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$multiply (aggregation)

Definition

$multiply

Multiplies numbers together and returns the result. Pass the arguments to $multiply in an array.

The $multiply expression has the following syntax:

{ $multiply: [ <expression1>, <expression2>, ... ] }

The arguments can be any valid expression as long as they resolve to numbers. For more information on expressions, see Expressions.

Starting in MongoDB 6.1 you can optimize the $multiply operation. To improve performance, group references at the end of the argument list. For example,

$multiply: [ 1, 2, 3, '$a', '$b', '$c' ]

Behavior

When input types are mixed, $multiply promotes the smaller input type to the larger of the two. A type is considered larger when it represents a wider range of values. The order of numeric types from smallest to largest is: integer → long → double → decimal

The larger of the input types also determines the result type unless the operation overflows and is beyond the range represented by that larger data type. In cases of overflow, $multiply promotes the result according to the following order:

If the larger input type is integer, the result type is promoted to long.
If the larger input type is long, the result type is promoted to double.
If the larger type is double or decimal, the overflow result is represented as + or - infinity. There is no type promotion of the result.

Example

Consider a sales collection with the following documents:

db.sales.insertMany( [
   { _id : 1, "item" : "abc", "price" : 10, "quantity": 2, date: ISODate("2014-03-01T08:00:00Z") },
   { _id : 2, "item" : "jkl", "price" : 20, "quantity": 1, date: ISODate("2014-03-01T09:00:00Z") },
   { _id : 3, "item" : "xyz", "price" : 5, "quantity": 10, date: ISODate("2014-03-15T09:00:00Z") }
] )

The following aggregation uses the $multiply expression in the $project pipeline to multiply the price and the quantity fields:

db.sales.aggregate(
   [
     { $project: { date: 1, item: 1, total: { $multiply: [ "$price", "$quantity" ] } } }
   ]
)

The operation returns the following results:

{ "_id" : 1, "item" : "abc", "date" : ISODate("2014-03-01T08:00:00Z"), "total" : 20 }
{ "_id" : 2, "item" : "jkl", "date" : ISODate("2014-03-01T09:00:00Z"), "total" : 20 }
{ "_id" : 3, "item" : "xyz", "date" : ISODate("2014-03-15T09:00:00Z"), "total" : 50 }

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