[SOLR-4445] Join queries should execute their "from" queries on all shards

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 4.1
• Component/s: query parsers, SolrCloud
• Assignee: Unassigned
• Reporter: Colin Bartolome
• Created: 11/Feb/13 23:08
• Updated: 11/Feb/13 23:36

Description

When running join queries on a collection with multiple shards, the "from" side of the query is executed on the shard that serves the request only, instead of on all shards. The matching documents are then passed to the "to" side of the query. This leads to the overall result set being a subset of what it would be, if the join query were run on a collection with only one shard.

That is, a four-shard collection will, on average, return 25% of the results a single-shard collection would.

The code should execute the "from" side of the query on all available shards before passing those matching documents to the "to" side of the query.

Note: LUCENE-3759 proposes an upgrade to JoinUtil to support joining when the documents matched by the "from" side of the query exist on multiple shards. Solr does not use that class for joining (nor does anything else?), so this would have to be implemented separately.