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 -- Sergey Petrunia, Software Developer Monty Program AB, http://askmonty.org Blog: http://s.petrunia.net/blog