-- blocks all writes
CREATE INDEX items_value_idx ON items USING GIN (value jsonb_path_ops);

-- only blocks other DDL
CREATE INDEX CONCURRENTLY items_value_idx ON items USING GIN (value jsonb_path_ops);

ALTER TABLE items ADD COLUMN last_update timestamptz;

SET lock_timeout TO '2s'
ALTER TABLE items ADD COLUMN last_update timestamptz;

-- blocks queries for a long time
ALTER TABLE items ADD PRIMARY KEY (id);

-- takes a long time, but doesn’t block queries
CREATE UNIQUE INDEX CONCURRENTLY items_pk ON items (id);
-- blocks queries, but only very briefly
ALTER TABLE items ADD CONSTRAINT items_pk PRIMARY KEY USING INDEX items_pk;

SELECT DISTINCT city FROM employees;

SELECT city FROM employees GROUP BY city;

CREATE TABLE people
  (name text, age int, interests text[]);

INSERT INTO people (name, age, interests)
  VALUES
  ('Jon', 12, ARRAY ['Golf', 'Food']),
  ('Jane', 45, ARRAY ['Art']);

SELECT row_to_json(ROW(name, age, interests)) FROM people;

row_to_json
-----------------------------------
{"f1":"Jon","f2":12,"f3":["Golf","Food"]}
{"f1":"Jane","f2":45,"f3":["Art"]}

SELECT row_to_json(q1) FROM
  (SELECT * FROM people LIMIT 1) q1;

row_to_json
-----------------------------------
{"name":"Jon","age":12,"interests":["Golf","Food"]}

SELECT * FROM blah WHERE timestampcol BETWEEN '2018-06-01' AND '2018-06-08'

SELECT * FROM blah WHERE timestampcol >= '2018-06-01' AND timestampcol < '2018-06-08'

SELECT pg_size_pretty(pg_database_size('n2'));

select pg_size_pretty(pg_database_size('n2'));

 pg_size_pretty
----------------
 446 MB
(1 row)