Thread: INSERT INTO .. SELECT nextval() ORDER BY - returns unexpectedly ordered values

It seems I have found a bug in the way postgres combines sequences and
ORDER BY with internal data ordering.

I have a table that has an `id`, where values were inserted somewhat
randomly. I wanted to re-order the rows a assign a new `id`, so I created a
sequence and did INSERT INTO .. SELECT. What I didn't understand is the
rows came out ordered correctly, but the new id's didn't. Instead of
incrementing correctly they seemed to follow the original ordering of the
rows.

I have reduced this to the following testcase:

CREATE TABLE test (
    name character varying(4),
    id smallint NOT NULL
);
CREATE TABLE test2 (like test);
ALTER TABLE test2 ADD old_id smallint;

INSERT INTO test VALUES ('c', 13);
INSERT INTO test VALUES ('d', 14);
INSERT INTO test VALUES ('a', 11);
INSERT INTO test VALUES ('b', 12);

CREATE TEMPORARY SEQUENCE tmp START 1;
INSERT INTO test2 SELECT name, nextval('tmp'), id FROM test ORDER BY id ASC;

SELECT * FROM test2;

What I expected:

name  id  old_id
a    1    11
b    2    12
c    3    13
d    4    14

What I got:

name  id  old_id
a    3    11
b    4    12
c    1    13
d    2    14

I have worked around this by clustering the old table on the new id before
SELECTing but this behavior doesn't seem to be documented, is this a bug?

Thanks,
Sjon

On 3. sep. 2015, at 14:00, Sjon Hortensius <sjon@hortensius.net> wrote:

> I have worked around this by clustering the old table on the new id before=
 SELECTing but this behavior doesn't seem to be documented, is this a bug?

Your nextval is run before the ordering. Otherwise, you wouldn't be able to o=
rder based on things computed in the select.=20

You can stack it though, such as:

INSERT INTO <...>
SELECT name, nextval(sequence), old_id FROM
( SELECT name, old_id FROM test ORDER BY old ASC ) as=20
 x;

That way, you sort before pulling a new value with nextval, giving you the r=
esult you want. Did that make sense?

Or simply put; not a bug. :-)

Terje=