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Displaying posts with tag: Blindspot (reset)
Jun
26
2023
Rows Examined Blindspot when Looking for non-Existing Data
Posted by Jean-François Gagné on Mon 26 Jun 2023 22:17 UTC
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When trying to understand queries in the slow log, an interesting metric to look at is rows examined.  Also, when trying to understand CPU spikes on a MySQL instance, the InnoDB Metric dml_reads and the global status Innodb_rows_read are interesting to check.  In a similar way, when trying to gather information about which queries are loading the system, SUM_ROWS_EXAMINED from the

Mar
08
2022
Rows Examined not Trustworthy because of Index Condition Pushdown
Posted by Jean-François Gagné on Tue 08 Mar 2022 05:00 UTC
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When Index Condition Pushdown is used, you cannot trust rows examined as reported by the slow query log (nor in some Performance Schema tables) and you cannot trust the global status Innodb_rows_read (nor the InnoDB Metrics dml_reads).  These can lead to an incorrect query complexity analysis, to a mistaken estimation of the total load of a MySQL server, and to a lot of wasted time.&

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