SQL injection in Postgres functions vs prepared queries

In Postgres, are prepared queries and user defined functions equivalent as a mechanism for guarding against SQL injection?

Are there particular advantages in one approach over the other?

It depends.
Prepared statements

A.k.a. "prepared queries". These only take values when executed with EXECUTE. No SQL-injection possible at this stage. But you have to defend against SQL-injection while generating / concatenating the statement to be fed to PREPARE in the first place. Prepared statements are very much equivalent to stored SQL functions / procedures in this regard.
SQL functions / procedures

With LANGUAGE sql they are generally safe.

Passed parameters are treated as values and SQL-injection is not possible - as long as you don't call unsafe functions from the body and pass parameters.
PL/pgSQL functions /procedures

With LANGUAGE plpgsql they are normally safe.

However, PL/pgSQL allows for dynamic SQL where passed parameters (or parts) are concatenated to a query string and executed with EXECUTE. This can convert user input to SQL code and make SQL injection possible. Tools are there to do it safely. You cannot tell from outside whether the function body handles it properly, you have to look at the code.

Plain SQL statements using parameters as values are safe against SQL injection, just like SQL functions. Only use dynamic SQL where needed and follow these guidelines:

Preferably pass values as values to EXECUTE with the USING clause. Makes SQL injection impossible on principal. Example.

Never just build strings from user input and execute. This includes identifiers, directly passed by a user or fetched from a system catalog.

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When concatenating values in the SQL string, wrap 'strings' in single-quotes safely. Use:

• format() with format specifier %L. Example.

• quote_literal() or quote_nullable(). Example.

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When concatenating identifiers in the SQL string, wrap them in double-quotes safely where required. Use:

• format() with format specifier %I. Example.

• quote_ident(). Example.

• a cast to a registered type - regclass for table names: _tbl::regclass. (Only works for existing objects!) Example.

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Concatenating other parts of the SQL code is never safe, when allowing unchecked user input in any way.

Related:

• Refactor a PL/pgSQL function to return the output of various SELECT queries

• Define table and column names as arguments in a plpgsql function?

More about performance implications in this related answer:

• Function Performance

Basics on SQL-injection:

• http://bobby-tables.com/

Similar considerations apply to other server-side languages that allow dynamic SQL.

StackExchange Database Administrators Q#49699, answer score: 58