Subject: Support for on conflict ignore returning

From: Brian McMahon <[email-masked]>
Date: Tue, 30 Jul 2024 08:53:55 -0700

Hi!

I have a question about support / if there's plans to support a feature, I
thought it would be best to send it to the "novice" mailing list because it
might be a naive question that's been answered before.

Is there a recommended way to achieve a ON CONFLICT DO NOTHING RETURNING
<column> so that conflicting columns still have information returned?
Currently it seems like the only simple way to achieve this is with a NOOP
update, so instead doing something like ON CONFLICT UPDATE SET name =
EXCLUDED.name RETURNING <column>.

As I understand it, this isn't desirable as there's various potentially
undesirable side effects since it's not treated as a NOOP, it's treated as
an update so triggers and all sorts of internal postgres logic are invoked
(which may or may not be fine for certain applications, but it's not
desirable).

Are there current features that support this functionality, or plans to
support something like this in the future?

I should probably add my use case, I need to insert a bunch of rows into
the database to ensure they exist, then for all those rows I need to update
corresponding tables that reference them with more data. If I can't make a
query that returns the primary key, then I either need to make a subsequent
query to pull all the rows from the database that weren't inserted to get
the primary keys, or I need to use the unique fields that caused the
conflict in the next query to update the tables that reference the table I
updated, which makes things a little messier (I'd rather just work with the
primary keys when possible).

Thanks in advance.

-- 
Sincerely,
Brian McMahon

---

From: "David G. Johnston" <[email-masked]>
Date: Tue, 30 Jul 2024 09:00:42 -0700

> EXCLUDED.name RETURNING <column>.

This is requested frequently.  As of now I'm not aware of anyone working on
a patch to make it work.

For the described "chained DML" use case my typical advice is to not rely
upon the "default" clause to produce your surrogate keys but to produce
them yourself.  Also, this is one of the few areas that iterating
row-by-row and doing one set of multi-table DML per transaction (or maybe
savepoint...) works out reasonably well.

David J.

---

From: Brian McMahon <[email-masked]>
Date: Tue, 30 Jul 2024 09:37:14 -0700

Thanks for the quick response David!

them yourself.

We do use auto generated uuids, and if I understand you advice correctly
our application could generate the surrogate keys, but they would need to
be generated by some hashing algorithm of the other unique identifiers,
otherwise the application wouldn't be able to know what the surrogate key
for a row is without re-querying that database. I've tried to avoid this
just because it makes changing constraints a bit more complicated since the
keys would also change and that would need to be rectified. Please dig into
this if my response doesn't reflect what you were suggesting.

> Also, this is one of the few areas that iterating row-by-row and doing
one set of multi-table DML per transaction (or maybe savepoint...) works
out reasonably well.

Just to make sure I'm understanding this correctly, would this be a single
sql statement that loops through the input data, inserts one row the
updates the other tables that need updating before moving onto the next
insert? Or is this sending multiple sql statements from the application?
Are there performance implications between looping over a data set and
inserting one by one vs doing a single insert statement with multiple rows?

Thanks in advace!

On Tue, Jul 30, 2024 at 9:01 AM David G. Johnston <
[email-masked]> wrote:

> This is requested frequently.  As of now I'm not aware of anyone working
> on a patch to make it work.
>

-- 
Sincerely,
Brian McMahon