27.12.20.5 Transaction Summary Tables 5060 Performance Schema Summary Tables • events_statements_histogram_global  has a  BUCKET_NUMBER  column. This table summarizes latencies globally across schema name and digest values, using a single histogram. The BUCKET_NUMBER  column indicates the bucket number within this global histogram. • BUCKET_TIMER_LOW ,  BUCKET_TIMER_HIGH •The value of  BUCKET_TIMER_LOW  for the first bucket ( BUCKET_NUMBER  = 0) is 0. •The value of  BUCKET_TIMER_LOW  for a bucket ( BUCKET_NUMBER  =  k ) is the same as BUCKET_TIMER_HIGH  for the previous bucket ( BUCKET_NUMBER  =  k − 1) •The last bucket is a catchall for statements that have a latency exceeding previous buckets in the histogram. • COUNT_BUCKET • COUNT_BUCKET_AND_LOWER • BUCKET_QUANTILE • events_statements_histogram_by_digest : •Unique index on ( SCHEMA_NAME ,  DIGEST ,  BUCKET_NUMBER ) •Primary key on ( BUCKET_NUMBER ) • events_statements_histogram_global : In addition, truncating  events_statements_summary_by_digest  implicitly truncates  events_statements_histogram_by_digest , and truncating events_statements_summary_global_by_event_name  implicitly truncates events_statements_histogram_global . TRUNCATE TABLE  is permitted for statement histogram summary tables. Truncation sets the COUNT_BUCKET  and  COUNT_BUCKET_AND_LOWER  columns to 0. The statement histogram summary tables have these indexes: The proportion of statements that fall into this or a lower bucket. This proportion corresponds by definition to  COUNT_BUCKET_AND_LOWER / SUM(COUNT_BUCKET)  and is displayed as a convenience column. The number of statements measured with a latency in the interval from 0 up to but not including BUCKET_TIMER_HIGH . The number of statements measured with a latency in the interval from  BUCKET_TIMER_LOW  up to but not including  BUCKET_TIMER_HIGH . A bucket counts statements that have a latency, in picoseconds, measured between BUCKET_TIMER_LOW  and  BUCKET_TIMER_HIGH : Each statement histogram summary table has these summary columns containing aggregated values: A histogram consists of  N  buckets, where each row represents one bucket, with the bucket number indicated by the  BUCKET_NUMBER  column. Bucket numbers begin with 0.