"description": "sub expression of field to aggregate"
}, {
"arg": "group_by",
"optional": true,
"description": "optional expression to use to group aggregate calculations"
}, {
"arg": "filter",
"optional": true,
"description": "optional expression to use to filter features used to calculate aggregate"
}, {
"arg": "order_by",
"optional": true,
"description": "optional expression to use to order features used to calculate aggregate. By default, the features will be returned in an unspecified order."