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.
The _all fields can be completely disabled. Explicit field mapping and object mapping can be excluded / included in the _all field. By default, it is enabled and all fields are included in it for ease of use.
One of the nice features of the _all field is that it takes into account specific fields boost levels. Meaning that if a title field is boosted more than content, the title (part) in the _all field will mean more than the content (part) in the _all field.
Here is a sample mapping:
{
"person" : {
"_all" : {"enabled" : true},
"properties" : {
"name" : {
"type" : "object",
"dynamic" : false,
"properties" : {
"first" : {"type" : "string", "store" : "yes", "include_in_all" : false},
"last" : {"type" : "string", "index" : "not_analyzed"}
}
},
"address" : {
"type" : "object",
"include_in_all" : false,
"properties" : {
"first" : {
"properties" : {
"location" : {"type" : "string", "store" : "yes", "index_name" : "firstLocation"}
}
},
"last" : {
"properties" : {
"location" : {"type" : "string"}
}
}
}
},
"simple1" : {"type" : "long", "include_in_all" : true},
"simple2" : {"type" : "long", "include_in_all" : false}
}
}
}
The _all fields allows for store, term_vector and analyzer (with specific index_analyzer and search_analyzer) to be set.
Highlighting
For any field to allow highlighting it has to be either stored or part of the _source field. By default _all field does not qualify for either, so highlighting for it does not yield any data.
Although it is possible to store the _all field, it is basically an aggregation of all fields, which means more data will be stored, and highlighting it might produce strange results.
