In MongoDB, the $log10 aggregation pipeline operator calculates the log base 10 of a number and returns the result as a double.

$log10 accepts any valid expression that resolves to a non-negative number.

The $log10 operator was introduced in MongoDB 3.2.

Example

Suppose we have a collection called test with the following document:

{ "_id" : 1, "data" : 0.5 }

We can use the $log10 operator to return the log base 10 of the data field:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        result: { $log10: "$data" }
      }
    }
  ]
)

Result:

{ "result" : -0.3010299956639812 }

Out of Range Values

As mentioned, the $log10 operator accepts any valid expression that resolves to a non-negative number. Values outside of that range will cause an error.

Suppose we add the following document to our collection:

{ "_id" : 2, "data" : -3 }

Let’s run the the $log10 operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 2 } },
    { $project: { 
        _id: 0,
        result: { $log10: "$data" }
      }
    }
  ]
)

Result:

uncaught exception: Error: command failed: {
	"ok" : 0,
	"errmsg" : "$log10's argument must be a positive number, but is -3",
	"code" : 28761,
	"codeName" : "Location28761"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:618:17
assert.commandWorked@src/mongo/shell/assert.js:708:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1046:12
@(shell):1:1

Null Values

Null values return null when using the $log10 operator.

Suppose we add the following document to our collection:

{ "_id" : 3, "data" : null }

Let’s run the the $log10 operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 3 } },
    { $project: { 
        _id: 0,
        result: { $log10: "$data" }
      }
    }
  ]
)

Result:

{ "result" : null }

We can see that the result is null.

NaN Values

If the argument resolves to NaN, $log10 returns NaN.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        result: { $log10: 1 * "$data" }
      }
    }
  ]
)

Result:

{ "result" : NaN }

Non-Existent Fields

If the $log10 operator is applied against a field that doesn’t exist, null is returned.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        result: { $log10: "$name" }
      }
    }
  ]
)

Result:

{ "result" : null }