maria-discuss team mailing list archive

[Roberto Spadim <roberto@xxxxxxxxxxxxx>]

but check my idea, for example...

select some_fields...
from table
where (where 2,3,4,5,6+ index could be used)
order by (where 1 index could be used)


optimizer prefer the order by index, i want that optimizer ignore the order
by index option (it doens't exists today)
does it make sense?


2015-06-26 1:38 GMT-03:00 Justin Swanhart <greenlion@xxxxxxxxx>:

> Ignore index
>
> Sent from my iPhone
>
> On Jun 25, 2015, at 7:44 PM, Roberto Spadim <roberto@xxxxxxxxxxxxx> wrote:
>
> could this be helped by a optimizer_switch to don't optimize using order
> by?
>
> 2015-06-23 10:18 GMT-03:00 Justin Swanhart <greenlion@xxxxxxxxx>:
>
>> Hi,
>>
>> You can't fix that with a flag that says an index is unique.  It is a big
>> problem.  Domas opened a bug on a variety of cases of it recently.
>>
>> On Mon, Jun 22, 2015 at 8:03 PM, Roberto Spadim <roberto@xxxxxxxxxxxxx>
>> wrote:
>>
>>> the other solution is  "force index()" but i'm trying to understand why
>>> it choose a 'bad' index
>>>
>>> 2015-06-23 0:01 GMT-03:00 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
>>>>
>>>
>>>
>>>
>>> --
>>> 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