maria-discuss team mailing list archive

[Roberto Spadim <roberto@xxxxxxxxxxxxx>]

ehhe that's the point, how to remove perverse order by, today i'm using
temporary table and after ordering, but i don't like this "solution"

2015-06-22 23:50 GMT-03:00 Justin Swanhart <greenlion@xxxxxxxxx>:

> If you have the PK in your query (unless the db does something perverse
> with order by (which is common)), then the PK will be used.  Sometimes the
> order by thing bites in other direction, and PK is used for desc or asc
> access to table when another index could be used.  Hash indexes are only on
> memory, and are preferred over b-tree unless ranges are performed because
> hash is O(1) and b-tree is O(log(n)).  rtree and fulltext indexes can only
> be used with particular functions.  non-clustered indexes are used in all
> other cases, sometimes with ICP.
>
> When you have more than one index to choose that could be effective, the
> histograms (statistics) for the tables are compared and the database tries
> to decide which index will perform best (cost based optimization).
>
> --Justin
>
> On Mon, Jun 22, 2015 at 7:47 PM, Roberto Spadim <roberto@xxxxxxxxxxxxx>
> wrote:
>
>> yeap
>>
>> at optimizer part of mariadb, is there some check about
>> clustered/nonclustered/rtree/hash index to select best index or not?
>>
>> 2015-06-22 23:45 GMT-03:00 Justin Swanhart <greenlion@xxxxxxxxx>:
>>
>>> you are probably thinking of b-trees which are unbalanced.  b+tree is
>>> o(log(n)) based on depth of tree and you always have to go to bottom to get
>>> leaf.
>>>
>>> On Mon, Jun 22, 2015 at 7:44 PM, Justin Swanhart <greenlion@xxxxxxxxx>
>>> wrote:
>>>
>>>> No, you have to look in the leaf!
>>>>
>>>> On Mon, Jun 22, 2015 at 7:43 PM, Roberto Spadim <roberto@xxxxxxxxxxxxx>
>>>> wrote:
>>>>
>>>>> yeap, i just want know if the unique allow stop the conquer at first
>>>>> result (unique key) or if it continue (non unique key)
>>>>>
>>>>> 2015-06-22 23:42 GMT-03:00 Justin Swanhart <greenlion@xxxxxxxxx>:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> b+tree search is binary search, which is divide and conquer.  Saving
>>>>>> scanning a key is like sneezing in a hurricane, you still are o(log(n)) in.
>>>>>>
>>>>>> On Mon, Jun 22, 2015 at 7:39 PM, Justin Swanhart <greenlion@xxxxxxxxx
>>>>>> > wrote:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> If you do a code inspection you will find that uniqueness is checked
>>>>>>> on insertion, and the database cares not about it after that.
>>>>>>>
>>>>>>> --Justin
>>>>>>>
>>>>>>> On Mon, Jun 22, 2015 at 7:20 PM, Roberto Spadim <
>>>>>>> roberto@xxxxxxxxxxxxx> wrote:
>>>>>>>
>>>>>>>> and there's optimization with this flag? i see that equal could be
>>>>>>>> optimized cause if we found 1 row we can return to user that row and stop
>>>>>>>> select
>>>>>>>>
>>>>>>>> in other words, if i see a index with primary key or unique
>>>>>>>> columns, could i change the index to unique and get some kind of
>>>>>>>> performace? today i think not ,cause unique key check if it's really unique
>>>>>>>> (for example at insert/update/replace), i'm wrong?
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Roberto Spadim
>>>>> SPAEmpresarial - Software ERP
>>>>> Eng. Automação e Controle
>>>>>
>>>>
>>>>
>>>
>>
>>
>> --
>> Roberto Spadim
>> SPAEmpresarial - Software ERP
>> Eng. Automação e Controle
>>
>
>


-- 
Roberto Spadim
SPAEmpresarial - Software ERP
Eng. Automação e Controle