Re: Rules and Command Status - update/insert/delete rule with series of commands in action

> On 31 May 2024, at 00:34, johnlumby(at)hotmail(dot)com wrote:
>
> On 5/30/24 4:56 PM, David G. Johnston wrote:

(…)

>> If anything is done it would have to be new syntax.
>>
>>
> A much bigger task surely.
>
> On 5/30/24 5:19 PM, Adrian Klaver wrote:
>>
>> 2) Use INSTEAD OF triggers:
>>
>>
>
> Unfortunately the same functionality as in my example with the RULE is not supported for triggers on views : from the manual
> INSTEAD OF triggers may only be defined on views, and only at row level;
>
> A RULE is essentially a statement-level operation which is what I need for this particular case. A row-level trigger would not work because it cannot "see" the query causing it to be fired, and also , (most importantly) is not fired at all if no rows match the original query, whereas a RULE is always in effect regardless of which rows are involved. before. I should add that the RULE I showed in my example is not the only RULE being used on this view - there are other conditional RULEs, and the combined effect is of being able to change the effect of the original statement into a set of new statements, one of which does what is needed.
>
> And if you are now inclined to say "well, maybe the application itself is poorly written and should be changed" - I would have to agree, but that is not mine to change.
>
> But I suppose that my next question, given what you both say about the RULE system being a dead-end, is whether there is any likelihood of supporting an INSTEAD OF trigger on a view at statement level? Maybe that stands more chance of going somewhere?

What you’re attempting to do boils down to adding a virtualisation layer over the database.

Several middleware products exist that provide data virtualisation, products that are accessed as a database (or as a web service, or both) that pass on queries to connected systems. The virtualisation layer rewrites those queries between the data sources and the user-visible virtual database connection and between generalised SQL and native dialects and languages.

If existing products support your particular use-case though, namely rewriting operational data-storage queries to data-source specific DML statements and then report the correct number of affected rows back, I don’t know.

However, an important reason that PG rules are deprecated (as I understand it) is that it is very hard to get right for generated columns, which are operations with side-effects (such as incrementing a sequence value, for example) that are included in those queries rewritten by the specified rules.
I doubt that a data virtualisation layer would be able to solve that particular problem.

Nevertheless, considering what path you’re on, they may be worth looking at. I don’t think there are any open-source initiatives (unfortunately), they’re all commercial products AFAIK, and not cheap. With a suitable use-case they can be rather valuable tools too though.

Regards,

Alban Hertroys
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