ex_data_table           Execute an 'rquery' pipeline with 'data.table'
                        sources.
ex_data_table_parallel
                        Execute an 'rquery' pipeline with 'data.table'
                        in parallel.
ex_data_table_step.default
                        default non-impementation.
ex_data_table_step.relop_drop_columns
                        Implement drop columns.
ex_data_table_step.relop_extend
                        Implement extend/assign operator.
ex_data_table_step.relop_natural_join
                        Natural join.
ex_data_table_step.relop_non_sql
                        Direct non-sql (function) node, not implemented
                        for 'data.table' case.
ex_data_table_step.relop_null_replace
                        Replace NAs.
ex_data_table_step.relop_order_expr
                        Order rows by expression.
ex_data_table_step.relop_orderby
                        Reorder rows.
ex_data_table_step.relop_project
                        Implement projection operator.
ex_data_table_step.relop_rename_columns
                        Rename columns.
ex_data_table_step.relop_select_columns
                        Implement drop columns.
ex_data_table_step.relop_select_rows
                        Select rows by condition.
ex_data_table_step.relop_set_indicator
                        Implement set_indicatoroperator.
ex_data_table_step.relop_sql
                        Direct sql node.
ex_data_table_step.relop_table_source
                        Build a data source description.
ex_data_table_step.relop_theta_join
                        Theta join (database implementation).
ex_data_table_step.relop_unionall
                        Bind tables together by rows.
make_dt_lookup_by_column
                        Lookup by column function factory.
rq_df_funciton_node     Helper to build data.table capable non-sql
                        nodes.
rq_df_grouped_funciton_node
                        Helper to build data.table capable non-sql
                        nodes.
rqdatatable-package     'rqdatatable': Relational Query Generator for
                        Data Manipulation Implemented by data.table
set_rqdatatable_as_executor
                        Set rqdatatable package as default rquery
                        executor
