Performance Schema Summary Tables •If a  events_statements_summary_by_digest  row already exists with the digest value for the statement that just completed, statistics for the statement are aggregated to that row. The  LAST_SEEN column is updated to the current time. •If no row has the digest value for the statement that just completed, and the table is not full, a new row is created for the statement. The  FIRST_SEEN  and  LAST_SEEN  columns are initialized with the current time. •If no row has the statement digest value for the statement that just completed, and the table is full, the statistics for the statement that just completed are added to a special “catch-all”  row with  DIGEST  = NULL , which is created if necessary. If the row is created, the  FIRST_SEEN  and  LAST_SEEN  columns are initialized with the current time. Otherwise, the  LAST_SEEN  column is updated with the current time. •A  DIGEST  =  NULL  row that has a  COUNT_STAR  value that represents 5% of all digests shows that the digest summary table is very representative; the other rows cover 95% of the statements seen. •A  DIGEST  =  NULL  row that has a  COUNT_STAR  value that represents 50% of all digests shows that the digest summary table is not very representative; the other rows cover only half the statements seen. Most likely the DBA should increase the maximum table size so that more of the rows counted in the  DIGEST  =  NULL  row would be counted using more specific rows instead. By default, the table is autosized, but if this size is too small, set the  performance_schema_digests_size  system variable to a larger value at server startup. •A row is added for an object when it is first used in the server. •The row for an object is removed when the object is dropped. •Statistics are aggregated in the row for an object as it executes. 5058 mysql>  SELECT * See also Section 27.4.3, “Event Pre-Filtering” . The Performance Schema maintains statement event summary tables that contain information about minimum, maximum, and average statement latency (see Section 27.12.20.3, “Statement Summary Tables” ). Those tables permit high-level assessment of system performance. To permit assessment at a more fine-grained level, the Performance Schema also collects histogram data for statement latencies. These histograms provide additional insight into latency distributions. Section 27.12.6, “Performance Schema Statement Event Tables”  describes the events on which statement summaries are based. See that discussion for information about the content of statement events, the current and historical statement event tables, and how to control statement event collection, which is partially disabled by default. Example statement histogram information: 27.12.20.4 Statement Histogram Summary Tables Stored Program Instrumentation Behavior The row with  DIGEST  =  NULL  is maintained because Performance Schema tables have a maximum size due to memory constraints. The  DIGEST  =  NULL  row permits digests that do not match other rows to be counted even if the summary table is full, using a common “other”  bucket. This row helps you estimate whether the digest summary is representative: For stored program types for which instrumentation is enabled in the  setup_objects  table, events_statements_summary_by_program  maintains statistics for stored programs as follows: