At the end of my talk “Troubleshooting MySQL Performance” at the LinuxPiter conference, a user asked me a question: “What does the EXPLAIN ‘filtered’ field mean, and how do I use it?” I explained that this is the percentage of rows that were actually needed, against the equal or bigger number of resolved rows. While the user was happy with the answer, I’d like to better illustrate this. And I can do it with help of EXPLAIN FORMAT=JSON and its
rows_examined_per_scan, rows_produced_per_join
statistics.
Let’s take a simple query that searches information about the …
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