more generic approach as for userdb? (was: Dict issue with PostgreSQL for last_login plugin (duplicate key))

[Steffen Kaiser]

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On Wed, 22 May 2019, John Fawcett via dovecot wrote:

> an update when insert fails seems to be a MySQL specific extension to
> standard Sql. So I think that it's clear that support for PostgreSql and
> Sqlite  needs to be implemented. The same issue likely exist in other
> plugins too, for example expire.
>
> My doubts are around the right solution to adopt. Initially I thought
> that there was a PostgreSql syntax similar to MySQL which could be
> easily added to the code, but closer inspection shows that the
> PostgreSql syntax requires specification of either a constraint name or
> the index column(s) for the primary/unique keys.

You mean the "target" in ON CONFLICT target action, right?
http://www.postgresqltutorial.com/postgresql-upsert/

> Constraint names are nowhere specified in the dictionary map syntax and
> it's not possible either to identify with 100% certainty the primary key
> column(s).

One could dive into Postgres-specifics to get it, but there are other 
SQLs, too; the quota plugin advertises to use TRIGGERs to turn an INSERT 
into an UPDATE silently, which is no general approach either.
https://wiki2.dovecot.org/Quota/Dict

> 1) logic which always tries to update and falls back to insert if the
> update fails (or viceversa) for all sql dictionaries.
>
> 2) updates to the map syntax so that either the constraint name or
> primary key columns can be specified.
>
> Ideas are welcome.

Maybe, one should drop the automatic at all and let the user specify the 
commands manually like with the userdb/passwd. Hence, the generic SQL 
preparation code is already present. There could/should/would be 
documented lots of "best practice" settings for various backends.

In fact, this approach would better fit into the open and more "general" 
base idea Dovecot uses in other places, IMHO.

Kind regards,

- -- 
Steffen Kaiser
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