[GitHub] [couchdb] garrensmith commented on issue #2856: $or and $in operators do not seem to be using indices

[GitBox <gi...@apache.org>]

garrensmith commented on issue #2856:
URL: https://github.com/apache/couchdb/issues/2856#issuecomment-658074431


   The `$in` can be used in an index. Say for the example of the index:
   
   ```
   {
      "index": {
         "fields": [
            "root.input"
         ]
      },
      "name": "foo-json-index",
      "type": "json"
   }
   ```
   With selector:
   ```
   {
      selector: {
        input.root: "aimer",
        input.option: {
          $in: ["option1", "option2"]
       }
      }
   }
   ```
   Mango will choose the above index because you have `input.root` in your selector. Then when it reads the docs from the index it will filter them out based on your fields in the `$in` section. 
   
   In another case if you had an index like this:
   
   ```
   {
      "index": {
         "fields": [
            "input.option"
         ]
      },
      "name": "foo-json-index",
      "type": "json"
   }
   ```
   
   And the same query:
   
   ```
   {
      selector: {
        input.root: "aimer",
        input.option: {
          $in: ["option1", "option2"]
       }
      }
   }
   ```
   
   Then mango will read everything from the `input.option` index and then filter each document with the full selector. 
   The better option is to have the first index as it will reduce the number of documents that need to be fetched from the index.
   Hope that clears it up a bit more.


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