Highlighting matched results on _all fields - lucene

Question

I want the matched results to be highlighted. This works for me if I mention the field name and it returns the highlighted text, however if I give the field as "_all", it is not returning any value.
This works for me:
curl -XGET "http://localhost:9200/my_index/my_type/_search?q=stackoverflow&size=999" -d '{
"highlight":{
"fields":{
"my_field":{}
}
}
}'
This returns the expected value as follows:
[highlight] => stdClass Object ( [my_field] => Array ( [0] => stackoverflow is the best website for techies ) )
But when I give this:
curl -XGET "http://localhost:9200/my_index/my_type/_search?q=stackoverflow&size=999" -d '{
"highlight":{
"fields":{
"_all":{}
}
}
}'
I get null value/no result.
[highlight] => stdClass Object ( [_all] => Array () )
How do I get it to work on any field so that I don't have to mention the field name?

Answer 1

To avoid the need to add _all as a stored field in your index
An alternative quick fix: use * instead of _all:
curl -XGET "http://localhost:9200/my_index/my_type/_search?q=stackoverflow&size=999" -d '{
"highlight":{
"fields":{
"*":{}
}
}
}'

Answer 2

If you are using ES 2.x then you need to set require_field_match option to false due to changes made, From the doc
The default value for the require_field_match option has changed from false to true, meaning that the highlighters will, by default, only take the fields that were queried into account.
This means that, when querying the _all field, trying to highlight on
any field other than _all will produce no highlighted snippets.
"highlight": {
"fields": {
"*": {}
},
"require_field_match": false
}

Answer 3

You need to map the _all field as stored. The mapping below should do the trick. Note though that this will add to the index size.
{
"my_type": {
"_all": {
"enabled": true,
"store": "yes"
}
}}

Answer 4

This library has functions for query highlighting including highlighting across all fields. The README explains how to create an elasticsearch index with _all field stored etc:
https://github.com/niranjan-uma-shankar/Elasticsearch-PHP-class