Re: Mini improvement: statement_cost_limit

Josh Berkus wrote:
> Tom,
>
>   
>> Wasn't this exact proposal discussed and rejected awhile back?
>>     
>
> We rejected Greenplum's much more invasive resource manager, because it 
> created a large performance penalty on small queries whether or not it was 
> turned on.  However, I don't remember any rejection of an idea as simple 
> as a cost limit rejection.
>
> This would, IMHO, be very useful for production instances of PostgreSQL.  
> The penalty for mis-rejection of a poorly costed query is much lower than 
> the penalty for having a bad query eat all your CPU.
>
>   
Greenplum's introduced a way to creating a cost "threshold" a bit like 
the way Simon was going to do "shared" work_mem. It did 2 things:

1/ Counted the cost of an about-to-be run query against the threshold, 
and made the query wait if it would exhaust it
2/ Aborted the query if its  cost was greater than the threshold

Initially there was quite a noticeable performance penalty with it 
enabled - but as the guy working on it (me) redid bits and pieces then 
penalty decreased massively. Note that in all cases, disabling the 
feature meant there was no penalty.

The latest variant of the code is around in the Bizgres repository 
(src/backend/utils/resscheduler I think) - some bits might be worth 
looking at!

Best wishes

Mark

P.s : I'm not working for Greenplum now.