sys Schema Views • statement_performance_analyzer.limit , • statement_performance_analyzer.view ,  @sys.statement_performance_analyzer.view • statement_truncate_len ,  @sys.statement_truncate_len 5140 28.4.3 sys Schema Views 28.4.2.3 The sys_config_update_set_user Trigger 28.4.2.2 The sys_config_insert_set_user Trigger The  sys  schema contains many views that summarize Performance Schema tables in various ways. Most of these views come in pairs, such that one member of the pair has the same name as the other member, plus a  x$  prefix. For example, the  host_summary_by_file_io  view summarizes file I/O grouped by host and displays latencies converted from picoseconds to more readable values (with units); The following sections describe  sys  schema views. The  sys_config_update_set_user  trigger for the  sys_config  table is similar to the sys_config_insert_set_user  trigger, but for  UPDATE  statements. For rows added to the  sys_config  table by  INSERT  statements, the  sys_config_insert_set_user trigger sets the  set_by  column to the current user. Second, to also ensure that procedure invocations within the current session use the changed value from the table, set the corresponding user-defined variable to  NULL : mysql>  SET @sys.debug = NULL; mysql>  UPDATE sys.sys_config SET value = ’OFF’ WHERE variable = ’debug’; mysql>  INSERT INTO sys.sys_config (variable, value) VALUES(’debug’, ’ON’); To change the debug setting in the table, do two things. First, modify the value in the table itself: Other options can be added to the  sys_config  table. For example, the  diagnostics()  and execute_prepared_stmt()  procedures use the  debug  option if it exists, but this option is not part of the  sys_config  table by default because debug output normally is enabled only temporarily, by setting the corresponding  @sys.debug  user-defined variable. To enable debug output without having to set that variable in individual sessions, add the option to the table: The maximum length of statements returned by the  format_statement()  function. Longer statements are truncated to this length. The default is 64. The custom query or view to be used by the  statement_performance_analyzer()  procedure (which is itself invoked by the  diagnostics()  procedure). If the option value contains a space, it is interpreted as a query. Otherwise, it must be the name of an existing view that queries the Performance Schema  events_statements_summary_by_digest  table. There cannot be any  LIMIT  clause in the query or view definition if the  statement_performance_analyzer.limit  configuration option is greater than 0. The default is  NULL  (no custom view defined). The maximum number of rows to return for views that have no built-in limit. (For example, the statements_with_runtimes_in_95th_percentile  view has a built-in limit in the sense that it returns only statements with average execution time in the 95th percentile.) The default is 100. @sys.statement_performance_analyzer.limit