Introduction to MongoDB $log Operator

The MongoDB $log operator is used to return the logarithm of a specified number. In the aggregation pipeline, the $log operator can be used to calculate the logarithm of a specified field for statistical and data analysis purposes.

Syntax

The syntax of the $log operator is as follows:

{ $log: { <expression>, <base> } }

Where <expression> is the numerical expression to calculate the logarithm, and <base> is the base of the logarithm.

Use Cases

The $log operator is commonly used to calculate the logarithm of a specified field for statistical and data analysis purposes. For example, the $log operator can be used to calculate the logarithm of the quantity of products purchased by a user to analyze the distribution of purchase quantities.

Examples

Sample Data

Assume that there is a collection named sales that stores sales data. Each sales data record contains the following fields:

• product: product name.
• price: unit price.
• quantity: quantity.

Here is an example record:

{
  "product": "A",
  "price": 10,
  "quantity": 5
}

Example 1

The following example demonstrates how to use the $log operator to calculate the logarithm of the quantity of sales, assuming the base is 10.

db.sales.aggregate([
  {
    $project: {
      _id: 0,
      product: 1,
      price: 1,
      quantity: 1,
      log_quantity: { $log: { $sum: ["$quantity"] }, base: 10 }
    }
  }
])

The above aggregation pipeline first uses the $project stage to select the fields to be returned, then uses the $log operator to calculate the logarithm of the quantity of sales and stores the result in a field named log_quantity.

Example 2

The following example demonstrates how to use the $log operator to calculate the logarithm of the sales amount, assuming the base is 2.

db.sales.aggregate([
  {
    $project: {
      _id: 0,
      product: 1,
      price: 1,
      quantity: 1,
      total: { $multiply: ["$price", "$quantity"] },
      log_total: { $log: ["$total", 2] }
    }
  }
])

The above aggregation pipeline first uses the $project stage to select the fields to be returned, then uses the $multiply operator to calculate the sales amount and uses the $log operator to calculate the logarithm of the sales amount, and stores the result in a field named log_total.

Conclusion

Using the $log operator can easily perform logarithmic operations in MongoDB. It can help us perform mathematical calculations, such as calculating the ratio or proportion of indicators. When performing mathematical calculations, we should use the $log operator carefully to ensure that the data does not contain negative or zero values to avoid calculation errors.

In addition, the $log operator can only be used in the aggregation pipeline and can only be used for fields of numerical types. When using it, we should read the official documentation carefully and use it according to the actual situation.

Through the introduction in this article, readers should have a certain understanding of the $log operator and be able to use it flexibly in actual projects.