Tables names from query

I'm currently developing a postgres DB backend to my current project and have
the following problem. If I execute a query I need to know the table the
returning fields belong to. For example :

SELECT x.y, z.y from x, y where x.key = z.key

The problem with the above is that I get back 2 fields called 'y' and have no
way of knowing from which tables they've come from. The query is arbitrary so I
cannot assume table order in the SQL statement.

What information from the 'C' or 'C++' APIs can help ?

Is there is a way of identifying the originating tables ?

Thanks
Philip
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Philip Shiels E-Mail:Philip(dot)Shiels(at)jrc(dot)it JRC Ispra, Italy, TP270
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