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This alarm fires when the number of temporary files created for sorts is high in comparison to the number of sorts performed.
When MySQL sorts some data – for instance, to resolve a query with an ORDER BY clause – it generates at least one disk file if the size of the SORT_BUFFER_SIZE is insufficient to sort all the data in memory.
If this happens, MySQL creates one or more disk files, and merges those files after all rows have been read.
If the average number of sort merge sort files is high, the sort will almost certainly benefit from more memory. If a sort generates only one disk file, adding more memory will improve the performance only if it allows the sort to avoid creating a disk file.