Re: Detecting change in event properties

>Marc Mamin-2 wrote
>> I would misuse GUC variables for this.
>> (using the functions current_setting and set_config)
>>
>> define a set get and switch fuction (I use operators for better
>> readability)
>> something like:
>>
>> select 'a' ==> 'foo'
>> 'a'
>> select 'b' <==> 'foo'
>> 'a'
>> select <== 'foo'
>> 'b'
>>
>>
>> and  in your query:
>>
>> SELECT
>>  case when test then col <==> 'foo' else <== 'foo' end
>
>Is it possible to alter GUC on a record-by-record basis?

yes, the underlying functions are volatile.
Which quite certainly imply that you cannot repeat the statement within a GROUP or ORDER BY...
And you must of course ensure that it get called on an ordered set.


>Is this something you have actually done?

yes

>
>Even if it does technically work this seems like a last-resort kind of
>solution.  The syntax (though that could be hidden in a wrapper function) is
>definitely unusual and the abuse of the GUC system in this manner is
>surprising.

Sure. There are a some caveat too:

 - it is quite slow

 - you should first initialize the GUC to avoid testing it in each call.
   This should be feasable with an extra function in a top WITH clause

 - GUC are stored as text. You need to define functions for each type you are interested in.
   This implies extra work for casting/decasting

 - NULLs are transformed to empty strings per default, so you may need to care for this in your functions.

Marc Mamin

>
>David J.
>
>