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Message #03851
index_merge, @@sort_buffer_size, and Sort_XXX counters
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?
BR
Sergey
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Sergey Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog
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