[MAHOUT-576] AbstractJDBCDiffStorage.updateItemPref is updating the AVG incorrectly in most cases

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.4
• Fix Version/s: 0.5
• Assignee: Sean R. Owen
• Reporter: Renaud Bruyeron
• Due: 14/Jan/11
• Created: 05/Jan/11 10:49
• Updated: 31/Jan/24 22:11
• Resolved: 15/Jan/11 16:30

Description

the JDBC version of the DiffStorage is not using a RunningAverage in the removePreference case, and ends up making incorrect calculations.
In a scenario where users are setting and removing a lot of preferences, the AVG stored in the diff table quickly diverges from the correct value because of this.

Right now, the input to updateItemPref comes from SlopeOneRecommender, and in the case of removePreference, it is the old preference value, not a delta. However, the code uses it as if it were a delta. Thus the calculation is off by PEER(removedpreference,userid)/count everytime a user removes a preference.

At first glance, the code should compute the old delta instead of the old preference, and use this in the updateItemPref