Re: people who buy A, also buy C, D, E - Mailing list pgsql-sql

On 6/24/2005 11:35 PM, Greg Sabino Mullane wrote:
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>> The goal of my query is: given a book, what did other people who
>> bought this book also buy?  I plan the list the 5 most popular such
>> books.

This sounds very much like one of the congestion problems given in the 
TPC-W benchmark. You might want to take a look at some of the published 
full disclosure reports or the PHP TPC-W implementation on pgfoundry to 
get some hints.


Jan

> 
> I've been playing with this a little bit, and I don't think you are
> going to get better than you already have. Certainly, the caching
> won't work either as any insert into the watch_list_element has
> the potential to change a very large number of pre-compiled lists.
> However, there are some minor optimizations that can be made to
> speed up the existing query quite a bit. One note first: the LIMIT
> should be 6 not 5 if you really want the five other "books" and the
> book itself will more than likely appear in the list. Picking it
> out is something the client app can do.
> 
> * Make sure the tables are freshly analyzed. Might want to bump
> up the default stats a bit too.
> 
> * Looks like you already have indexes on the watch_list_element
> table. The watch_list_element_element_id index could be broken
> into multiple conditional indexes, but your explain shows this
> would not really gain us much:
> 
> actual time=37.957..41.789
> 
> * One big gain would be to cluster the table on watch_list_id:
> 
> CREATE INDEX watch_index ON watch_list_element (watch_list_id);
> CLUSTER watch_index ON watch_list_element;
> 
> I got about a 25% speedup on my queries by doing this. YMMV, as I
> don't know enough about your conditions to do more than make an
> approximate test database. But it should help this query out.
> 
> * Finally, you should upgrade if at all possible. Going from
> 7.4.7 to 8.0.1 gave me a 10% speed increase, while going from
> 8.0.1 to 8.1.0 (e.g. the upcoming version) gave me an additional
> 25% speed boost, mostly due to the new bitmap stuff. So, making
> the jump to 8.0.1 will be good practice for the 8.1.0 jump, right? :)
> 
> Overall, I was able to get the query to go about a third faster
> than when I started. Hope this helps.
> 
> - --
> Greg Sabino Mullane greg@turnstep.com
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