According to clause 3 on 
http://www.postgresql.org/docs/9.3/static/typeconv-union-case.html regarding 
union type matching:

3. If the non-unknown inputs are not all of the same type category, fail.

So a query "SELECT 1 UNION SELECT 1.1" works because 1 and 1.1 are of the same 
category, and one type has an implicit cast to the other, but the query "SELECT 
'1' UNION SELECT 2" fails because '1' is a string literal and 2 is a number and 
so they are different categories. Right?

Is this an artificial limitation of postgres or is there an underlying 
technical reason for this behaviour? For my purposes it would be better if the 
restriction was removed and that the union would work as long as there was an 
implicit cast that allowed conversion of all fields to the same type. 

MSSQL doesn't have this restriction and I'd prefer if I didn't have to rewrite 
these queries (or create a complete set of mssql compatible types in the same 
category) when porting applications.

Thanks

James


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