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https://issues.apache.org/jira/browse/SOLR-17131?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17811367#comment-17811367
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Kevin Risden commented on SOLR-17131:
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https://github.com/apache/solr/pull/2221
> Optimize rows=0 queries since score/sort isn't necessary
> --------------------------------------------------------
>
> Key: SOLR-17131
> URL: https://issues.apache.org/jira/browse/SOLR-17131
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Kevin Risden
> Assignee: Kevin Risden
> Priority: Major
>
> If a query has rows=0 Solr does not need to sort the results or even compute
> the score. This means the filterCache can be more efficient to compute the
> result and Solr doesn't spend time on unnecessarily sorting and scoring large
> result sets for it to be thrown away.
> This was found while looking at
> https://issues.apache.org/jira/browse/SOLR-14765 and trying to reason through
> the case of rows=0 that are sometimes used to just determine count of
> matching results. This change adds the check for rows=0 before other checks
> to ensure that we can short circuit scoring even if there is a scoring query.
> We also don't add the query to the filter cache since that could result in
> filter cache thrashing.
> In some tests, we found that queries with rows=0 p99 improved by 50% and CPU
> dropped by 5%.
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