Queries vs Filters - Order of execution

Question

I've read this question and a colleague of mine made me doubt:

In a filtered query, when is the filter applied ? Before or after executing the query ? When is the result cached ?

If the filter is applied beforehand, wouldn't it be a a good thing to duplicate the query part in the filters ? If the filter is applied afterward, then i'm having trouble understanding what is cached.

Answer 1

Luckily, ES provides two types of filters for you to work with:

{
  "query" : {
    "field" : { "title" : "Catch-22" }
  },
  "filter" : {
    "term" : { "year" : 1961 }
  }
}


{
  "query": {
    "filtered" : {
      "query" : {
        "field" : { "title" : "Catch-22" }
      },
      "filter" : {
        "term" : { "year" : 1961 }
      }
    }
  }
}

In the first case, filters are applied to all documents found by the query. In the second case, the documents are filtered before the query runs. This yields better performance.

Quoted from: http://www.packtpub.com/elasticsearch-server-for-fast-scalable-flexible-search-solution/book

About cache, I'm not sure about cache mechanism of filters. My guessing would be: First case, since the filter is against a set of results returned by query, the cache is kind of specific for this return set. Second case, the filter is applied first, the cache is stored for the indices you checked against, thus, this cache is more reusable because it does not rely on the content of the query, but at larger memory cost and query time for first time(before the cache is generated).

Answer 2

Let me explain you search query execution-

First thing is that there is always a Complete document of reference in which you want to search.

If you have filter query included with search query then it will just make that document smaller or in other words filter queries are cached results of same query. Now you have a smaller tree to search from with your query text.

Now your doubt part- Duplicating the query in filters will only increase overhead of cache mechanism and There are many guide lines on what to include in filter query and what to ignore. It's all play of relevancy.