another idea for updatable fields [LUCENE-4272]

I've been reviewing the ideas for updatable fields and have an alternative
proposal that I think would address my biggest concern:

- not slowing down searching

When I look at what Solr and Elasticsearch do here, by basically reindexing from stored fields, I think they solve a lot of the problem: users don't have to "rebuild" their document from scratch just to update one tiny piece.

But I think we can do this more efficiently: by avoiding reindexing of the unaffected fields.

The basic idea is that we would require term vectors for this approach (as the already store a serialized indexed version of the doc), and so we could just take the other pieces from the existing vectors for the doc.

I dont think we should discard the idea because vectors are slow/big today, this seems like something we could fix.

Personally I like the idea of not slowing down search performance to solve the problem, I think we should really start from that angle and work towards making the indexing side more efficient, not vice-versa.

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Migrated from [LUCENE-4272](https://issues.apache.org/jira/browse/LUCENE-4272) by Robert Muir (@rmuir), 4 votes, updated May 02 2013
Linked issues:
- #5664
- #5328
- #2963

No files, tests, or entry points are named. Start by reviewing the linked issues and the proposal's requirement for term vectors, then determine whether the updatable-fields design has been accepted; completion criteria are not defined in this issue.

Written by the indexing model from the issue text.