PL/pgSQL Constants

Summary: in this tutorial, you will learn about PL/pgSQL constants whose values cannot be changed.

Introduction to PL/pgSQL constants

In PL/pgSQL, constants are identifiers whose values cannot be changed during the execution of the code.

Typically, you use constants to assign meaningful names to values that remain constants throughout the execution of a block, a function, or a stored procedure.

The following are the reasons to use constants:

First, constants make code more readable and maintainable. Suppose you have the following formula:

selling_price = net_price + net_price * 0.1;

In this formula, the magic value 0.1 does not convey any meaning.

However, when using the following formula, the meaning for determining the selling price becomes clear:

selling_price = net_price + net_price * vat;

Second, constants help reduce maintenance effort.

Suppose you have a formula that calculates the selling price throughout a function. When the VAT changes from 0.1 to 0.12, you’ll need to update all of these hard-coded values.

By using a constant, you only need to modify its value in one place where you define the constant.

So how do you define a constant in PL/pgSQL?

Defining constants

To define a constant in PL/pgSQL, you use the following syntax:

constant_name constant data_type = expression;

In this syntax:

First, specify the name of the constant. The name should be as descriptive as possible.
Second, add the constant keyword after the name and specify the data type of the constant.
Third, initialize a value for the constant after the assignment operator (=).

PL/pgSQL constants example

The following example declares a constant named vat that stores the value-added tax and calculates the selling price from the net price:

do $$
declare
   vat constant numeric = 0.1;
   net_price    numeric = 20.5;
begin
   raise notice 'The selling price is %', net_price * ( 1 + vat );
end $$;

Output:

NOTICE:  The selling price is 22.55

Now, if you try to change the value of the constant as follows:

do $$
declare
   vat constant numeric = 0.1;
   net_price    numeric = 20.5;
begin
   raise notice 'The selling price is %', net_price * ( 1 + vat);
   vat = 0.05; -- error
end $$;

You will get the following error message:

ERROR: "vat" is declared CONSTANT
SQL state: 22005
Character: 155

Similar to the default value of a variable, PostgreSQL evaluates the value for the constant when the block is entered at run-time, not compile-time. For example:

do $$
declare
   started_at constant time := clock_timestamp();
begin
   -- pause 3s
   perform pg_sleep(3);

   -- show the current time
   raise notice '3s later';
   raise notice 'Current time: %', clock_timestamp();

   -- pause 3s
   perform pg_sleep(3);
   -- show the value of the started_at
   -- later than the one above
   raise notice 'Started at: %', started_at;
end $$;

Output:

NOTICE:  3s later
NOTICE:  Current time: 2024-03-19 09:30:09.28782+07
NOTICE:  Started at: 09:30:06.276246
DO

Summary

A constant holds a value that cannot be changed.
Use the constant keyword to define a constant in PL/pgSQL.