PostgreSQL add SERIAL column to existing table with values based on ORDER BY - postgresql

Question

I have a large table (6+ million rows) that I'd like to add an auto-incrementing integer column sid, where sid is set on existing rows based on an ORDER BY inserted_at ASC. In other words, the oldest record based on inserted_at would be set to 1 and the latest record would be the total record count. Any tips on how I might approach this?

Answer 1

Add a sid column and UPDATE SET ... FROM ... WHERE:
UPDATE test
SET sid = t.rownum
FROM (SELECT id, row_number() OVER (ORDER BY inserted_at ASC) as rownum
FROM test) t
WHERE test.id = t.id
Note that this relies on there being a primary key, id.
(If your table did not already have a primary key, you would have to make one first.)
For example,
-- create test table
DROP TABLE IF EXISTS test;
CREATE TABLE test (
id int PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY
, foo text
, inserted_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
);
INSERT INTO test (foo, inserted_at) VALUES
('XYZ', '2019-02-14 00:00:00-00')
, ('DEF', '2010-02-14 00:00:00-00')
, ('ABC', '2000-02-14 00:00:00-00');
-- +----+-----+------------------------+
-- | id | foo | inserted_at |
-- +----+-----+------------------------+
-- | 1 | XYZ | 2019-02-13 19:00:00-05 |
-- | 2 | DEF | 2010-02-13 19:00:00-05 |
-- | 3 | ABC | 2000-02-13 19:00:00-05 |
-- +----+-----+------------------------+
ALTER TABLE test ADD COLUMN sid INT;
UPDATE test
SET sid = t.rownum
FROM (SELECT id, row_number() OVER (ORDER BY inserted_at ASC) as rownum
FROM test) t
WHERE test.id = t.id
yields
+----+-----+------------------------+-----+
| id | foo | inserted_at | sid |
+----+-----+------------------------+-----+
| 3 | ABC | 2000-02-13 19:00:00-05 | 1 |
| 2 | DEF | 2010-02-13 19:00:00-05 | 2 |
| 1 | XYZ | 2019-02-13 19:00:00-05 | 3 |
+----+-----+------------------------+-----+
Finally, make sid SERIAL (or, better, an IDENTITY column):
ALTER TABLE test ALTER COLUMN sid SET NOT NULL;
-- IDENTITY fixes certain issue which may arise with SERIAL
ALTER TABLE test ALTER COLUMN sid ADD GENERATED BY DEFAULT AS IDENTITY;
-- ALTER TABLE test ALTER COLUMN sid SERIAL;