Determine the version of postgres a statement is run on.

postgres_version.sql

select version();
-- PostgreSQL 9.6.12 on x86_64-pc-linux-musl, compiled by gcc (Alpine 8.2.0) 8.2.0, 64-bit

SHOW server_version;
-- 9.6.12

SELECT current_setting('server_version_num');
-- 90612

select current_setting('server_version');
-- 9.6.12

-- major version
select (regexp_matches(current_setting('server_version'), '(\d+)\.(\d+)\.(\d+)'))[1];
-- 9

-- minor version
select (regexp_matches(current_setting('server_version'), '(\d+)\.(\d+)\.(\d+)'))[2];
-- 6