PostgreSQL: Return auto-generated ids from COPY FROM insertion

I have a non-empty PostgreSQL table with a GENERATED ALWAYS AS IDENTITY column id. I do a bulk insert with the C++ binding pqxx::stream_to, which I'm assuming uses COPY FROM. My problem is that I want to know the ids of the newly created rows, but COPY FROM has no RETURNING clause. I see several possible solutions, but I'm not sure if any of them is good, or which one is the least bad:

Provide the ids manually through COPY FROM, taking care to give the values which the identity sequence would have provided, then afterwards synchronize the sequence with setval(...).
First stream the data to a temp-table with a custom index column for ordering. Then do something like
```
INSERT INTO foo (col1, col2)
SELECT ttFoo.col1, ttFoo.col2 FROM ttFoo
ORDER BY ttFoo.idx RETURNING foo.id
```
and depend on the fact that the identity sequence produces ascending numbers to correlate them with ttFoo.idx (I cannot do RETURNING ttFoo.idx too because only the inserted row is available for that which doesn't contain idx)
Query the current value of the identity sequence prior to insertion, then check afterwards which rows are new.

I would assume that this is a common situation, yet I don't see an obviously correct solution. What do you recommend?

Solution

You can find out which rows have been affected by your current transaction using the system columns. The xmin column contains the ID of the inserting transaction, so to return the id values you just copied, you could:

BEGIN;

COPY foo(col1,col2) FROM STDIN;

SELECT id FROM foo
WHERE xmin::text = (txid_current() % (2^32)::bigint)::text
ORDER BY id;

COMMIT;

The WHERE clause comes from this answer, which explains the reasoning behind it.

I don't think there's any way to optimise this with an index, so it might be too slow on a large table. If so, I think your second option would be the way to go, i.e. stream into a temp table and INSERT ... RETURNING.