There is a trick to putting computed fields in the payload. Since a "computed field" by definition does not exist in the dataset, the technique required for their creation and use is to build the content of the INDEX beforehand.

The following example code (contained in IndexPayloadFetch.ECL) illustrates how to accomplish this by building the content of some computed fields (derived from related child records) in a TABLE on which the INDEX is built:

IMPORT $;

PersonFile := $.DeclareData.Person.FilePlus;
AcctFile   := $.DeclareData.Accounts;
IDXname := '~$.DeclareData::EXAMPLEDATA::KEYS::Person.PersonID.CompPay';

r1 := RECORD
  PersonFile.PersonID;
  UNSIGNED8 AcctCount := 0;
  UNSIGNED8 HighCreditSum := 0;
  UNSIGNED8 BalanceSum := 0;
  PersonFile.RecPos;
END;

t1 := TABLE(PersonFile,r1);
st1 := DISTRIBUTE(t1,HASH32(PersonID));


r2 := RECORD
  AcctFile.PersonID;
  UNSIGNED8 AcctCount := COUNT(GROUP);
  UNSIGNED8 HighCreditSum := SUM(GROUP,AcctFile.HighCredit);
  UNSIGNED8 BalanceSum := SUM(GROUP,AcctFile.Balance);
END;

t2 := TABLE(AcctFile,r2,PersonID);
st2 := DISTRIBUTE(t2,HASH32(PersonID));

r1 countem(t1 L, t2 R) := TRANSFORM
  SELF := R;
  SELF := L;
END;

j := JOIN(st1,st2,LEFT.PersonID=RIGHT.PersonID,countem(LEFT,RIGHT),LOCAL);

Bld := BUILDINDEX(j,
                  {PersonID},
                  {AcctCount,HighCreditSum,BalanceSum,RecPos},
                  IDXname,OVERWRITE);


i := INDEX(PersonFile,
           {PersonID},
           {UNSIGNED8 AcctCount,UNSIGNED8 HighCreditSum,UNSIGNED8 BalanceSum,RecPos},
           IDXname);

f := FETCH(PersonFile,i(PersonID BETWEEN 1 AND 100),RIGHT.RecPos);

Get := OUTPUT(f,ALL);

SEQUENTIAL(Bld,Get);

The first TABLE function gets all the key field values from the Person dataset for the INDEX and creates empty fields to contain the computed values. Note well that the RecPos virtual(fileposition) field value is also retrieved at this point.

The second TABLE function calculates the values to go into the computed fields. The values in this example are coming from the related Accounts dataset. These computed field values will allow the final payload INDEX into the Person dataset to produce these child recordset values without any additional code (or disk access).

The JOIN operation moves combines the result from two TABLEs into its final form. This is the data from which the INDEX is built.

The BUILDINDEX action writes the INDEX to disk. The tricky part then is to declare the INDEX against the base dataset (not the JOIN result). So the key to this technique is to build the INDEX against a derived/computed set of data, then declare the INDEX against the base dataset from which that data was drawn.

To demonstrate the use of a computed-field payload INDEX, this example code just does a simple FETCH to return the combined result containing all the fields from the Person dataset along with all the computed field values. In "normal" use, this type of payload key would generally be used in a half-keyed JOIN operation.