[AQUMV] Use view's TupleDesc to construct final columns.

[avamingli]

We once used a SQL like: SELECT * FROM mv to fetch target materialized view's attributes, which is ineffective.
We don't need to do a full palnner of that SQL, use TupleDesc to fetch mv's attributes instead.

Authored-by: Zhang Mingli avamingli@gmail.com

fix #ISSUE_Number

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