PostgreSQL PL/pgSQL random value from array of values

How can I declare an array like variable with two or three values and get them randomly during execution?

a := [1, 2, 5] -- sample sake
select random(a) -- returns random value

Any suggestion where to start?

CREATE OR REPLACE FUNCTION random_pick()
  RETURNS int
  LANGUAGE sql VOLATILE PARALLEL SAFE AS
$func$
SELECT ('[0:2]={1,2,5}'::int[])[trunc(random() * 3)::int];
$func$;

random() returns a value x where 0.0 <= x < 1.0. Multiply by 3 and truncate it with trunc() (slightly faster than floor()) to get 0, 1, or 2 with exactly equal chance.

Postgres indexes are 1-based by default (as per SQL standard). This would be off-by-1. We could increment by 1 every time - or declare the array index to start with 0 instead. Slightly faster, yet. See:

• Normalize array subscripts so they start with 1
• The manual on mathematical functions.

PARALLEL SAFE for Postgres 9.6 or later. See:

• PARALLEL label for a function with SELECT and INSERT
• When to mark functions as PARALLEL RESTRICTED vs PARALLEL SAFE?

You can use the plain SELECT statement if you don't want to create a function:

SELECT ('[0:2]={1,2,5}'::int[])[trunc(random() * 3)::int];