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Whati is a filter

A filter is an object that defines criteria for selecting objects
Filters are instances of PntSqlFilter, PntSqlJoinFilter or PntSqlCombiFilter (situated in the pnt/db/query classFolder). They are usually used for generating SQL that is used to retrieve the objects from the database that meet the criteria defined by the filter. 

Filters can also be used by peanuts to select other peanuts. The peanuts that use the filter then must implement the actual selection. This is the case with global filters.