Statement Digest Memory Use •Server features other than the Performance Schema use normalized statement digests that take up to max_digest_length  bytes. •The Performance Schema does not further truncate normalized statement digests that it stores, but allocates more memory than  max_digest_length  bytes per digest, which is unnecessary. •If  max_digest_length  equals  performance_schema_max_digest_length : •Server features other than the Performance Schema use normalized statement digests that take up to max_digest_length  bytes. •The Performance Schema does not further truncate normalized statement digests that it stores, and allocates the same amount of memory as  max_digest_length  bytes per digest. •If  max_digest_length  is greater than  performance_schema_max_digest_length : •Server features other than the Performance Schema use normalized statement digests that take up to max_digest_length  bytes. •The Performance Schema further truncates normalized statement digests that it stores, and allocates less memory than  max_digest_length  bytes per digest. •The need to have long normalized statement digests available to server features outside the Performance Schema •Many concurrent sessions, each of which allocates digest-computation memory •The need to limit memory consumption by the Performance Schema statement event tables when storing many statement digests Because the Performance Schema statement event tables might store many digests, setting performance_schema_max_digest_length  smaller than  max_digest_length  enables administrators to balance these factors: The  performance_schema_max_digest_length  setting is not per session, it is per statement, and a session can store multiple statements in the  events_statements_history  table. A typical number of statements in this table is 10 per session, so each session consumes 10 times the memory indicated by the  performance_schema_max_digest_length  value, for this table alone. Also, there are many statements (and digests) collected globally, most notably in the events_statements_history_long  table. Here, too,  N  statements stored consumes  N  times the memory indicated by the  performance_schema_max_digest_length  value. To assess the amount of memory used for SQL statement storage and digest computation, use the  SHOW ENGINE PERFORMANCE_SCHEMA STATUS  statement, or monitor these instruments: mysql>  SELECT NAME FROM performance_schema.setup_instruments WHERE NAME LIKE ’%.sqltext’; +------------------------------------------------------------------+ | NAME | +------------------------------------------------------------------+ | memory/performance_schema/events_statements_history.sqltext | | memory/performance_schema/events_statements_current.sqltext | | memory/performance_schema/events_statements_history_long.sqltext | +------------------------------------------------------------------+ mysql>  SELECT NAME 4937