gruuya commented on PR #20819:
URL: https://github.com/apache/datafusion/pull/20819#issuecomment-4065785422
> This approach doesn't handle queries like:
>
> `SELECT a, b FROM t1 UNION ALL SELECT count(*), count(*) FROM t2;`
>
> (Error during planning: Projections require unique expression names but
the expression "count(Int64(1)) AS count(*)" at position 0 and "count(Int64(1))
AS count(*)" at position 1 have the same name. Consider aliasing ("AS") one of
them.)
>
> Or other non-column-name expressions in the target list of non-first
queries.
Fair point, this occurs because of the `is_literal_value` condition for
aliasing.
> I wonder if it would be cleaner to arrange not to check for duplicate
column names in set operation queries that aren't the left-most query? Since
the column names for such queries are discarded anyway, it seems a bit
laborious to first rewrite them to ensure they are unique, and then check that
they are indeed unique, before discarding them anyway.
Agreed, i actually thought about this first, but it seemed less clean and
more invasive in the end, so i ended up with this approach. Let me rethink
about it.
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