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From Emmanuel Lécharny <elecha...@gmail.com>
Subject Re: [index] Presence index usage
Date Thu, 22 Mar 2012 16:28:50 GMT
Le 3/22/12 5:11 PM, hyc@symas.com a écrit :
> On Thu, Mar 22, 2012 at 04:40:17PM +0100, Emmanuel Lécharny wrote:
>> Hi guys,
>>
>> I will split my mails about index in smaller mails, in order to
>> focus on one element at a time.
>>
>> Let's first talk about the presence index.
>>
>> Yesterday, I posted a question about the need to restrain this index
>> to hold only the AT which are indexed. Alex explain that it was
>> rational as it will cost less to store only the indexed AT in this
>> index.
>>
>> I have now a few more things to discuss about this index :
>>
>> 2) What about storing all the AT into this index ?
>>  From the performance POV, this is not a good idea. We have around
>> 1000 different AT in a schema, and each entry with, say, N different
>> AT will need to update the forward table for every single of those N
>> AT. Costly.
> My rule of thumb is only index attrs that are actually used in search
> filters, and only if their frequency in the db is low. (If an attr is
> present in 100% of entries, then the index is totally superfluous.)
Makes sense. The Admin must be smart, and think before creating the 
indexes...
>
>> Now, if we consider the fact that having all the AT stored in the
>> index will allow us to know what will be the impacted entries if an
>> AT is removed from the schema, then it can be a good thing to have a
>> complete index with all the AT.
> It's an interesting idea, if the admin was going to index it anyway.
> Otherwise, IMO you're optimizing for a very infrequent case, which
> is self-defeating.
Here, it's not about optimization, really.

The idea is much more about bieng able to see if an AT removal from the 
schema is likely to impact the data, without doing a full scan.

Not sure it's a sane politic though : removing an AT from a production 
server sounds a bad idea...

-- 
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com


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