Allow sorting by multi-valued fields

[martijnvg]

Add sorting support for fields that have multiple values per document (For example: string array or double array).
By default when sorting on a multi-valued field the lowest or highest value will be picked from the field values depending on the sort order.

It is possible to define what value should be picked when a document has multiple values in a field via the sort_mode option. The following options can be specified:

• min - Pick the lowest value.
• max - Pick the highest value.
• sum - Use the sum of all values as sort value.
• avg - Use the average of all values as sort value.

Example usage

curl -XPOST 'localhost:9200/_search' -d '{
   "query" : {
    ...
   },
   "sort" : [
      {"price" : {"order" : "asc", "sort_mode" : "avg"}}
   ]
}'

In the above example the field price has multiple prices per document. In this case the result hits will be sort by price ascending based on the average price per document.

[clintongormley]

Any chance of making the min or max value configurable? eg:

sort: [ { foo: { order: "asc", value: "max"} ]