SQL vs. PLPGSQL in PostgreSQL Functions: When Should I Use Each?

Differences between SQL and PLPGSQL in PostgreSQL functions: When to use which?

When using PostgreSQL functions, a key decision is whether to use LANGUAGE SQL or LANGUAGE PLPGSQL. While both methods provide ways to create functions, there are significant differences between them that determine the scenarios in which each method is best suited.

LANGUAGE SQL function

LANGUAGE SQL function is simpler and more direct. Their syntax is similar to SQL SELECT statements, making them easier to understand and implement. These functions are a good choice for simple scalar queries that don't require complex logic or variables.

Example:

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

LANGUAGE PLPGSQL function

LANGUAGE PLPGSQL functions provide greater flexibility and control over code execution. They allow the use of procedural elements such as variables, loops, and conditionals. These functions are ideal for situations that require more complex logic or dynamic behavior, such as building dynamic SQL statements or handling errors.

Example:

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

When to use each language

The following guidelines can help you decide when to use LANGUAGE SQL or LANGUAGE PLPGSQL:

Recommended situations to use the LANGUAGE SQL function:

• Requires simple scalar queries, no procedural logic required.
• Functions are rarely called within a session, so plan caching is not required.
• Functions can be inlined into larger queries, improving performance.
• Prefer a cleaner coding style.

It is recommended to use the LANGUAGE PLPGSQL function:

• Requires a procedural element or variable.
• Use dynamic SQL statements.
• Function needs to be reused in multiple places, and CTE is not enough.
• Functions are called repeatedly and query plans can benefit from caching.
• Need to catch errors and handle them accordingly.
• Use trigger functions.
• Contains DDL statements that change objects or system directories.

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