[jira] [Created] (SOLR-5783) Can we stop opening a new searcher when the index hasn't changed?

["Hoss Man (JIRA)" <ji...@apache.org>]

Hoss Man created SOLR-5783:
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             Summary: Can we stop opening a new searcher when the index hasn't changed?
                 Key: SOLR-5783
                 URL: https://issues.apache.org/jira/browse/SOLR-5783
             Project: Solr
          Issue Type: Improvement
            Reporter: Hoss Man


I've been thinking recently about how/when we re-open searchers -- and what the overhead of that is in terms of caches and what not -- even if the underlying index hasn't changed.  

The particular real world case that got me thinking about this recently is when a deleteByQuery gets forwarded to all shards in a collection, and then the subsequent (soft)Commit (either auto or explicit) opens a new searcher -- even if that shard was completley uneffected by the delete.

It got me wondering: why don't re-use the same searcher when the index is unchanged?

>From what I can tell, we're basically 99% of the way there (in {{<nrtMode/>}})...

* IndexWriter.commit is already smart enough to short circut if there's nothing to commit
* SolrCore.openNewSearcher already uses DirectoryReader.openIfChanged to see if the reader can be re-used.
* for "realtime" purposes, SolrCore.openNewSearcher will return the existing searcher if it exists and the DirectoryReader hasn't changed

...The only reason I could think of for not _always_ re-using the same searcher when the underlying DirectoryReader is identical (ie: that last bullet above) is in the situation where the "live" schema has changed -- but that seems pretty trivial to account for.

Is there any other reason why this wouldn't be a good idea for improving performance?




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