[Monetdb-developers] Single column select vs aggregation

Stefan de Konink stefan at konink.de
Thu Feb 18 19:40:15 CET 2010


Op 18-02-10 19:22, Stefan Manegold schreef:
> "It" (in fact we) choose to do a hash select, and since there is no hash
> table, yet, we need to build it, which is infact more expensive than a
> simple scan select for this very operation (later operation *might* then
> benefit from the hash table ...):

Question about that then; if we make an on the fly hash. Why isn't it 
'maintained' between queries (or does this depend on the chosen 
pipeline?) Because the query doesn't seem to get faster when running it 
multiple time?

> For now, you can just locally disable/remove that alternative in the above code,
> try again, and report the result.

Cold:
sql>select kvk from kvk where kvk = 412657690010;
+--------------+
| kvk          |
+==============+
| 412657690010 |
+--------------+
1 tuple
Timer    1174.737 msec 1 rows

Hot:
sql>select kvk from kvk where kvk = 412657690010;
+--------------+
| kvk          |
+==============+
| 412657690010 |
+--------------+
1 tuple
Timer      23.741 msec 1 rows
sql>select kvk from kvk where kvk = 412657690010;


Thanks for this 20x performance increase! (And it gets even better, 
because numbers that doesn't exist are excluded in ~13ms.)


Stefan




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