On 3/21/2013 3:07 PM, Shawn Heisey wrote:
This might be a requirement of the lower-level Lucene API, or it might
be a requirement that was instituted at the Solr level because a problem
was found when docs did not contain the field. Google seems reluctant
to tell me, and I haven't figured out the right way to ask.
Some poking around the Lucene API has turned up an interesting notation
on all the different types on DocValues:
http://lucene.apache.org/core/4_1_0/core/org/apache/lucene/index/DocValues.Type.html#FIXED_INTS_16
They all say that if a value isn't present, zero (or an empty string) is
assumed, and that there is no way to distinguish this from the same
value that is intentionally indexed.
So it appears that Solr *could* use docValues without the "required or
default value" restriction, but there is a strong possibility that the
behavior will not be what the user expects. When docValues is not
turned on, there is a clear difference between a default value and a
missing field. The sort mechanism without docValues can sort documents
with the field missing either before or after the other values. That
would be impossible with docValues.
By including the restriction, the dev team has made it less likely that
the Solr admin will be surprised by the new behavior, because they have
to change the field definition to make docValues work.
Thanks,
Shawn
