index and query items in an array with mango query for cloudant and couchdb 2.0

问题

I have the following db structure:

{"_id": "0096874","genre": ["Adventure","Comedy", "Sci-Fi" ]}
{"_id": "0099088","genre": ["Comedy", "Sci-Fi", "Western"]}

and like to query it like I could do in mongodb

db.movies.find({genre: {$in: ["Comedy"]}})

It works when i use a text index for the whole database, but that seems very wasteful:

// index
    {
      "index": {},
      "type": "text"
    }
//query
{
  "selector": {
    "genre": {
      "$in": ["Comedy"]
    }
  },
  "fields": [
    "_id",
    "genre"
  ]
}

The following index does not work:

{
  "index": {
    "fields": [
      "genre"
    ]
  },
  "type": "json"
}

What is the correct index for cloudant query, which does not index the whole db? Thanks for your help

回答1:

You had it almost correct. Your index is right, but you need to throw in a selector to get all IDs https://cloudant.com/blog/mango-json-vs-text-indexes/.

This isn't a great solution performance-wise, as Tony says,

The reason this works is, again, because Mango performs the above $in operation as a filtering mechanism against all the documents. As we saw in the conclusion of the previous section on JSON syntax, the performance tradeoff with the query above is that it, essentially, performs a full index scan and then applies a filter.

{
  "selector": {
    "_id": {
      "$gt": null
    }, 
    "genre": {
      "$in": ["Western"]
    }
  },
  "fields": [
    "_id",
    "genre"
  ],
  "sort": [
    {
      "_id": "asc"
    }
  ]
}

来源：https://stackoverflow.com/questions/34107333/index-and-query-items-in-an-array-with-mango-query-for-cloudant-and-couchdb-2-0