Idempotent PostgreSQL DDL scripts

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

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

Answer 2

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 3

You should be able to use plpgsql:

create language plpgsql;
create function f() ... as $$
<plpgsql code>
$$language plpgsql;
select f();