Calcite now has support for nested column structure in parsing and validation, by representing the inner-level columns as a RexFieldAccess based on a RexInputRef. Meanwhile it does not flatten the inner level structure in wildcard expansion, which would then cause an UnsupportedOperationException in Avatica.
The idea is to take into account this nested structure in column resolving, but to flatten the structure when translating to RelNode/RexNode.
For example, if the table structure is defined as
, it should be viewed as a flat type like
, so that:
1) Column reference "K0" is translated as $0
2) Column reference "F0.C1" is translated as $3
3) Wildcard "*" is translated as: $0, $1, $2, $3, $4, $5
4) Complex-column wildcard "F1.*", which is translated as $2, $3
And we would like to resolve columns based on the following rules (here we only consider the "suffix" part of the qualified names, which means the table resolving is already done by this time):
a) A two-part column name is matched with its first-level column name and its second-level column name. For example, "F1.C0" corresponds to $4; "F1,X" will throw a column not found error.
b) A single-part column name is matched against non-nested columns first, and if no matches, it is then matched against those second-level column names. For example, "C1" will be matched as "$1" instead of "$3", since non-nested columns have a higher priority; "C2" will be matched as "$5"; "C0" will lead to an ambiguous column error, since it exists under both "F0" and "F1".
c) We would also like to have a way for defining "default first-level column" so that it has a precedence in column resolving over other first-level columns. For example, if "F0" is defined as default, "C0" will not cause an ambiguous column error, but instead be matched as "$2".
d) Reference to first-level column only without wildcard is not allowed, e.g., "F1".