Re: Memory usage - indexes

On 29/09/10 19:41, Tobias Brox wrote:
> I just got this crazy, stupid or maybe genius idea :-)
>
>
> Now, my idea is to drop that fat index and replace it with conditional
> indexes for a dozen of heavy users - like those:
>
>    acc_trans(trans_type, created) where customer_id=224885;
>    acc_trans(trans_type, created) where customer_id=643112;
>    acc_trans(trans_type, created) where customer_id=15;
>
> or maybe like this:
>
>    acc_trans(customer_id, trans_type, created) where customer_id in ( ... );
>
> Any comments?
>
> My sysadmin is worried that it would be a too big hit on performance
> when doing inserts.  It may also cause more overhead when planning the
> queries.  Is that significant?  Is this idea genius or stupid or just
> somewhere in between?
>
>

Yeah, I think the idea of trying to have a few smaller indexes for the
'hot' customers is a good idea. However I am wondering if just using
single column indexes and seeing if the bitmap scan/merge of smaller
indexes is actually more efficient is worth testing - i.e:

acc_trans(trans_type);
acc_trans(created);
acc_trans(customer_id);

It may mean that you have to to scrutinize your effective_cache_size and
work_mem parameters, but could possibly be simpler and more flexible.

regards

Mark