MySQL 5.7 Reference Manual
MySQL 5.7 Release Notes
The
table_io_waits_summary_by_index_usage
table aggregates all table index I/O wait events, as
generated by the
wait/io/table/sql/handler instrument. The
grouping is by table index.
The columns of
table_io_waits_summary_by_index_usage
are nearly identical to
table_io_waits_summary_by_table.
The only difference is the additional group column,
INDEX_NAME, which corresponds to the name
of the index that was used when the table I/O wait event was
recorded:
A value of
PRIMARYindicates that table I/O used the primary index.A value of
NULLmeans that table I/O used no index.Inserts are counted against
INDEX_NAME = NULL.
TRUNCATE TABLE is permitted
for table I/O summary tables. It resets the summary columns
to zero rather than removing rows. This table is also
truncated by truncation of the
table_io_waits_summary_by_table
table. A DDL operation that changes the index structure of a
table may cause the per-index statistics to be reset.