THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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- The column name
- The column data type
- The column expression of SQL-style string that looks like that how user writes them(except if the expression contains a UDF, we should expand it's schema path for a complement reference)
When deserializing, we use SqlParser to parse the expression strings into SqlNode, then converts it to RexNode and apply the projections.
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