The post_filter allows any filter that it holds to be executed as last filter, because of this the post_filter only has affect on the search hits and not facets.

There are several reasons why to specify filters as post_filter. One reason is to force expensive filters to be executed as last filter, so that these filters only operate on the docs that match with the rest of the query. An example of for what filter a post_filter should be used for this reason is the geo_distance filter. The geo_distance filter is in general an expensive filter to execute and to reduce the execution time for this filter, one can choose to specify it as post_filter, so it runs on documents that are very likely to be end up as matching documents.

Another important reason is when doing things like facet navigation, sometimes only the hits are needed to be filtered by the chosen facet, and all the facets should continue to be calculated based on the original query. The post_filter element within the search request can be used to accomplish it.

Note, this is different compared to creating a filtered query with the filter, since this will cause the facets to only process the filtered results.

For example, let’s create two tweets, with two different tags:

curl -XPUT 'localhost:9200/twitter/tweet/1' -d '
{
    "message" : "something blue",
    "tag" : "blue"
}
'

curl -XPUT 'localhost:9200/twitter/tweet/2' -d '
{
    "message" : "something green",
    "tag" : "green"
}
'

curl -XPOST 'localhost:9200/_refresh'

We can now search for something, and have a terms facet.

curl -XPOST 'localhost:9200/twitter/_search?pretty=true' -d '
{
    "query" : {
        "term" : { "message" : "something" }
    },
    "facets" : {
        "tag" : {
            "terms" : { "field" : "tag" }
        }
    }
}
'

We get two hits, and the relevant facets with a count of 1 for both green and blue. Now, let’s say the green facet is chosen, we can simply add a filter for it:

curl -XPOST 'localhost:9200/twitter/_search?pretty=true' -d '
{
    "query" : {
        "term" : { "message" : "something" }
    },
    "post_filter" : {
        "term" : { "tag" : "green" }
    },
    "facets" : {
        "tag" : {
            "terms" : { "field" : "tag" }
        }
    }
}
'

And now, we get only 1 hit back, but the facets remain the same.

Note, if additional filters are required on specific facets, they can be added as a facet_filter to the relevant facets.