Re: New GUC to sample log queries

On 11/30/18 7:42 AM, Nikolay Samokhvalov wrote:
> On Thu, Nov 29, 2018 at 1:49 PM Alvaro Herrera <alvherre@2ndquadrant.com 
> <mailto:alvherre@2ndquadrant.com>> wrote:
> 
>     Thanks!  I pushed this with two changes -- one was to reword the docs a
>     bit more, and the other was to compute in_sample only if it's going to
>     be used (when exceeded is true).  I hope this won't upset any
>     compilers ...
> 
> 
> This is a great news – I can imaging how helpful this feature will be 
> for query analysis and
> troubleshooting.
> 
> At the same time, there is an approach, when we use application (client) 
> code or pgbouncer's
> connect_query parameter to perform sampled logging (with 
> log_min_duration_statement = 0)
> of n% of all *sessions* or *transactions*.

Good idead! Unfortunately it require pgbouncer, but I keep this trick in 
mind.

> 
> If you use single-query transactions only, new parameter will do 
> equivalent job for you, while
> significantly simplifying you life (pgbouncer is not required and you 
> don't need to patch application
> code). However, if you have multi-statement transaction, 
> log_statement_sample_rate will not
> be enough for troubleshooting – logging just a single statement of a 
> multi-statement transaction
> won't really help to troubleshoot in many cases.
> 
> That being said, I wonder, does it make sense to think about extending 
> the functionality
> just committed, with some options to to log all statements of n% of 
> transactions (or sessions)?
> In other words, allowing to choose, at which level perform sampling – 
> statement, transaction, or
> session?
> 

+1, I like this idea.