[389-devel] RFC: New Design: Fine Grained ID List Size

[Rich Megginson]

On 09/12/2013 07:08 PM, David Boreham wrote:
> On 9/11/2013 11:41 AM, Howard Chu wrote:
>>
>> Just out of curiosity, why is keeping a count per key a problem? If 
>> you're using BDB duplicate key support, can't you just use 
>> cursor->c_count() to get this? I.e., BDB already maintains key counts 
>> internally, why not leverage that?
>>
>
> afaik you need to pass the DB_RECNUM flag at DB creation time to get 
> record counting behavior, and it imposes a performance and concurrency 
> penalty on writes. Also afaik 389DS does not set that flag except on 
> VLV indexes (which need it, and coincidentally were the original 
> reason for the feature being added to BDB).

I'm using bdb 4.7 on RHEL 6.
Looking at the code, it appears the dbc->count method for btree is 
__bamc_count() in bt_cursor.c.  I'm not sure, but it looks as though 
this function has to iterate each page counting the duplicates on each 
page, which makes it a non-starter.  Unless I'm mistaken, it doesn't 
look as though it keeps a counter on each update, then simply returns 
the counter.  I don't see any code which would make the behavior 
different depending on if DB_RECNUM is used when the database is created.

>
>
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