SQL vs. PL/pgSQL for PostgreSQL Functions: When Should I Use Which?

PostgreSQL Functions: SQL vs. PL/pgSQL Selection Guide

PostgreSQL provides two main languages ​​for writing user-defined functions: SQL and PL/pgSQL. While both can achieve similar output, understanding their key differences is critical to making informed choices in different scenarios.

SQL Function

SQL functions are generally preferred when:

• Simple scalar queries are required because they have minimal overhead and can be easily inlined.
• Expect fewer calls per session as there is no benefit from plan caching.
• No procedural elements required.
• No dynamic SQL required.
• Computations do not need to be reused or cannot be expressed using CTE.

PL/pgSQL functions

PL/pgSQL functions are a better choice when:

• Requires a procedural element or variable.
• Requires dynamic SQL.
• Computationally complex and requires repeated use or cannot be expressed using CTE.
• The function is called repeatedly and requires query plan caching.
• Requires error catching.
• Requires trigger function.

How to choose the right language

When choosing between SQL and PL/pgSQL functions, consider the following factors:

• Simplicity: SQL functions are generally easier to write for people who are familiar with SQL.
• Performance: PL/pgSQL functions may be faster for repeated calls due to plan caching.
• Features: PL/pgSQL provides more advanced features such as procedural control flow and exception handling.
• Development Cost: Writing and debugging PL/pgSQL functions may require a higher level of expertise and may be more time-consuming than writing SQL functions.

Example differences

Consider the following two examples:

SQL function (f1):

CREATE OR REPLACE FUNCTION f1(istr varchar)
RETURNS text AS $$
SELECT 'hello! '::varchar || istr;
$$ LANGUAGE SQL;

PL/pgSQL functions (f2):

CREATE OR REPLACE FUNCTION f2(istr varchar)
RETURNS text AS $$
BEGIN
    RETURN 'hello! '::varchar || istr;
END;
$$ LANGUAGE plpgsql;

Calling these two functions using select f1('world') and select f2('world') will produce different results. f1 returns "hello! world", while f2 generates an error due to lack of appropriate result data target in PL/pgSQL.

Conclusion

Understanding the differences between SQL and PL/pgSQL functions enables developers to use the most appropriate language for each use case, maximizing performance and functionality. SQL functions are suitable for simple queries and one-time calls, while PL/pgSQL functions are more suitable for complex operations, dynamic SQL, and scenarios that require caching and procedural elements.

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