Re: Extension to rewrite queries before execution

Jeff, although it would be tedious, you could write a trigger of the form
BEFORE INSERT ON yatta_yatta
EXECUTE PROCEDURE check_rewrite;

Then in check_rewrite, you could examine the contents of
pg_stat_activity.query column for the current process
and act accordingly.

I acknowledge that it will impact performance, but I see no other way to
accomplish what you wish.

At the very least, you would be able to issue the "set enable_* =off;" or
"set work_mem=*; inside the procedure.

That is the best I can suggest.

On Fri, Aug 14, 2015 at 11:53 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> > On Thu, Aug 13, 2015 at 1:37 PM, Melvin Davidson <melvin6925(at)gmail(dot)com>
> wrote:
>
>
>> On Thu, Aug 13, 2015 at 3:49 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>
>>> I am looking for an extension or a technique that will allow me to
>>> intercept a query by the exact query text, and replace that query with a
>>> different one.
>>>
>>> The context is running a third-party app which issues queries I have no
>>> control over. I'd like to intercept a specific query (which has no bind
>>> parameters) and either replace the query text with a different text which,
>>> for example, swaps out an "in list" clause to instead be an "exists
>>> (subquery)".
>>>
>>> Or just wrap the query in a "set enable_* =off;" or "set work_mem=*;"
>>> before and a reset of it after.
>>>
>>> Is there anything out there like this? This would be for 9.4.
>>>
>>> I'm willing to put the query text, and its replacement, directly into
>>> the extension source code and compile it, but of course something more
>>> flexible would be ideal.
>>>
>>>
>
> You have not stated which Version or PostgreSQL, nor the O/S involved.
>> That being said, depending on what the specific query is, you might
>> consider using a Rule or Trigger to handle it. If you use a Trigger ( which
>> is the preferred method) you can also embed "set" commands the associated
>> function.
>>
>
> Sure I did, 9.4. OS would be Linux, although I would hope a solution
> would be work. If you mean the built-in-to-core rules or triggers, those
> wouldn't work. Neither one allows you to rewrite a where clause as far as
> I can tell. Rules allows you add one, but not more than that. And
> triggers don't exists for select queries.
>
> Cheers,
>
> Jeff
>

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.