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https://issues.apache.org/jira/browse/SOLR-11891?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16338160#comment-16338160
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David Smiley commented on SOLR-11891:
-------------------------------------

convertLuceneDocToSolrDoc should be innocent enough; it merely iterates the 
IndexableField instances in a collection and potentially calls name() on them 
which is always a simple getter.  We can't avoid putting all fields on the 
SolrDocument because of the _potential_ for a document transformer to need it 
-- and that's not known by RetrieveFieldsOptimizer.  But it should be innocent 
enough because if no such transformer requests the value, then it shouldn't 
actually be loaded (it's lazy).

Something else must be going on.  I wonder who the caller is and what doc 
transformers you may have.

> BinaryResponseWriter fetches unnecessary fields
> -----------------------------------------------
>
>                 Key: SOLR-11891
>                 URL: https://issues.apache.org/jira/browse/SOLR-11891
>             Project: Solr
>          Issue Type: Improvement
>      Security Level: Public(Default Security Level. Issues are Public) 
>          Components: Response Writers
>    Affects Versions: 5.4, 6.4.2, 6.6.2
>            Reporter: wei wang
>            Priority: Major
>
> We observe that solr query time increases significantly with the number of 
> rows requested,  even all we retrieve for each document is just fl=id,score.  
> Debugged a bit and see that most of the increased time was spent in 
> BinaryResponseWriter,  converting lucene document into SolrDocument.  Inside 
> convertLuceneDocToSolrDoc():   
> [https://github.com/apache/lucene-solr/blob/df874432b9a17b547acb24a01d3491839e6a6b69/solr/core/src/java/org/apache/solr/response/DocsStreamer.java#L182]
>  
> I am a bit puzzled why we need to iterate through all the fields in the 
> document. Why can’t we just iterate through the requested field list?    
> [https://github.com/apache/lucene-solr/blob/df874432b9a17b547acb24a01d3491839e6a6b69/solr/core/src/java/org/apache/solr/response/DocsStreamer.java#L156]
>  
> e.g. when pass in the field list as 
> sdoc = convertLuceneDocToSolrDoc(doc, rctx.getSearcher().getSchema(), fnames)
> and just iterate through fnames,  there is a significant performance boost in 
> our case.  



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