is a 'removed-casts' problem generally faced
>> > in
>> > 8.3, but I may be wrong. Will adding some cast resolve this?
>>
>> yes
>>
>> postgres=#  select '' union all select * from (select ''::text ) as s;
>>  ?column?
>> ----------
>>
>>
>> (2 rows)
>>
>> regards
>> Pavel Stehule
>>
>> >
>> > Best regards,
>> > --
>> > gurjeet[.singh]@EnterpriseDB.com
>> > singh.gurjeet@{ gmail | hotmail | indiatimes | yahoo }.com
>> >
>> > EnterpriseDB      http://www.enterprisedb.com
>> >
>> > Mail sent from my BlackLaptop device
>> >
>
>
>
> --
> gurjeet[.singh]@EnterpriseDB.com
> singh.gurjeet@{ gmail | hotmail | indiatimes | yahoo }.com
>
> EnterpriseDB      http://www.enterprisedb.com
>
> Mail sent from my BlackLaptop device
>

On Tue, Jan 6, 2009 at 2:04 AM, Gurjeet Singh <singh.gurjeet@gmail.com> wrote:
> I took your cue, and have formulated this solution for 8.3.1 :

Is there a good reason you're running against a db version with known
bugs instead of 8.3.5?  Seriously, it's an easy upgrade and running a
version missing over a year of updates is not a best practice.

On Tue, Jan 6, 2009 at 2:24 AM, Gurjeet Singh <singh.gurjeet@gmail.com> wrote:
> On Tue, Jan 6, 2009 at 2:43 PM, Scott Marlowe <scott.marlowe@gmail.com>
> wrote:
>>
>> On Tue, Jan 6, 2009 at 2:04 AM, Gurjeet Singh <singh.gurjeet@gmail.com>
>> wrote:
>> > I took your cue, and have formulated this solution for 8.3.1 :
>>
>> Is there a good reason you're running against a db version with known
>> bugs instead of 8.3.5?  Seriously, it's an easy upgrade and running a
>> version missing over a year of updates is not a best practice.
>
> That's just a development instance that I have kept for long; actual issue
> was on EDB 8.3.0.12, which the customer is using. As noted in the PS of
> previous mail, the solution that worked for PG 8.3.1 didn't work on EDB
> 8.3.0.12, so had to come up with a different code for that!

Ahh, ok.  I was just worried you were ignoring updates.  I don't know
anything about the numbering scheme for EDB.  What does 8.3.0.12
translate to in regular pgsql versions?

"Gurjeet Singh" <singh.gurjeet@gmail.com> writes:
> create cast (unknown as text) with function unknown2text( unknown ) as
> implicit;

This is a horrendously bad idea; it will bite your *ss sooner or later,
probably sooner.

            regards, tom lane

"Gurjeet Singh" <singh.gurjeet@gmail.com> writes:
>> This is a horrendously bad idea; it will bite your *ss sooner or later,
>> probably sooner.

> Can you please let us know how this would be problematic?

The point is that it's going to have unknown, untested effects on the
default coercion rules, possibly leading to silent changes in the
behavior of queries that used to work.  If you'd rather retest every one
of your other queries than fix this one, then go ahead.

            regards, tom lane

On Tue, Jan 06, 2009 at 11:13:59PM +0530, Gurjeet Singh wrote:
> If we consider the second branch of UNION ALL of both the queries above, if
> "select '' " yields a text column, then so should a "select * from (select
> '')".

The problem is ofcourse that "select ''" doesn't produce a text column
in postgres. This generally works fine, except in the case of UNION
where none of the branches provide the necessary type info.

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
> boarding. Thank you for flying nlogn airlines.