README: SQL that supports re-execution

Make the SQL work in automated deployments where it will be repeated many times.

README.md

@@ -178,7 +178,27 @@ data with the non-superuser. Only creating the views will leave out the most
 important bits of data.
 
 ```sql
-CREATE USER postgres_exporter PASSWORD 'password';
+-- To use IF statements, hence to be able to check if the user exists before
+-- attempting creation, we need to switch to procedural SQL (PL/pgSQL)
+-- instead of standard SQL.
+-- More: https://www.postgresql.org/docs/9.3/plpgsql-overview.html
+-- To preserve compatibility with <9.0, DO blocks are not used; instead,
+-- a function is created and dropped.
+CREATE OR REPLACE FUNCTION __tmp_create_user() returns void as $$
+BEGIN
+  IF NOT EXISTS (
+          SELECT                       -- SELECT list can stay empty for this
+          FROM   pg_catalog.pg_user
+          WHERE  usename = 'postgres_exporter') THEN
+    CREATE USER postgres_exporter;
+  END IF;
+END;
+$$ language plpgsql;
+
+SELECT __tmp_create_user();
+DROP FUNCTION __tmp_create_user();
+
+ALTER USER postgres_exporter WITH PASSWORD 'password';
 ALTER USER postgres_exporter SET SEARCH_PATH TO postgres_exporter,pg_catalog;
 
 -- If deploying as non-superuser (for example in AWS RDS), uncomment the GRANT
@@ -187,7 +207,7 @@ ALTER USER postgres_exporter SET SEARCH_PATH TO postgres_exporter,pg_catalog;
 CREATE SCHEMA IF NOT EXISTS postgres_exporter;
 GRANT USAGE ON SCHEMA postgres_exporter TO postgres_exporter;
 
-CREATE FUNCTION get_pg_stat_activity() RETURNS SETOF pg_stat_activity AS
+CREATE OR REPLACE FUNCTION get_pg_stat_activity() RETURNS SETOF pg_stat_activity AS
 $$ SELECT * FROM pg_catalog.pg_stat_activity; $$
 LANGUAGE sql
 VOLATILE