Jan Wieck <[EMAIL PROTECTED]> writes:
> Checked it against HEAD and 8.2:
> postgres=# select 1, 1, 1 union select * from (select 2, null, 2) two;
> ERROR:  failed to find conversion function from "unknown" to integer

It's always done that.   The SQL spec would tell you that you have to
cast the null to some specific type.  We allow untyped nulls in contexts
where we can derive a type from reasonably nearby context, but an
integer two levels up and over in another union arm isn't very nearby
IMHO.  So I'm not particularly concerned about making this work ...

                        regards, tom lane

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TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not

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