Prodataset child table records are filled with more records if the child table contains a date field with a null value in an index

The child table in the prodataset relation contains an index that includes a date field:

/* child */
CREATE TEMP-TABLE hTempTable2.
hTempTable2:ADD-NEW-FIELD('CalculationMethod':U,'logical':U,?,'Satz/Fakt':U,TRUE,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('Deductible':U,'decimal':U,?,'zz9.99%':U,100.0,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('DeductibleVAT':U,'decimal':U,?,'zz9.99%':U,100.0,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('TaxRate_Obj':U,'character':U,?,'x(60)':U,'':U,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('TaxTerritory_Obj':U,'character':U,?,'x(60)':U,'':U,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('TaxCategory':U,'logical':U,?,'MwSt/VSt':U,TRUE,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('Valid_from':U,'date':U,?,'99.99.9999':U,?,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('Valid_to':U,'date':U,?,'99.99.9999':U,?,'':U,'':U).
hTempTable2:ADD-NEW-FIELD('TaxRate':U,'decimal':U,?,'z9.999999%':U,0,'':U,'':U).
hTempTable2:ADD-NEW-INDEX('Main':U, YES, YES).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'SBM_TaxTerritory_Obj':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'Valid_from':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'Valid_to':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'TaxRate':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'TaxCategory':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'CalculationMethod':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'Deductible':U).
hTempTable2:ADD-INDEX-FIELD('Main':U, 'DeductibleVAT':U).

The source buffer database field for this temp table that is used when filling the prodataset, has a null value.
When the dataset it filled this null value can cause the child temp table buffer to have more records than it should have.
In this specific case after the FILL method is run on the prodataset, the child buffer contained two records with exactly the same values, when it should have only contained one.  
The date field with the null value doesn't have any other reference in the code, so it's not clear why this particular date field is causing the problem.
 

Assigning a valid data value (e.g. TODAY) to the datasource buffer record resolves the problem.  
Removing the date field from the child temp table index resolves the problem.  

This is expected behavior.  If a field that participates in a unique index contains null values, a duplicate record with a null value in the same field does not generate a unique key conflict.  For example the following code works without error:

DEFINE TEMP-TABLE tt1 
  FIELD f1 AS CHARACTER
  FIELD F2 AS CHARACTER 
  INDEX IDX1 AS UNIQUE PRIMARY F1 F2 ASC.  
  
CREATE TT1.
ASSIGN 
  F1 = "1"
  F2 = ?.
  
CREATE TT1.
ASSIGN 
  F1 = "1"
  F2 = ?.   
  
FOR EACH TT1:
  DISPLAY TT1.
END.

This behaviour is documented in OpenEdge Web Paper: Use ABL Database Triggers and Indexes:Version 12.2 17, "Indexes and Unknown values", as follows:

Indexes and Unknown values
If the default FILL-MODE of "MERGE is used for the buffer object, then because the null value does not generate a unique key conflict, the duplicate record is added to the prodataset.  


 

Replace the default FILL-MODE "MERGE" for the buffer object with "REPLACE":

...

hTempTable2:DEFAULT-BUFFER-HANDLE:FILL-MODE="REPLACE".

hDataset:GET-BUFFER-HANDLE('datasetBuffer-001':U):fill() .
...

 

References to Other Documentation:
OpenEdge Web Paper: Use ABL Database Triggers and Indexes:Version 12.2 17, "Indexes and Unknown values"
OpenEdge Documentation: Datasets:Version 12.2, Chapter 4: Populate a dataset using FILL, "Use FILL-MODE to control how a dataset is populated"