Views, Stored Procedures, and Check Constraints
This post will take a quick look at views, stored procedures, and check constraints. What are they and what are their use cases? This post is specific to PostgreSQL.
A database view is kind of a virtual table, based on a
SELECT query to an existing table. How are they used? One use case would be for security, limiting access to a table for a particular user to read only, and to a subset of the rows. This would be achieved by creating a view, and then granting permission to the view for that user (role).
Basic View Example
We'll put together a table for storing earnings by hour. Each record belongs to an employee. We'll insert some records with
employee_id values between 1 and 5. We'll call the employees with IDs of 3 or less "special". Views are currently read only.
test# insert into earnings (employee_id, hour, total) VALUES (1, date_trunc('hour', now()), 10); test# insert into earnings (employee_id, hour, total) VALUES (1, date_trunc('hour', now() + interval '1 hour'), 10); ... (insert some more and change the employee_id value)
Now we'll create the view for the special employees. The view will be called
special_employee_earnings and it limits result set rows by using a SELECT query against the earnings table.
CREATE VIEW special_employee_earnings AS SELECT * from earnings where employee_id <= 3;
Now a user may be configured to have no access to the
earnings table, and read only access to
special_employee_earnings. When they issue a
select * against the view, they see the expected rows.
"The stored procedures define functions for creating triggers or custom aggregate functions." PL/pgSQL is a safe choice. One of the advantages is re-use across various applications (by moving logic to the database) as well as performance by having the logic execute in the database as opposed to the application layer.
Stored procedures are how functions are defined within the database. One language option is PL/pgSQL for PostgreSQL. I made a repo called db-stuff with like
get_sum(a,b) from the Stored Procedures Tutorial.
Why have these in the database versus the application layer? One reason might be to share this function across applications, potentially with different language stacks, by putting into a shared database.
Check constraints are a way to do data validations within the database.
A check constraint might be part of a column definition. The documentation has an example for a
price column of
numeric type, where we want to check that the price is greater than zero.
CREATE TABLE products ( price numeric CHECK (price > 0) );
Violating this constraint produces a descriptive error message:
INSERT INTO PRODUCTS (price) VALUES (-1); ERROR: new row for relation "products" violates check constraint "products_price_check" DETAIL: Failing row contains (-1).