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MySQL 5.7 Reference Manual
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The read row probes are triggered at a storage engine level each time a row read operation occurs. These probes are specified within each storage engine (as opposed to the *row-start probes which are in the storage engine interface). These probes can therefore be used to monitor individual storage engine row-level operations and performance. Because these probes are triggered around the storage engine row read interface, they may be hit a significant number of times during a basic query.

read-row-start(database, table, scan_flag)
  • read-row-start: Triggered when a row is read by the storage engine from the specified database and table. The scan_flag is set to 1 (true) when the read is part of a table scan (that is, a sequential read), or 0 (false) when the read is of a specific record.

  • read-row-done: Triggered when a row read operation within a storage engine completes. The status returns 0 on success, or a positive value on failure.

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