An executor that materializes the result of a set of expressions.
The expressions are evaluated on Insert/UpdateInsert rows and the results are stored in a state table.
When a Delete/UpdateDelete row is received, the corresponding result row is popped from the state table
without the need to re-evaluate the expressions.
ProjectExecutor project data with the expr. The expr takes a chunk of data,
and returns a new data chunk. And then, ProjectExecutor will insert, delete
or update element into next operator according to the result of the expression.
ProjectSetExecutor projects data with the expr. The expr takes a chunk of data,
and returns a new data chunk. And then, ProjectSetExecutor will insert, delete
or update element into next operator according to the result of the expression.