PostgreSQL Tutorial: Schema

Summary: In this tutorial, you will learn about PostgreSQL schema and how to use the schema search path to resolve objects in schemas.

Table of Contents

What is a PostgreSQL schema

In PostgreSQL, a schema is a namespace that contains named database objects such as tables, views, indexes, data types, functions, stored procedures and operators.

To access an object in a schema, you need to qualify the object by using the following syntax:

schema_name.object_name

A database can contain one or multiple schemas and each schema belongs to only one database. Two schemas can have different objects that share the same name.

For example, you may have sales schema that has staff table and the public schema which also has the staff table. When you refer to the staff table you must qualify it as follows:

public.staff

Or

sales.staff

Why do you need to use schemas

There are some scenarios that you want to use schemas:

• Schemas allow you to organize database objects e.g., tables into logical groups to make them more manageable.
• Schemas enable multiple users to use one database without interfering with each other.

The public schema

PostgreSQL automatically creates a schema called public for every new database. Whatever object you create without specifying the schema name, PostgreSQL will place it into this public schema. Therefore, the following statements are equivalent:

CREATE TABLE table_name(
  ...
);

and

CREATE TABLE public.table_name(
   ...
);

The schema search path

In practice, you will refer to a table without its schema name e.g., staff table instead of a fully qualified name such as sales.staff table.

When you reference a table using its name only, PostgreSQL searches for the table by using the schema search path, which is a list of schemas to look in.

PostgreSQL will access the first matching table in the schema search path. If there is no match, it will return an error, even the name exists in another schema in the database.

The first schema in the search path is called the current schema. Note that when you create a new object without explicitly specifying a schema name, PostgreSQL will also use the current schema for the new object.

The current_schema() function returns the current schema:

SELECT current_schema();

Here is the output:

 current_schema
----------------
public
(1 row)

This is why PostgreSQL uses public for every new object that you create.

To view the current search path, you use the SHOW command in psql tool:

SHOW search_path;

The output is as follows:

 search_path
-----------------
"$user", public
(1 row)

In this output:

• The "$user" specifies that the first schema that PostgreSQL will use to search for the object, which has the same name as the current user. For example, if you use the postgres user to login and access the staff table. PostgreSQL will search for the staff table in the postgres schema. If it cannot find any object like that, it continues to look for the object in the public schema.
• The second element refers to the public schema as we have seen before.

To create a new schema, you use the CREATE SCHEMA statement:

CREATE SCHEMA sales;

To add the new schema to the search path, you use the following command:

SET search_path TO sales, public;

Now, if you create a new table named staff without specifying the schema name, PostgreSQL will put this staff table into the sales schema:

CREATE TABLE staff(
    staff_id SERIAL PRIMARY KEY,
    first_name VARCHAR(45) NOT NULL,
    last_name VARCHAR(45) NOT NULL,
    email VARCHAR(100) NOT NULL UNIQUE
);

The following picture shows the new schema sales and the staff table that belongs to the sales schema:

To access the staff table in the sales schema you can use one of the following statement:

SELECT * FROM staff;

Or

SELECT * FROM sales.staff;

The public schema is the second element in the search path, so to access the staff table in the public schema, you must qualify the table name as follows:

SELECT * FROM public.staff;

If you use the following command, you will need to explicitly refer to objects in the public schema using a fully qualified name:

SET search_path TO public;

The public schema is not a special schema, therefore, you can drop it too.

PostgreSQL schemas and privileges

Users can only access objects in the schemas that they own. It means they cannot access any objects in the schemas that do not belong to them.

To allow users to access the objects in the schema that they do not own, you must grant the USAGE privilege of the schema to the users:

GRANT USAGE ON SCHEMA schema_name 
TO role_name;

To allow users to create objects in the schema that they do not own, you need to grant them the CREATE privilege of the schema to the users:

GRANT CREATE ON SCHEMA schema_name 
TO user_name;

Note that, by default, every user has the CREATE and USAGE on the public schema.

PostgreSQL schema operations

• To create a new schema, you use the CREATE SCHEMA statement.
• To rename a schema or change its owner, you use the ALTER SCHEMA statement.
• To drop a schema, you use the DROP SCHEMA statement.

In this tutorial, you have learned about the PostgreSQL schema and how PostgreSQL uses the search path to resolve object names.