SQL vs. PL/pgSQL in PostgreSQL: When Should I Use Each Function Type?

PostgreSQL Functions: Best Practices for SQL and PL/pgSQL

Functions in a PostgreSQL database can be implemented using SQL or PL/pgSQL (Procedural Language/PostgreSQL). Although both techniques can achieve similar results, which method to choose depends on the specific requirements of the problem.

SQL Function

Advantages:

• Simple and direct when used in scalar queries
• Minimum overhead when called occasionally
• Since there is no block structure, the code is concise

Usage scenarios:

• Simple query without procedural elements or variables
• Single or infrequent call
• Execute inline within SQL query
• Easy to use for users who are familiar with SQL but not familiar with PL/pgSQL

PL/pgSQL functions

Advantages:

• Supports procedural logic, variables and dynamic SQL
• Reuse calculation results in multiple executions
• Improving performance with query plan caching
• Catch errors and handle exceptions
• Trigger function
• Use DDL statements to modify database objects

Usage scenarios:

• Computational tasks that cannot be expressed in SQL alone
• Dynamic SQL generation and execution
• Complex processes or error handling
• Optimize repeated calls through query plan caching
• Synchronous execution of database modifications

Choice between SQL and PL/pgSQL

To make an informed decision, consider the following factors:

• Simplicity: SQL functions are simpler to write and understand.
• Performance: PL/pgSQL functions may perform better after the first call due to plan caching.
• Flexibility: PL/pgSQL provides greater functionality through procedural elements and dynamic SQL.
• Specific Requirements: Consider the specific needs of your application, such as variable manipulation, error handling, or database modifications.

Example of function implementation

Use the example provided in the query:

• f1 (SQL function):

<code class="language-sql">CREATE OR REPLACE FUNCTION f1(istr varchar)
RETURNS text AS $$ 
SELECT 'hello! '::varchar || istr;
$$ LANGUAGE SQL;</code>

• f2 (PL/pgSQL function):

<code class="language-sql">CREATE OR REPLACE FUNCTION f2(istr varchar)
RETURNS text AS $$ 
BEGIN
 RETURN 'hello! ';  -- 类型默认为 text
END
$$ LANGUAGE plpgsql;</code>

In this example, the two functions return the same result for the input value "world", but the underlying implementation is different. f1 uses a simple SQL query, while f2 uses PL/pgSQL's BEGIN-END block and implicit returns.

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