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Re: Order by speed optimization


On 01/11/2013 09:33 PM, Matthew Lagoe wrote:
> I was talking with a few people about a query such as
>     |select * from table order by X, Y, Z limit G;|
> As the "ORDER BY" for me could change to any number of 12 values in any
> order I am not able to use a multicolumn index as such I was looking at
> how to optimize in such case where sorting is dynamic.
> On my dataset the query takes ~7 seconds for a full table scan but only
> .3 seconds when using the column index and returns ~80,000 records for
> the query (before the LIMIT)
> I was wondering if there is any reason that the order by syntax doesn't
> use the following logic, I was thinking of looking at the code but was
> told such a simple optimization must have a reason that its not already
> in the code, so I am asking here before I spend hours being stupid.
> This logic block will be based on the above query.
>    1. If index exists for column X and there is a limit on the query
>       then use the index and sort the list, then return the first G
>       values, continue past G until you hit a different value for X then
>       the value that was at G
>    2. Use this smaller subset of data for then ordering the data based on Y
>    3. Order the data again based on Z
>    4. Return the sorted data like normal
> If there is a reason this optimization isn't in the code please someone
> let me know, if not I think I will take a look at adding it.


It would be nice if you could support such an optimization for
when only a prefix  of the <columns> list is compatible with some
index or with its major components.

> -Botanic
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