Parses and evaluates a Postgres R-Aggregator initialization string.

Description

An RAggregator is a Postgres data-type whose functionality is implemented in R. Such an object is initialized by a user-specified Postgres string which is expected to create an R object (usually a closure) that can be used to incrementally compute some statistic by being passed each record within an SQL query one at a time. This function converts the Postgres initialization string into an R object which can be used as the aggregator.

Usage

evalPostgresAggregatorInitialization(txt)

Arguments

Value

An object which can be used in the plRaggregate procedural language within Postgres. This typically is a list returned from a closure definition. By default, the first element of the list should be the function that is called for each record and which updates the internal state of the closure. It is called with a single argument - the value of the record. The second function is typically called with no arguments to retrieve the final result when all the records have been processed by Postgres and passed to the update function. The positions of these functions can be changed depending on how the aggregator is defined within Postgres. It is much simpler to user this ordering.

Author(s)

Duncan Temple Lang

References

http://www.postgresql.org, http://www.omegahat.org/DBMS/Postgres/R

Examples

 evalPostgresAggregatorInitialization('jsumGenerator()')
 evalPostgresAggregatorInitialization('(function() { 
                                          minVal <- Inf
                                          update <- function(recordValue) {
                                            minVal <- min(minVal, recordValue)
                                          }
                                          getMinVal <- function() {
                                                minVal
                                          }

                                          return(list(update, getMinVal))
                                         })()')