Summary: in this tutorial, you will learn how to use the PostgreSQL LOG() function to calculate the logarithm of a number

Introduction to the PostgreSQL LOG() function

In PostgreSQL, the LOG() function allows you to calculate the logarithm of a number. PostgreSQL offers two LOG() functions:

Base-10 logarithm: The base-10 logarithm is the most commonly used logarithm in science and engineering applications.

Here’s the syntax of the LOG() function that calculates the base-10 logarithm of a number:

LOG(n)

In this syntax:

The LOG() function returns the base-10 logarithm with the same type as the type of the input number (n), which is numeric and double precision respectively. If n is NULL the LOG() function returns NULL.

If n is a text string, the LOG() function will attempt to convert it into a number before calculating the logarithm. It raises an error if the conversion fails.

The second LOG() function allows you to calculate the logarithm of a number with a specified base:

LOG(b, n)

In this syntax:

  • b is the base of the logarithm. b can be a value of the numeric type.
  • n has the same meaning in the LOG(n) function.

To calculate the natural logarithm of a number, you use the LN() function instead.

PostgreSQL LOG() function examples

Let’s take some examples of using the LOG() function.

1) Basic PostgreSQL LOG() function examples

The following example uses the LOG() function to calculate the base-10 logarithm of 100:

SELECT LOG(100);

Output:

log
-----
   2

The following statement uses the second form of the LOG() function to calculate the base-10 logarithm of 100:

SELECT LOG(10,100);

Output:

log
--------------------
 2.0000000000000000

The following statement uses the LOG() function to calculate the base-2 logarithm of 8:

SELECT LOG(2,8);

Output:

log
--------------------
 3.0000000000000000

2) Using LOG() function with text

The following statement uses the LOG() function to calculate the base-2 logarithm of the text '64':

SELECT LOG(2, '64');

Output:

log
--------------------
 6.0000000000000000
(1 row)

In this example, the LOG() function converts the text '64' into a number and calculate the base-2 logarithm of 64.

The following example raises an error because the LOG() function cannot convert the string '64x' into a number for calculation:

SELECT LOG('64x');

Error:

ERROR:  invalid input syntax for type double precision: "64x"
LINE 1: SELECT LOG('64x');                  ^

Summary

  • Use LOG(n) function to calculate the base-10 logarithm of the number n.
  • Use LOG(b, n) function to calculate the logarithm of the number n with the base b.