In simple usage Solr DataImportHandler deletes old document and replace it with new. Is it possible to use DataImportHandler for updating only part of existing document fields and left other intact? I want to do second data import for part of fields, which will use light SQL queries.
This link would help you to resolve the issue.
Look at http://solr.pl/en/2012/07/09/solr-4-0-partial-documents-update/