The kind of PlanNode][google.spanner.v1.PlanNode]. Distinguishes
between the two different kinds of nodes that can appear in a query
Denotes a Relational operator node in the expression tree.
Relational operators represent iterative processing of rows
during query execution. For example, a TableScan
operation that reads rows from a table.
Denotes a Scalar node in the expression tree.
Scalar nodes represent non-iterable entities in
the query plan. For example, constants or
arithmetic operators appearing inside predicate
expressions or references to column names.