Query multiple collections with different fields in solr

Question

Given the following (single core) query\'s:

http://localhost/solr/a/select?indent=true&q=*:*&rows=100&start=0&wt=json
http://localhost/solr/b         


        

Answer 1

What you need is - what I call - a unification core. That schema itself will have no content, it is only used as a sort of wrapper to unify those fields you want to display from both cores. In there you will need

• a schema.xml that wraps up all the fields that you want to have in your unified result
• a query handler that combines the two different cores for you

An important restriction beforehand taken from the Solr Wiki page about DistributedSearch

Documents must have a unique key and the unique key must be stored (stored="true" in schema.xml) The unique key field must be unique across all shards. If docs with duplicate unique keys are encountered, Solr will make an attempt to return valid results, but the behavior may be non-deterministic.

As example, I have shard-1 with the fields id, title, description and shard-2 with the fields id, title, abstractText. So I have these schemas

schema of shard-1

schema of shard-2

To unify these schemas I create a third schema that I call shard-unification, which contains all four fields.

Now I need to make use of this combined schema, so I create a query handler in the solrconfig.xml of the solr-unification core

  
    edismax
    *:*
    id title description abstractText
    *,score
    100%
  

That's it. Now some index-data is required in shard-1 and shard-2. To query for a unified result, just query shard-unification with appropriate shards param.

http://localhost/solr/shard-unification/select?q=*:*&rows=100&start=0&wt=json&shards=localhost/solr/shard-1,localhost/solr/shard-2

This will return you a result like

{
  "responseHeader":{
    "status":0,
    "QTime":10},
  "response":{"numFound":2,"start":0,"maxScore":1.0,"docs":[
      {
        "id":1,
        "title":"title 1",
        "description":"description 1",
        "score":1.0},
      {
        "id":2,
        "title":"title 2",
        "abstractText":"abstract 2",
        "score":1.0}]
  }}

Fetch the origin shard of a document

If you want to fetch the originating shard into each document, you just need to specify [shard] within fl. Either as parameter with the query or within the requesthandler's defaults, see below. The brackets are mandatory, they will also be in the resulting response.

  
    edismax
    *:*
    id title description abstractText
    *,score,[shard]
    100%
  

Working Sample

If you want to see a running example, checkout my solrsample project on github and execute the ShardUnificationTest. I have also included the shard-fetching by now.