Performance: Distributed Search should skip GET_FIELDS stage if EXECUTE_QUERY 
stage gets all fields

                 Key: SOLR-1880
             Project: Solr
          Issue Type: Improvement
          Components: search
    Affects Versions: 1.4
            Reporter: Shawn Smith

Right now, a typical distributed search using QueryComponent makes two HTTP 
requests to each shard:
# STAGE_EXECUTE_QUERY executes one HTTP request to each shard to get top N ids 
and sort keys, merges the results to produce a final list of document IDs 
# STAGE_GET_FIELDS executes a second HTTP request to each shard to get the 
document field values for the final list of document IDs (PURPOSE_GET_FIELDS).

If the "fl" param is just "id" or just "id,score", all document data to return 
is already fetched by STAGE_EXECUTE_QUERY.  The second STAGE_GET_FIELDS query 
is completely unnecessary.  Eliminating that 2nd HTTP request can make a big 
difference in overall performance.

Also, the "fl" param only gets id, score and sort columns, it would probably be 
cheaper to fetch the final sort column data in STAGE_EXECUTE_QUERY which has to 
read the sort column data anyway, and skip STAGE_GET_FIELDS.

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