How to use a query to set a custom option?

Question

I tried this but it does not work:

CREATE OR REPLACE FUNCTION answer()
  RETURNS bigint
  LANGUAGE sql AS
$$
SELECT 42
$$;
SET session.answer = select answer();

Answer 1

The SET command only accepts constants. The manual:

constants, identifiers, numbers, or comma-separated lists of these

It does not take parameters, nor can it evaluate subqueries. Parameter substitution only works for basic DML commands (SELECT, INSERT, UPDATE, DELETE, MERGE).

The function set_config() provides equivalent functionality;

SELECT set_config('session.answer', answer()::text, false);

Note the cast to ::text. Postgres parameters only store text.

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• Set custom constant in Postgres script