FAQ
With 8.4RC1, I get the following error with the SQL below.

ERROR: failed to find conversion function from unknown to character varying
SQL state: XX000


SELECT 0 AS "C1", cast('a' as varchar) AS "C2"
UNION ALL
SELECT 1 AS "C1", "X"."XX" AS "C2"
FROM ( SELECT NULL AS "XX" ) AS "X"
WHERE TRUE = FALSE


Compare that SQL to this:


SELECT 0 AS "C1", cast('a' as varchar) AS "C2"
UNION ALL
SELECT 1 AS "C1", NULL AS "C2"
WHERE TRUE = FALSE

The second works without the nested subquery.

I ran into SQL like the above when trying to use the Npgsql .Net driver 2.0.5 via Microsoft's Entity Framework (edmgen.exe). It happened against two different databases. Both have UTF-8 encoding and English, United States language.

--
Eric

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  • Tom Lane at Jun 26, 2009 at 12:50 am

    Eric Miller writes:
    With 8.4RC1, I get the following error with the SQL below.
    ERROR: failed to find conversion function from unknown to character varying
    Every PG version since 7.1 complains about that query, because it's
    underspecified. Cast the NULL to something specific in the sub-SELECT.

    regards, tom lane

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postedJun 25, '09 at 11:44p
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