Understanding SELECT vs Function Debate: A More Efficient Approach with UNION ALL

Understanding the SELECT vs Function Debate

In PostgreSQL, Using a Function with Nested INSERT Can Lead to Unexpected Behavior

When it comes to writing database functions that interact with tables, developers often face challenges when deciding how to structure their queries. Two common approaches are using a SELECT statement within a function or using a separate function to perform an INSERT operation. In this article, we’ll delve into the intricacies of these two methods and explore why one might be considered “faster” than the other in certain situations.

The Problem with Dynamic SQL

The example provided by the Stack Overflow user demonstrates a function that checks if a row exists in a table based on an input user_id. The function uses dynamic SQL to execute a SELECT query and then inserts a new row if necessary. However, this approach has several issues:

Dynamic SQL with EXECUTE: Using EXECUTE with dynamic SQL can lead to performance problems because it doesn’t allow the database to optimize the query plan.
SELECT * in the function: The use of SELECT * within the function is not necessary and can slow down the execution.
No PRIMARY KEY or UNIQUE constraint on (id): Without a primary key or unique constraint, the insert operation becomes less efficient.

A Better Approach with UNION ALL

To fix these issues, a more effective approach involves using UNION ALL to combine two separate queries:

CREATE OR REPLACE FUNCTION test_function(_user_id int)
  RETURNS test_table LANGUAGE sql AS
$func$
   WITH ins AS (
      INSERT INTO test_table(id, tvalue)
      VALUES (_user_id, TRUE) 
      ON CONFLICT DO NOTHING
      RETURNING *
      )
   TABLE ins
   UNION ALL
   SELECT * FROM test_table WHERE id = _user_id
   LIMIT 1
$func$;

This revised function uses UNION ALL to combine the insert operation with a separate query that checks for existing rows. By doing so, it avoids some of the performance issues associated with dynamic SQL and ensures that the insert operation is performed efficiently.

Why This Approach Works

So, why does this approach work better? Let’s break down the key differences:

No need for dynamic SQL: By using a separate query to check for existing rows, we avoid the need for dynamic SQL, which can improve performance.
Improved visibility: The use of UNION ALL ensures that both the insert operation and the subsequent query are visible in the query plan, making it easier to optimize performance.
Efficient insert operation: By using ON CONFLICT DO NOTHING, we ensure that the insert operation is performed efficiently without affecting existing rows.

Additional Considerations

While this revised function resolves many of the issues with the original code, there are still additional considerations to keep in mind:

Race conditions for concurrent calls: If multiple threads or processes call the function simultaneously, there’s a risk of race conditions. To mitigate this issue, consider using locks or other concurrency control mechanisms.
Best practices for functions and triggers: When writing database functions that interact with tables, it’s essential to follow best practices for both functions and triggers. This includes ensuring that functions are properly optimized and that triggers are used effectively.

Conclusion

When working with database functions in PostgreSQL, understanding the nuances of different approaches is crucial for optimizing performance and avoiding common pitfalls. By using UNION ALL to combine separate queries, we can create more efficient and effective functions that interact with tables. Additionally, by following best practices for functions and triggers, developers can ensure that their codebase remains scalable, maintainable, and performant.

Example Use Cases

To demonstrate the benefits of this revised function, let’s consider some example use cases:

-- Insert a new row into test_table
SELECT * FROM test_function(1);

-- Retrieve all rows from test_table where id = 1
SELECT * FROM test_function(2);

In these examples, we’re using the test_function to perform two separate operations: inserting a new row and retrieving existing rows. By following best practices for functions and triggers, developers can create more efficient and effective code that interacts with tables in PostgreSQL.

For those interested in learning more about database functions, triggers, and query optimization, here are some related topics:

Return a value if no record is found: This article explores the different ways to handle scenarios where no records are found using SELECT queries.
How to use RETURNING with ON CONFLICT in PostgreSQL?: In this article, we’ll dive into the details of using RETURNING and ON CONFLICT together to improve performance when inserting rows into a table.

By following these tips and best practices, developers can create more efficient and effective database functions that interact with tables in PostgreSQL.

Last modified on 2023-06-25