[[search-request-search-type]]
=== Search Type

There are different execution paths that can be done when executing a
distributed search. The distributed search operation needs to be
scattered to all the relevant shards and then all the results are
gathered back. When doing scatter/gather type execution, there are
several ways to do that, specifically with search engines.

One of the questions when executing a distributed search is how much
results to retrieve from each shard. For example, if we have 10 shards,
the 1st shard might hold the most relevant results from 0 till 10, with
other shards results ranking below it. For this reason, when executing a
request, we will need to get results from 0 till 10 from all shards,
sort them, and then return the results if we want to ensure correct
results.

Another question, which relates to the search engine, is the fact that each
shard stands on its own. When a query is executed on a specific shard,
it does not take into account term frequencies and other search engine
information from the other shards. If we want to support accurate
ranking, we would need to first gather the term frequencies from all
shards to calculate global term frequencies, then execute the query on
each shard using these global frequencies.

Also, because of the need to sort the results, getting back a large
document set, or even scrolling it, while maintaining the correct sorting
behavior can be a very expensive operation. For large result set
scrolling, it is best to sort by `_doc` if the order in which documents
are returned is not important.

Elasticsearch is very flexible and allows to control the type of search
to execute on a *per search request* basis. The type can be configured
by setting the *search_type* parameter in the query string. The types
are:

[[query-then-fetch]]
==== Query Then Fetch

Parameter value: *query_then_fetch*.

The request is processed in two phases. In the first phase, the query
is forwarded to *all involved shards*. Each shard executes the search
and generates a sorted list of results, local to that shard. Each
shard returns *just enough information* to the coordinating node
to allow it merge and re-sort the shard level results into a globally
sorted set of results, of maximum length `size`. 

During the second phase, the coordinating node requests the document
content (and highlighted snippets, if any) from *only the relevant
shards*.

NOTE: This is the default setting, if you do not specify a `search_type`
      in your request.

[[dfs-query-then-fetch]]
==== Dfs, Query Then Fetch

Parameter value: *dfs_query_then_fetch*.

Same as "Query Then Fetch", except for an initial scatter phase which
goes and computes the distributed term frequencies for more accurate
scoring.


