Creating Mapping/Type in ElasticSearch

Basically two forms:

1. creating index while creating mapping

curl -XPUT localhost:9200/testIndex -d'
{
"mappings":{
"testType": {
    "properties": {
        "id": {
                "type": "string"
            },
            "xml": {
                "type" : "string", "index" : "no" , "include_in_all":"false"
            },
            "json": {
                "type": "nested"
            },
            "comment": {
                "type": "string"
            }
        }
    }
}
}'

2. creating or updating mapping in existing index

curl -XPUT localhost:9200/testIndex/_mapping/testType2 -d'
{

    "properties": {
        "id": {
                "type": "string"
            },
            "xml": {
                "type" : "string", "index" : "no" , "include_in_all":"false"
            },
            "json": {
                "type": "nested"
            },
            "comment": {
                "type": "string"
            }
        }

}'

Left most index key

It should be most selective(more distinct values) because this reduces the number of database pages that must be read by the database engine while traversing the index, in order to satisfy the query.

Keep in mind that SQL Server's index has an entry for each of the row in underlying table, no matter they are unique or duplicate.