Re: understanding explain data

[Sim Zacks <sim@xxxxxxxxxxxxxx>]

Now you're talking about data warehouse design and not optimizing 
queries, though they are obviously interrelated. A human looking at the 
explain data would not be able to determine that it would be better to 
have a summary table either.

However, first you would want to optimize your queries and if that 
didn't bring the desired performance, then you would consider adding 
denormalization statistics. An experienced DBA would know when to put in 
the statistics tables to begin with.

Alban Hertroys wrote:
> Sim Zacks wrote:
> > I disagree with you that a human brain would be better then a machine 
> > for optimizing purposes. If the system is programmed to optimize 
> > correctly, then it will when to stick data into a temp table and 
> > update columns instead of doing a select because x number of joins are 
> > too much for the select. Humans may not know the optimal number of 
> > joins before the query becomes inefficent.
>
> You're thinking about single queries here ;) A piece of software can 
> very well optimize a single query, but it will be limited to that.
>
> I was referring to changing parts of your database so that the query can 
> be written in a simpler, more optimal way.
>
> As an example, say that you have a tree structure of factories, each 
> with a multitude of departments, and you want to query the number of 
> employees (sum of all departments) in a specific factory.
>
> You could write a query to read in all those departments and then count 
> all the employees in them.
>
> You could also keep track of the the employee-count for each department 
> when you insert or delete employee records, and summarize these values 
> for each factory record (using a trigger, most likely). If you do that, 
> you only need to read the employee-count of the factory record.
>
> Which do you think is faster? And how do you think a piece of software 
> can get to this solution only from reading the explain output of your 
> query?