Prefix Query

Matches documents that have fields containing terms with a specified prefix. The prefix query maps to Lucene PrefixQuery. The following matches documents where the user field contains a term that starts with ki:

{
    "prefix" : { "user" : "sh" }
}

A boost can also be associated with the query:

{
    "prefix" : { "user" :  { "value" : "ki", "boost" : 2.0 } }
}

Or :

{
    "prefix" : { "user" :  { "prefix" : "ki", "boost" : 2.0 } }
}