Does Postgres preserve insertion order of records?

For example when I'm using query which returns record ids

INSERT INTO projects(name)
VALUES (name1), (name2), (name3) returning id;

Which produce output:

1
2
3

Will this ids point to corresponding inserted values?

1 -> name1
2 -> name2
3 -> name3

The answer for this simple case is: "Yes". Rows are inserted in the provided order in the VALUES expression. And if your id column is a serial type, values from the underlying sequence will be fetched in that order.

But this is an implementation detail and there are no guarantees. In particular, the order is not necessarily maintained in more complex queries with WHERE conditions or joins.

You might also get gaps or other rows mixed in if you have concurrent transactions writing to the same table at the same time. Unlikely, but possible.

There is no "natural" order in a database table. While the physical order of rows (which is reflected in the system column ctid) will correspond to their inserted order initially, that may change any time. UPDATE, DELETE, VACUUM and other commands can change the physical order of rows. To SELECT rows in any particular order, you must add an ORDER BY clause. Values for id, once generated, are stable and decoupled from any physical order, of course.

StackExchange Database Administrators Q#95822, answer score: 33