Idempotent PostgreSQL DDL scripts - database

Question

I'm looking for a way to script postgreSQL schema changes in an idempotent manner.
In MSSQL I could do something like this:
if(not exists(select * from information_schema.columns where table_name = 'x' and column_name = 'y'))
begin
alter table x add y int
end
go
PostgreSQL doesn't seem to allow ad-hoc pl/pgsql in the same way MSSQL does with T-SQL so I can't use control structures in a SQL script and run it with psql -f x.sql.
I know PostgreSQL will throw an error if the object already exists but I don't want to have to ignore errors.
I could use some schema versioning technique like dbdeploy but I really like the simplicity of running a set of files through psql without incurring any unwanted side effects.
Is this possible?
Thanks,
Mark

Answer 1

Disclaimer: I know, this is a very old question and already has an accepted answer.
But I'd like to register here a totally idempotent script, without external links.
A simple script demonstrating PostgreSQL 9.5+ idempotent built-in capabilities. You can run this script as many times as you wish.
Here we go:
--Table
CREATE TABLE IF NOT EXISTS person (
id integer NOT NULL,
person_name character varying(40) NOT NULL,
updated_date date,
CONSTRAINT person_pkey PRIMARY KEY (id)
);
--Index
CREATE INDEX IF NOT EXISTS idx_person_name ON person (person_name);
--Sequence
CREATE SEQUENCE IF NOT EXISTS seq_person_inc;
--Function
CREATE OR REPLACE FUNCTION simple_sum(a_integer int, b_integer int) RETURNS INT
AS $$ SELECT a_integer+b_integer $$
LANGUAGE SQL;
--View
CREATE OR REPLACE VIEW vw_select_1 AS
SELECT 1;
--Role
DO $$
BEGIN
CREATE ROLE rick_deckard;
EXCEPTION
WHEN duplicate_object THEN
RAISE NOTICE 'Role already exists. Ignoring...';
END$$;
--Simple insert
INSERT INTO person (id, person_name) VALUES (1, 'HAL-9000');
--Upsert (insert + update)
INSERT INTO person (id, person_name) VALUES (1, 'Betrayer') ON CONFLICT ON CONSTRAINT person_pkey DO UPDATE SET person_name = EXCLUDED.person_name;
--Upsert (ignoring duplicate error)
INSERT INTO person (id, person_name) VALUES (1, 'HAL-9000') ON CONFLICT ON CONSTRAINT person_pkey DO NOTHING;
--Upsert (ignoring any error)
INSERT INTO person (id, person_name) VALUES (1, 'HAL-9000') ON CONFLICT DO NOTHING;
--Field
DO $$
BEGIN
ALTER TABLE person ADD COLUMN id_another_person INTEGER;
EXCEPTION
WHEN duplicate_column THEN
RAISE NOTICE 'Field already exists. Ignoring...';
END$$;
--Constraint
DO $$
BEGIN
ALTER TABLE person ADD CONSTRAINT person_id_another_person_fkey FOREIGN KEY (id_another_person) REFERENCES person (id);
EXCEPTION
WHEN duplicate_object THEN
RAISE NOTICE 'Constraint already exists. Ignoring...';
END$$;
--Trigger
CREATE OR REPLACE FUNCTION person_trigger_function() RETURNS trigger AS $BODY$
BEGIN
--Something complex here =)
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;
DO $$
BEGIN
CREATE TRIGGER person_trigger BEFORE INSERT OR UPDATE ON person FOR EACH ROW EXECUTE PROCEDURE person_trigger_function();
EXCEPTION
WHEN duplicate_object THEN
RAISE NOTICE 'Trigger already exists. Ignoring...';
END$$;
--Drop
DROP TRIGGER IF EXISTS person_trigger ON person;
DROP INDEX IF EXISTS idx_person_name;
ALTER TABLE person DROP COLUMN IF EXISTS person_name;
ALTER TABLE person DROP CONSTRAINT IF EXISTS person_id_another_person_fkey;
DROP ROLE IF EXISTS rick_deckard;
DROP VIEW IF EXISTS vw_select_1;
DROP FUNCTION IF EXISTS simple_sum(integer, integer);
DROP FUNCTION IF EXISTS person_trigger_function();
DROP TABLE IF EXISTS person;
DROP SEQUENCE IF EXISTS seq_person_inc;

Answer 2

you might find this blogpost helpful: http://www.depesz.com/index.php/2008/06/18/conditional-ddl/

Answer 3

You should be able to use plpgsql:
create language plpgsql;
create function f() ... as $$
<plpgsql code>
$$language plpgsql;
select f();