SQL vs. PL/PgSQL Functions in PostgreSQL: When Should I Use Each?

PostgreSQL: SQL vs. PL/PgSQL Functions – A Practical Guide

Understanding the differences between SQL and PL/PgSQL functions in PostgreSQL is crucial for writing efficient and robust database code. This guide clarifies their distinct strengths and when to use each.

SQL Functions: Simplicity and Efficiency

SQL functions are ideal for:

• Simple, scalar queries needing minimal logic.
• Infrequent function calls within a session; prepared statement caching isn't a primary concern.
• Inline use within larger queries due to their concise nature.
• Developers less familiar with procedural languages.
• Minimizing overhead; they offer a more streamlined approach than PL/PgSQL.

PL/PgSQL Functions: Power and Flexibility

PL/PgSQL functions shine when:

• Procedural elements or variables are necessary, exceeding SQL's capabilities.
• Dynamic SQL is required (constructing and executing statements at runtime – always protect against SQL injection!).
• Complex computations unsuitable for CTEs (Common Table Expressions).
• Frequent function calls benefit from query plan caching for optimized performance.
• Robust error handling is critical.
• Creating trigger functions.
• Dynamically modifying database objects or system catalogs; PL/PgSQL's sequential execution is advantageous here, unlike SQL functions.

Addressing a Common Issue

The error encountered with the example function f2() stems from the missing RETURN statement. Correct implementation in PL/PgSQL requires explicit return value specification:

CREATE FUNCTION f2(istr varchar)
  RETURNS text AS
$func$
BEGIN
   RETURN 'hello! ';  -- Explicit return, though 'text' is the default
END
$func$ LANGUAGE plpgsql;

Remember, PL/PgSQL offers diverse return mechanisms, as detailed in the PostgreSQL documentation. Choosing between SQL and PL/PgSQL hinges on the complexity of your task and performance requirements.

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