Обсуждение: Estimate time without running the query

On 9/13/18 4:55 PM, Neto pr wrote:
> 
> 
> Em qui, 13 de set de 2018 às 19:53, David G. Johnston 
> <david.g.johnston@gmail.com <mailto:david.g.johnston@gmail.com>> escreveu:
> 
>     On Thu, Sep 13, 2018 at 3:30 PM, Neto pr <netoprbr9@gmail.com
>     <mailto:netoprbr9@gmail.com>>wrote:
> 
>         The problem is that using the explain analyze <query> I have to
>         wait for the query to execute.
>         I would like to estimate the time without having to wait for the
>         query execution.
>         Does anyone know how to estimate the time without waiting for
>         the query to be executed?
> 
> 
>     On the machine in question you have to experiment to obtain data to
>     construct a formula to convert cost to time.  Then when using the
>     function remember that lots of things can play into individual
>     executions taking more time (and sometimes less too I suspect) such
>     as locks, caching, physical data locality.
> 
>     It seems more useful to log actual execution times and look for
>     trends.  If you are writing a query odds are it needs to be run
>     regardless of how efficient it may be - or used in a relative
>     comparison to an alternate query.
> 
> 
> Okay, David, but does not it have some SQL statement that returns a time 
> estimate, without having to execute the query?

To get close to a true time you need to run the actual query. An analogy 
based on running 10K under the following conditions:

1) Cool day, flat course.

2) Hot day, up a 10% grade.

You can reasonably predict that 1) will yield a faster time then 2), 
however you will not know the actual times until you run them.

> 
>     David J.
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com