Elasticsearch questions: search, performance and caching

Question

I'm new to elasticsearch, have been reading their API and some things are not clear to me

1) It is said that filters are cached. what does that mean? if i send a query with a filter on it, what gets cached? The results of that query? If i send a different query with the same filter, will the cache help me somehow?
I know the question is kinda vague, but so is ElasticSearch's documentation for this.

2) Is there a real performance difference between a query matching a term X to the "_all" field or to a specific field? As far i understand, both queries will be compared against all documents that contain X in one of their fields, and the only difference is in how many fields will be matched against X, in these documents. is that correct?

Answer 1

1) For your first question take a look at this link. To quote from the post "Filters don’t score documents – they simply include or exclude. If a document matches a filter, it is represented with a one in the BitSet; otherwise a zero. This means that Elasticsearch can store an entire segment’s filter state (“who matches this particular filter?”) in a single, compact BitSet.

The first time Elasticsearch executes a filter, it parses Lucene segment data structures to determine what matches your filter. Instead of throwing away this information, it caches it inside a BitSet.

The next time the same filter is executed, Elasticsearch can reference the compact BitSet instead of the Lucene segments. This has huge performance benefits."

2) "The idea of the _all field is that it includes the text of one or more other fields within the document indexed. It can come very handy especially for search requests, where we want to execute a search query against the content of a document, without knowing which fields to search on. This comes at the expense of CPU cycles and index size."link So if you know what fields you are going to query use specifics fields to search on.