- Postgresql insert trigger how to#
- Postgresql insert trigger update#
However, on the default Read Committed isolation level, reading the sum of salaries in the application does not guarantee that the sum will be the same at the end of the transaction. Many times, application developers try to enforce these rules in the application layer by using a read-modify-write data access pattern. If you enjoyed this article, I bet you are going to love my Book and Video Courses as well.ĭatabase trigger functions are very useful when it comes to applying consistency rules that involve multiple tables.
Postgresql insert trigger update#
Luckily, the check_department_budget trigger function will not validate the UPDATE statement, and an exception will be thrown, and Alice’s transaction will be rolled back. However, when Alice wants to execute the UPDATE, the budget is now 99000, so, if the UPDATE succeeds, the new budget value will be 108900. When Bob hires Dave, the budget was 90000, so his INSERT statement is validated by the check_department_budget trigger function. However, thanks to the check_department_budget trigger function, one of the transaction will be rolled back, as illustrated by the following diagram: If both Alice and Bob commit their transactions, we risk going over the budget.
Bob wants to hire Dave for 9000, therefore raising the budget from 90000 to 99000. Alice wants to give a year-end raise of 10% to all employees in the IT department, which should raise the budget from 90000 to 99000. Now, let’s consider that Alice and Bob want to run the following operations: Notice that the current sum of salaries is 90000, so, currently, we are 10000 under budget. Testing timeĪssuming we have the following IT department with a budget of 100000:Īnd, we have three employees currently working in the IT department: We now have a trigger in place that, on every employee table INSERT or UPDATE, checks whether the sum of salaries does not exceed the department budget. Now, we also need to define a PostgreSQL trigger that is executed after each INSERT or UPDATE on the employee table:ĬREATE TRIGGER check_department_budget_triggerįOR EACH ROW EXECUTE PROCEDURE check_department_budget() Īnd, that’s it. Notice that the check_department_budget PostgreSQL function returns a TRIGGER object since we want this trigger function to be executed in the context of table row INSERT or UPDATE events. RAISE EXCEPTION 'Overbudget department by ', Therefore, we are going to create the following check_department_budget trigger function, which verifies that the sum of salaries in a given department does not exceed the allocated budget.ĬREATE OR REPLACE FUNCTION check_department_budget() If we want to implement a more complex data integrity rule, then a database trigger is a much more suitable alternative. However, CHECK constraints are limited to the columns of the table for which we defined the custom constraint. PostgreSQL supports the SQL standard CHECK constraints, and we used them for the SINGLE_TABLE JPA inheritance strategy. Employees have a salary column, and the sum of salaries in a given department should not exceed the budget column value of the associated department table record. The department is the parent table while the employee is the child table. We are going to reuse the department and employee database tables from the article showing the difference between 2PL (Two-Phase Locking) and MVCC (Multi-Version Concurrency Control) when it comes to handling the Write Skew anomaly: Postgresql insert trigger how to#
In this article, we are going to see how to implement a non-trivial consistency check using a PostgreSQL INSERT and UPDATE trigger.īy using a database trigger after executing an INSERT or UPDATE, we can ensure that the sum of salaries in a given department does not exceed the maximum budget allocated for the given department.
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