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From Julian Hyde <jh...@apache.org>
Subject ProjectMergeRule
Date Tue, 21 Jul 2015 18:04:30 GMT
ProjectMergeRule currently refuses to reduce identity projects if the
fields have different names.

For instance suppose you have a table Dept (deptno, name) and the algebra

  2: Project($1 as X, $0 as Y)
    1: Project($1, $0)
      0: Scan(Dept)

Observe that if you combine projects #1 and #2 you end up with

  3: Project($0 as X, $1 as Y)
    0: Scan(Dept)

Although the new project (#3) is an identity, it renames the fields.
ProjectMergeRule will return the new project (#3), but it could return
Scan(Dept) (#0).

Does anyone think they will break if I make it return #0?

Julian

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