> On Wed, 21 May 2008 16:10:53 +0200, H. Hall wrote:
>> Does anyone know if there is a source that provides "Big O" notation
>> for postgres's aggregate functions and operations? For example is
>> count(*) = O(1) or O(n)?
>> Do the developers for postgres use Big O when selecting algorithms?
>> If so, is the info easily available?
> You can't do any better than O( n rows examined by the aggregate )
> except for max() and min() on an indexed expression, which in this
> case aren't really aggrgates anymore since they are internally
> rewritten as an index lookup to get the value you want... but stuff
> like sum() or avg() or count() will always have to see all the rows
> selected (and some more) unless you use clever hacks like materialized
> views etc, in which case the thing in the O() will change, or at least
> the O() constant will change...
Thank you PFC and also Jonah, and Richard for your replies.
It occurs to me that Big O might be a useful way to understand/explain
what is happening with situations like Albert's earlier today:
I've got a query similar to this:
> > select * from t1, t2 where t1.id > 158507 and t1.id = t2.id;
> > That took > 84 minutes (the query was a bit longer but this is the part
> > that made the difference) after a little change the query took ~1 second:
> > select * from t1, t2 where t1.id > 158507 and t2.id > 158507 and t1.id =
> > t2.id;
BTW, anyone reading this and not familiar with Big O notation might want
to check out these links. All are intro type articles:
* An informal introduction to O(N) notation:
* Analysis of Algorithms and Selection of Algorithms:
* Complexity and Big-O Notation
ReedyRiver Group LLC
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