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Re: index_merge, @@sort_buffer_size, and Sort_XXX counters


On 01/19/2011 02:32 PM, Sergey Petrunya wrote:
> Hello,
> index_merge uses @@sort_buffer_size as a guide of how much space it can use for
> sorting. I think there is an issue with the way it is done.
> Before index_merge was introduced, @@sort_buffer_size was used only by
> filesort (also for couple of other cases like GROUP_CONCAT with sorting, but I
> consider them to be too rare to be of practical importance).  This allowed the
> following:
> 1. One can set @@sort_buffer_size to control how much space is to be allocated
> 2. SHOW STATUS will produce these numbers:
>   Sort_merge_passes
>   Sort_range
>   Sort_scan
> Sort_range+Sort_scan  gives the total number of filesort invocations.
> Sort_merge_passes is the number of merges. One can get average amount of merges
> per sort operation by calculating:
>   Sort_merge_passes / (Sort_range + Sort_scan)      (1)
> If the number is high, then it means that it could be useful to increase 
> @@sort_buffer_size.
> Now, if we take index_merge into the picture, the counters are much less
> useful. index_merge causes Sort_merge_passes to be incremented, but we
> dont have a counter for index_merge operations, so formula (1) becomes invalid.
> Possible solutions are:
> Option#1:  introduce a counter Sort_index_merge_count (name subject to
> discussion), which will mean "number of index_merge sort union/intersection
> scans".
> Option#2: make it so that index_merge does not cause Sort_merge_passes to be
> incremented. I think that in this case we'll need a separate variable to
> control index_merge's buffer size. it could have @@sort_buffer_size value as
> default.
> Any comments?

IMHO it does not make sense to think about it BEFORE 5.3 beta is out.


> BR
>  Sergey