The getter needs to check if the view is backing a secondary index.
Currentl it's done inside the handle_computed_column() method, but it's
more convenient if this bit is known during construction, so move it
there. There are no places that can change this property between
view_getter is created and the method in question is called.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
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