[GitHub] garrensmith commented on issue #1549: Slow `_find` query when no documents match selector

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garrensmith commented on issue #1549: Slow `_find` query when no documents match selector
URL: https://github.com/apache/couchdb/issues/1549#issuecomment-411657758
 
 
   @wohali is right that the query that takes long is doing a full index scan on `_all_docs` as you can see from the stats:
   
   ```
   "execution_stats": {
           "total_keys_examined": 0,
           "total_docs_examined": 24856,
           "total_quorum_docs_examined": 0,
           "results_returned": 0,
           "execution_time_ms": 34390.737
       }
   ```
   
   The one that is faster is because it uses the `"publishTime"` index. Mango needs a way to know to select that index on your queries. The best way to do that is to add a publishTime into your selector even if its something like `publishTime: {$gt: null}`. That way it should always select that index. You can also add in the `use_index` to specify that index.
   
   But running _explain on your queries will give you some information around what is happening internally and which index is being selected.
   
   Since you using `league` in your query it might be worth adding that into the index as well even if you only use it with the $in selector. That way only the docs with a league field and a publishTime field will be in your index.

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