PostgreSQL Temporary Tables

PostgreSQL, an open-source relational database management system, offers a variety of features to efficiently manage and manipulate data. Temporary tables are one such feature that plays a crucial role in handling temporary data within a session. 

In this article, we'll delve into PostgreSQL temporary table, exploring their usage, benefits, and best practices.

What are PostgreSQL Temporary Tables?

Temporary tables in PostgreSQL are session-specific tables that exist only for the duration of a database session. Unlike regular tables, temporary tables are automatically dropped at the end of the session or when the connection that created them is closed. These tables are particularly useful for storing intermediate results, temporary data, or isolating data within a specific session.

Syntax:

You can create temporary tables using the CREATE TEMPORARY TABLE statement. 

CREATE TEMPORARY TABLE table_name (
    column1 datatype,
    column2 datatype,
    ...
);

Explanation of the Parameters:

• CREATE TEMPORARY TABLE: This part of the statement tells PostgreSQL that you're creating a temporary table. Temporary tables are session-local; they exist only for the duration of the database session or the current transaction, depending on how they are defined.
• table_name: This is where you specify the name of the temporary table you want to create. Replace table_name with the desired name for your temporary table.
• (column1 datatype, column2 datatype, ...): This section defines the structure of the temporary table. You list the columns you want the table to have, along with their respective data types. Each column definition consists of a column name followed by its data type. You can have as many columns as needed, separated by commas.

Creating a simple PostgreSQL temporary table to store employee data and insert some sample records into it. Then, we'll retrieve the data from the temporary table to see the output.

-- Create a temporary table to store employee data
CREATE TEMPORARY TABLE temp_employee (
    emp_id SERIAL PRIMARY KEY,
    emp_name VARCHAR(100),
    emp_salary NUMERIC
);

-- Insert sample data into the temporary table
INSERT INTO temp_employee (emp_name, emp_salary) VALUES
    ('John Doe', 50000),
    ('Jane Smith', 60000),
    ('Alice Johnson', 55000);

-- Retrieve data from the temporary table
SELECT * FROM temp_employee;

Output:

 emp_id |   emp_name    | emp_salary 
--------+---------------+------------
      1 | John Doe      |      50000
      2 | Jane Smith    |      60000
      3 | Alice Johnson |      55000
(3 rows)

In this example, we created a temporary table named temp_employee with three columns: emp_id, emp_name, and emp_salary then inserted three sample records into the temporary table using the INSERT INTO statement. Finally, we retrieved all the data from the temporary table using a SELECT query, which returned three rows containing the sample employee data.

Temporary Table with Index

The PostgreSQL temporary tables are created within a session and are automatically dropped at the end of the session or when the connection is closed. Indexes can also be created on temporary tables to improve query performance. However, it's important to note that these indexes are also temporary and are automatically dropped when the temporary table is dropped.

Here's a brief outline of how to create a temporary table with an index in PostgreSQL:

-- Create a temporary table
CREATE TEMPORARY TABLE temp_table (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100)
);

-- Create an index on the temporary table
CREATE INDEX idx_name ON temp_table (name);

-- Insert sample data into the temporary table
INSERT INTO temp_table (name) VALUES 
    ('John'),
    ('Alice'),
    ('Bob'),
    ('Charlie'),
    ('David'),
    ('Emma');

-- Query the temporary table
SELECT * FROM temp_table WHERE name = 'Alice';

Output:

 id | name  
----+-------
  2 | Alice
(1 row)

This output shows that only the record with the name Alice is returned from the temporary table.

Temporary Table with Aggregation

The temporary tables in PostgreSQL can be used in conjunction with aggregation functions to store and manipulate intermediate results within a session. This approach is particularly useful when dealing with complex queries that involve aggregating data from multiple tables or performing calculations on large datasets.

Creating a temporary table named temp_sales, inserts sample data into it, and then performs aggregation to calculate the total sales amount for each product_id.

-- Create a temporary table
CREATE TEMPORARY TABLE temp_sales (
    product_id INT,
    sales_amount NUMERIC
);

-- Insert sample data into the temporary table
INSERT INTO temp_sales (product_id, sales_amount) VALUES 
    (1, 100),
    (2, 200),
    (1, 150),
    (3, 300),
    (2, 250);

-- Perform aggregation on the temporary table
SELECT 
    product_id, 
    SUM(sales_amount) AS total_sales
FROM 
    temp_sales
GROUP BY 
    product_id;

Output:

 product_id | total_sales 
------------+-------------
          1 |         250
          2 |         450
          3 |         300
(3 rows)

This output shows the total sales amount for each product_id aggregated from the temporary table temp_sales.

Temporary Table with Join

Temporary tables can be effectively utilized alongside joins to combine data from multiple tables within a session in PostgreSQL. This approach is particularly useful when complex queries involve joining multiple tables or when intermediate results need to be stored temporarily for further processing.

Let's creates two temporary tables (temp_orders and temp_products), inserts sample data into them, and then performs a join operation between the two temporary tables based on the product_id.

-- Create a temporary table
CREATE TEMPORARY TABLE temp_orders (
    order_id INT,
    product_id INT,
    quantity INT
);

-- Insert sample data into the temporary table
INSERT INTO temp_orders (order_id, product_id, quantity) VALUES 
    (1, 101, 2),
    (2, 102, 3),
    (3, 101, 1),
    (4, 103, 4);

-- Create another temporary table
CREATE TEMPORARY TABLE temp_products (
    product_id INT,
    product_name VARCHAR(100),
    price NUMERIC
);

-- Insert sample data into the second temporary table
INSERT INTO temp_products (product_id, product_name, price) VALUES 
    (101, 'Product A', 10),
    (102, 'Product B', 20),
    (103, 'Product C', 15);

-- Perform a join between the two temporary tables
SELECT 
    t.order_id,
    t.product_id,
    p.product_name,
    t.quantity,
    p.price,
    (t.quantity * p.price) AS total_price
FROM 
    temp_orders t
JOIN 
    temp_products p ON t.product_id = p.product_id;

Output:

 order_id | product_id | product_name | quantity | price | total_price 
----------+------------+--------------+----------+-------+-------------
        1 |        101 | Product A    |        2 |    10 |          20
        2 |        102 | Product B    |        3 |    20 |          60
        3 |        101 | Product A    |        1 |    10 |          10
        4 |        103 | Product C    |        4 |    15 |          60
(4 rows)

This output displays the joined data from the temporary tables temp_orders and temp_products, including order details such as order ID, product ID, product name, quantity, price, and total price (quantity multiplied by price).

Using Temporary Table in Subquery

Temporary tables within subqueries in PostgreSQL allows for the creation of more complex and versatile queries. This approach is beneficial when intermediate results need to be generated and manipulated before being used in the main query.

Let's consider a scenario where we have two tables: students and grades. We want to find the average grade for each student along with their names.

-- Create students table
CREATE TABLE students (
    student_id SERIAL PRIMARY KEY,
    student_name VARCHAR(100)
);

-- Insert sample data into the students table
INSERT INTO students (student_name) VALUES
    ('Alice'),
    ('Bob'),
    ('Charlie'),
    ('David');

-- Create grades table
CREATE TABLE grades (
    grade_id SERIAL PRIMARY KEY,
    student_id INT,
    grade NUMERIC
);

-- Insert sample data into the grades table
INSERT INTO grades (student_id, grade) VALUES
    (1, 85),
    (2, 90),
    (3, 75),
    (4, 80),
    (1, 95),
    (2, 88);

-- Using a temporary table within a subquery
SELECT 
    s.student_name,
    (SELECT AVG(grade) FROM grades WHERE student_id = s.student_id) AS average_grade
FROM 
    students s;

Output:

 student_name | average_grade 
--------------+---------------
 Alice        |          90.0
 Bob          |          89.0
 Charlie      |          75.0
 David        |          80.0
(4 rows)

This output displays the average grade for each student, with the student names retrieved from the students table and the average grade computed from the grades table using a temporary table within a subquery.

Benefits of Temporary Tables:

1. Isolation: Temporary tables isolate data within a session, preventing interference from other sessions or users.
2. Performance: Temporary tables can improve performance by storing intermediate results or frequently accessed data within the session.
3. Flexibility: They support a wide range of operations and features, similar to regular tables, allowing for complex data manipulation.
4. Automatic Cleanup: Temporary tables are automatically dropped at the end of the session, reducing the need for manual cleanup and improving resource management.

Best Practices for Using Temporary Tables:

1. Limit Usage: Use temporary tables only when necessary, as they consume resources and can impact performance, especially in high-concurrency environments.
2. Drop Tables Explicitly: Although temporary tables are automatically dropped at the end of the session, it's good practice to explicitly drop them when they are no longer needed to release resources promptly.
3. Avoid Long Transactions: Long-running transactions holding locks on temporary tables can impact concurrency and lead to performance issues. Keep transactions short and release locks promptly.
4. Optimize Queries: Optimize queries involving temporary tables to improve performance, considering indexes, query structure, and data volume.
5. Consider Alternatives: In some cases, alternatives such as Common Table Expressions (CTEs) or temporary views may offer similar functionality without the overhead of temporary tables.

Conclusion

PostgreSQL temporary tables provide a powerful mechanism for managing temporary data within a session. By understanding their usage, benefits, and best practices, developers can leverage temporary tables effectively to improve performance, isolate data, and streamline data manipulation workflows in PostgreSQL databases.