What does 'Using filesort' in EXPLAIN Extra indicate?
- A The query reads directly from a file
- B MySQL must perform an additional sorting pass rather than reading rows in index order
- C The table is stored on a slow disk
- D A temporary file has been created for the output
Answer
MySQL must perform an additional sorting pass rather than reading rows in index order
Despite the name it does not necessarily involve disk, but it does mean extra work. An index matching the ORDER BY can often eliminate it.
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