Anshum Gupta created SOLR-6248:
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Summary: MoreLikeThis Query Parser
Key: SOLR-6248
URL: https://issues.apache.org/jira/browse/SOLR-6248
Project: Solr
Issue Type: New Feature
Reporter: Anshum Gupta
MLT Component doesn't let people highlight/paginate and the handler comes with
an cost of maintaining another piece in the config. Also, any changes to the
default (number of results to be fetched etc.) /select handler need to be
copied/synced with this handler too.
Having an MLT QParser would let users get back docs based on a query for them
to paginate, highlight etc. It would also give them the flexibility to use this
anywhere i.e. q,fq,bq etc.
A bit of history about MLT (thanks to Hoss)
MLT Handler pre-dates the existence of QParsers and was meant to take an
arbitrary query as input, find docs that match that
query, club them together to find interesting terms, and then use those
terms as if they were my main query to generate a main result set.
This result would then be used as the set to facet, highlight etc.
The flow: Query -> DocList(m) -> Bag (terms) -> Query -> DocList(y)
The MLT component on the other hand solved a very different purpose of
augmenting the main result set. It is used to get similar docs for each of the
doc in the main result set.
DocSet(n) -> n * Bag (terms) -> n * (Query) -> n * DocList(m)
The new approach:
All of this can be done better and cleaner (and makes more sense too) using an
MLT QParser.
An important thing to handle here is the case where the user doesn't have
TermVectors, in which case, it does what happens right now i.e. parsing stored
fields.
Also, in case the user doesn't have a field (to be used for MLT) indexed, the
field would need to be a TextField with an index analyzer defined. This
analyzer will then be used to extract terms for MLT.
In case of SolrCloud mode, '/get-termvectors' can be used after looking at the
schema (if TermVectors are enabled for the field). If not, a /get call can be
used to fetch the field and parse it.
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