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-done(status)
read-row-start: Triggered when a row is
read by the storage engine from the specified
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.