PostgreSQL CREATE PROCEDURE

Summary: in this tutorial, you will learn how to use the PostgreSQL CREATE PROCEDURE statement to create new stored procedures.

Introduction to PostgreSQL CREATE PROCEDURE statement

So far, you have learned how to define user-defined functions using the create function statement.

A drawback of user-defined functions is that they cannot execute transactions. In other words, inside a user-defined function, you cannot start a transaction, and commit or rollback it.

PostgreSQL 11 introduced stored procedures that support transactions.

To define a new stored procedure, you use the create procedure statement with the following syntax:

create [or replace] procedure procedure_name(parameter_list)
language plpgsql
as $$
declare
-- variable declaration
begin
-- stored procedure body
end; $$

In this syntax:

• First, specify the name of the stored procedure after the create procedure keywords.
• Second, define parameters for the stored procedure. A stored procedure can accept zero or more parameters.
• Third, specify plpgsql as the procedural language for the stored procedure. Note that you can use other procedural languages for the stored procedure such as SQL, C, etc.
• Finally, use the dollar-quoted string constant syntax to define the body of the stored procedure.

Parameters in stored procedures can have the in and inout modes but cannot have the out mode.

A stored procedure does not return a value. You cannot use the return statement with a value inside a store procedure like this:

return expression;

However, you can use the return statement without the expression to stop the stored procedure immediately:

return;

If you want to return a value from a stored procedure, you can use parameters with the inout mode.

PostgreSQL CREATE PROCEDURE statement examples

We will use the following accounts table for the demonstration:

drop table if exists accounts;

create table accounts (
    id int generated by default as identity,
    name varchar(100) not null,
    balance dec(15,2) not null,
    primary key(id)
);

insert into accounts(name,balance)
values('Bob',10000);

insert into accounts(name,balance)
values('Alice',10000);

The following statement shows the data from the accounts table:

select * from accounts;

Output:

id | name  | balance
----+-------+----------
  1 | Bob   | 10000.00
  2 | Alice | 10000.00
(2 rows)

The following example creates a stored procedure named transfer that transfers a specified amount of money from one account to another.

create or replace procedure transfer(
   sender int,
   receiver int,
   amount dec
)
language plpgsql
as $$
begin
    -- subtracting the amount from the sender's account
    update accounts
    set balance = balance - amount
    where id = sender;

    -- adding the amount to the receiver's account
    update accounts
    set balance = balance + amount
    where id = receiver;

    commit;
end;$$;

Calling a stored procedure

To call a stored procedure, you use the CALL statement as follows:

call stored_procedure_name(argument_list);

For example, this statement invokes the transfer stored procedure to transfer $1,000 from Bob’s account to Alice’s account.

call transfer(1,2,1000);

The following statement verifies the data in the accounts table after the transfer:

SELECT * FROM accounts;

Output:

id | name  | balance
----+-------+----------
  1 | Bob   |  9000.00
  2 | Alice | 11000.00
(2 rows)

The output shows that the transfer has been successful.

Summary

• Use create procedure statement to define a new stored procedure.
• Use the call statement to invoke a stored procedure.