•Index on ( EVENT_NAME ) 

TRUNCATE TABLE  is permitted for file I/O summary tables. It resets the summary columns to zero rather than removing rows. 

The MySQL server uses several techniques to avoid I/O operations by caching information read from files, so it is possible that statements you might expect to result in I/O events do not do so. You may be able to ensure that I/O does occur by flushing caches or restarting the server to reset its state. 

## 27.12.20.8 Table I/O and Lock Wait Summary Tables 

## The table_io_waits_summary_by_table Table 

The following sections describe the table I/O and lock wait summary tables: 

• table_io_waits_summary_by_index_usage : Table I/O waits per index 

• table_io_waits_summary_by_table : Table I/O waits per table 

• table_lock_waits_summary_by_table : Table lock waits per table 

The table_io_waits_summary_by_table  table aggregates all table I/O wait events, as generated by the wait/io/table/sql/handler  instrument. The grouping is by table. 

The table_io_waits_summary_by_table  table has these grouping columns to indicate how the table aggregates events: OBJECT_TYPE , OBJECT_SCHEMA , and OBJECT_NAME . These columns have the same meaning as in the events_waits_current  table. They identify the table to which the row applies. 

table_io_waits_summary_by_table  has the following summary columns containing aggregated values. As indicated in the column descriptions, some columns are more general and have values that are the same as the sum of the values of more fine-grained columns. For example, columns that aggregate all writes hold the sum of the corresponding columns that aggregate inserts, updates, and deletes. In this way, aggregations at higher levels are available directly without the need for user-defined views that sum lower- level columns. 

• COUNT_STAR , SUM_TIMER_WAIT , MIN_TIMER_WAIT , AVG_TIMER_WAIT , MAX_TIMER_WAIT 

These columns aggregate all I/O operations. They are the same as the sum of the corresponding xxx _READ  and xxx _WRITE  columns. 

• COUNT_READ , SUM_TIMER_READ , MIN_TIMER_READ , AVG_TIMER_READ , MAX_TIMER_READ 

These columns aggregate all read operations. They are the same as the sum of the corresponding xxx _FETCH  columns. 

• COUNT_WRITE , SUM_TIMER_WRITE , MIN_TIMER_WRITE , AVG_TIMER_WRITE , MAX_TIMER_WRITE 

These columns aggregate all write operations. They are the same as the sum of the corresponding xxx _INSERT , xxx _UPDATE , and xxx _DELETE  columns. 

• COUNT_FETCH , SUM_TIMER_FETCH , MIN_TIMER_FETCH , AVG_TIMER_FETCH , MAX_TIMER_FETCH 

These columns aggregate all fetch operations. 

• COUNT_INSERT , SUM_TIMER_INSERT , MIN_TIMER_INSERT , AVG_TIMER_INSERT , MAX_TIMER_INSERT 

