# [erlang-questions] Get a range of tuples from a Mnesia table

Greg Burri greg.burri@REDACTED
Mon Oct 6 16:08:48 CEST 2008

```Thanks for you reply. I haven't try yet but I keep your answer for later
use.

On Sat, Oct 4, 2008 at 8:11 PM, Edwin Fine
<erlang-questions_efine@REDACTED>wrote:

> Looking through the docs, I see that if you are using an mnesia table that
> is ram_copies or disc_copies, you can do certain fast operations on the
> associated ets table. One such operation (which is recommended only for
> debugging, but what the heck) is ets:slot(Tab, I), which returns the I'th
> record in the table. So maybe you could find a way to do that on the ETS
> table that Mnesia is using, perhaps via the mnesia:ets() function. I have
> never tried this so it's just a guess. This is also kind of a hack but other
> than reading through the mnesia code, I can't locate something better. Maybe
> one of the real Mnesia gurus will have an idea.
>
> 2008/10/4 Greg Burri <greg.burri@REDACTED>
>
>> Hi,
>> I want to retrieve N tuple with an offset O from the end of a table. N and
>> O are integer(). I consider that the acces is based on an ordered table.
>>
>> Here is an example, I have such tuples in my table t :
>> [{t, 1, _},
>> {t, 2, _},
>> {t, 3, _},
>> {t, 4, _},
>> {t, 5, _},
>> {t, 6, _},
>> {t, 7, _}]
>>
>> For N=2 and O=3 the result will be :
>> [{t, 4, _}, {t, 5, _}]
>>
>> For now I have to use an ordered_set table and the functions mnesia:last/1
>> and mnesia:prev/2, it works but the complexity is O(O). If I have to get
>> some tuple from the middle of the table it will take long time.
>> Is there an another solution than mine ?
>>
>> Second question : How change the type of a table from set to ordered_set
>> on an existing table ?
>>
>>
>> /Greg Burri
>>
>> _______________________________________________
>> erlang-questions mailing list
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>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>
>
>
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