Aggregate

Executes an external function for all tuples of a relation.

Usage

aggregate (indata, func-expr, indata-attr, new-attr : outdata)

Details

All tuples of the relation indata are passed to the function specified in func-expr. The function's return value is then joined back to the packed relation indata as new-attr to produce outdata (i.e., a dependent join). The packed relation indata is contained in the new attribute indata-attr. A func-expr is a call to the remote function that specifies how attributes in the indata relation are to be passed.

Notes

Functions should be written in Java and adhere to the following constraints:
- Functions must be statically declared.
- Functions must consist of only one java.util.ArrayList input parameter and must return either java.lang.String, java.util.Date, int, or double types. The values returned are automatically converted into CHAR, DATE, NUMBER, and NUMBER (respectively). Any other types are automatically coverted into strings (CHAR).
Function calls can accept multiple parameters (attributes) from the input relation indata - use commas to separate each parameter.

Known Bugs

Functions cannot accept literals.

Example

Using the input:

      RELATION books: title char, author char, pub_date date, pages number
      Title1|Author1|09-01-1991|34
      Title2|Author2|12-23-1954|479
      Title3|Author2|05-09-2002|733
      Title4|Author3|01-01-1968|32
      Title5|Author2|07-03-2001|1152

and the externally compiled MyFunctions.java that contains:

      import java.text.*;
      import java.util.*;
  
      public class MyFunctions
      {
        public static double average (ArrayList a_data)
        {
          double sum = 0;
          for (int i=0; i<a_data.size(); i++) {
            sum += Double.parseDouble(a_data.get(i).toString());
          }
          return sum/((double)a_data.size());
        }
      }

when executing the plan:

      PLAN test
      {
        INPUT: stream books
        OUTPUT: stream result
  
        BODY
        {
          aggregate (books, "MyFunctions.average(pages)", "old_rel", "avg_pages" : post-agg)
          project (post-agg, "avg_pages" : result)
        }
      }

generates the following output:

      ----------------------------------------------
      RELATION: uaggregate1_result
         attrs: avg_pages
      ----------------------------------------------
      486.0
      ----------------------------------------------