Add system column support to the USING clause #3
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Add system column support to the USING clause
Problem: The parser can't handle USING joins with system columns, such as xmin.
Solution:
constructing Var objects, as it correctly handles system columns.
invoked in scanNSItemForColumn().
user-defined column indices into l_colnos and r_colnos.
with system attributes, analogous to buildVarFromNSColumn(), since
scanNSItemForColumn() is complex and has side effects.
with system attributes.
res_nscolumns is allocated with system columns in mind.