Re: Guidance on INSERT RETURNING order

To clarify, the work around (for SQLAlchemy) does not require the RETURNING values to be ordered; they will be reordered code side.  Longer term matching the order up would be an added bonus!

On Thu, 13 Apr 2023 at 04:02, John Howroyd <jdhowroyd@googlemail.com> wrote:

The ideal solution would be that the RETURNING values are in the same order as the declared rows in the table literal "FROM (VALUES (10, 11, 1), (12, 13, 2)) AS sel(p1, p2, num)".  But before any of you jump down my throat about about sets and a bar to parallel inserts, I think it's a small matter of some internal smoke and mirrors: simply marshal the RETURNING results through and in memory/temporary table maintaining the order from the SELECT and return these to the client upon commit.  At this moment, I believe the folk at SQLAlchemy are looking for a work around that can work for most use cases with either an identity column or a fake "sentinel" column for matching back to the ORM objects.  There may be an upper limit of 1000 to the number rows in a single insert (this arises in  MS SQL Server, who absolutely do not guarantee the order of their OUTPUT clause -- it's tied to the physical row insert rather than marshalled).

My feeling is that PostgreSQL may already do this.  See the commit:

https://github.com/postgres/postgres/commit/9118d03a8cca3d97327c56bf89a72e328e454e63

and the original message thread

https://www.postgresql.org/message-id/flat/CADBwkHv1nUx5bqxqRLp26k8iUWSnY3uW-1_0EMYxKns0t2JnUA%40mail.gmail.com

and references to undesired behaviour prior to PostgreSQL 9.6 such as in https://www.postgresql.org/docs/current/sql-select.html.

The test cases in the above commit use:

SELECT nextval('sequence'), datavals FROM ... ORDER BY something;

type syntax.  And I presume that's exactly what's happening internally when there's a serial identity column (via query rewrite rules or another mechanism).

So really, that just needs confirming.  What happens with triggers?  Presumably the same as long as query rewrite rules are used, but this is presumably getting deeper into the code for actual inserts after the initial select.  The jump to the output being ordered, is just a guess but there's some bits and pieces that seem to suggest that there may indeed be a marshalling process going on in the background (whether or not that is linked to the original order is another matter).

I have set up a PostgreSQL server to test if I can break this hypothesis and see what query explains can allude to.  Does anyone have a test case where the order is not preserved?

Might I also point out that if the implementation of parallel inserts does create a bar then doing so may end up with the programmatic interfaces (such as SQLAlchemy) not being able to use that feature (possibly reverting back to single inserts).  Ur, so what would be the problem being solved with parallel inserts?

On Thu, 13 Apr 2023 at 02:27, Mike Bayer <mike_mp@zzzcomputing.com> wrote:

We do something different for uuids.  These are usually created client side anyway or are very easy to create client side if not and then we also have a separate sentinel column option.

Overall the whole thing is an optimization that can be turned off for uncommon cases.   We just can't go back to having insert of 1000 rows be 3-5x slower for all tables w server generated primary keys which is what statement at a time gets us.  Surrogate integer Pks w sequence or identity is the big case.

On Wed, Apr 12, 2023, at 8:58 PM, Thorsten Glaser wrote:

> On Wed, 12 Apr 2023, Mike Bayer wrote:

>

>>ascending values for "mytable_id_seq" will correspond to the ascending

>>values for "num".

>

> But, again, what if it’s uuid or something instead of a sequence?

>

> bye,

> //mirabilos

> --

> 15:41⎜<Lo-lan-do:#fusionforge> Somebody write a testsuite for helloworld :-)