For applications with many indexed fields, the norms cause memory problems both during indexing and querying.
This patch makes norms optional on a per-field basis, in the same way that term vectors are optional per-field.
Overview of changes:
- Field.omitNorms that defaults to false
- backward compatible lucene file format change: FieldInfos.FieldBits has a bit for omitNorms
- IndexReader.hasNorms() method
- During merging, if any segment includes norms, then norms are included.
- methods to get norms return the equivalent 1.0f array for backward compatibility
The patch was designed for backward compatibility:
- all current unit tests pass w/o any modifications required
- compatible with old indexes since the default is omitNorms=false
- compatible with older/custom subclasses of IndexReader since a default hasNorms() is provided
- compatible with older/custom users of IndexReader such as Weight/Scorer/explain since a norm array is produced on demand, even if norms were not stored
If this patch is accepted (or if the direction is acceptable), performance for scoring could be improved by assuming 1.0f when hasNorms(field)==false.