[Monetdb-developers] Bug report: Poor query performance

Martin Kersten Martin.Kersten at cwi.nl
Sun Aug 31 20:43:47 CEST 2008


Elad . wrote:
>  
> Hi,
>  
> I have a database with 52 columns and 7 million rows.
dear Elad,

Thanks for using MonetDB. The problem you describe is known. No easy 
solution exists,
because the front-end optimizer does not use, nor rely on table 
statistics to derive the best
plan.
A solution will emerge, but don't hold your breath

regards, Martin
>  
> The columns are Date (timestamp data type), Customer (real data 
> type) and a bunch of other columns that indicate values for each 
> customer on each date (all of these columns are of real data type).
>  
> The 'Date' column contains 5 unique values.
>  
> The 'Customer' column contains 1.7 million unique values.
>  
> 1. The following query returns instantly: SELECT "CUSTOMER" FROM "TABLE".
>  
> 2. The following query takes a long time (stopped after a few 
> minutes): SELECT "CUSTOMER" FROM "TABLE" GROUP BY "CUSTOMER".
>  
> 3. The following query returns instantly: SELECT "DATE", "CUSTOMER" 
> FROM "TABLE" GROUP BY "DATE", "CUSTOMER".
>  
> 4. The following query takes a long time (stopped after a few 
> minutes): SELECT "CUSTOMER", "DATE" from "TABLE" GROUP BY "CUSTOMER", 
> "DATE".
>  
> * Note that the only difference between query 1 and query 2 are the 
> addition of a group by clause to query 2.
>  
> * Note that the only difference between query 3 and query 4 are the 
> order of columns in the select and group by clauses.
>  
> Is there a way to work around this?  I'm really stuck.
>  
> Thanks in advance,
>  
> Elad
>
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