Table Maintenance Statements •When running  CHECK TABLE  on large  InnoDB  tables, other threads may be blocked during  CHECK TABLE  execution. To avoid timeouts, the semaphore wait threshold (600 seconds) is extended by 2 hours (7200 seconds) for  CHECK TABLE  operations. If  InnoDB  detects semaphore waits of 240 seconds or more, it starts printing  InnoDB  monitor output to the error log. If a lock request extends beyond the semaphore wait threshold,  InnoDB  aborts the process. To avoid the possibility of a semaphore wait timeout entirely, run  CHECK TABLE QUICK  instead of  CHECK TABLE . • CHECK TABLE  functionality for  InnoDB SPATIAL  indexes includes an R-tree validity check and a check to ensure that the R-tree row count matches the clustered index. • CHECK TABLE  supports secondary indexes on virtual generated columns, which are supported by InnoDB . •As of MySQL 8.0.14,  InnoDB  supports parallel clustered index reads, which can improve  CHECK TABLE  performance.  InnoDB  reads the clustered index twice during a  CHECK TABLE  operation. The second read can be performed in parallel. The  innodb_parallel_read_threads  session variable must be set to a value greater than 1 for parallel clustered index reads to occur. The default value is 4. The actual number of threads used to perform a parallel clustered index read is determined by the innodb_parallel_read_threads  setting or the number of index subtrees to scan, whichever is smaller. • CHECK TABLE  updates key statistics for  MyISAM  tables. •If  CHECK TABLE  output does not return  OK  or  Table is already up to date , you should normally run a repair of the table. See Section 7.6, “MyISAM Table Maintenance and Crash Recovery” . •If none of the  CHECK TABLE  options  QUICK ,  MEDIUM , or  EXTENDED  are specified, the default check type for dynamic-format  MyISAM  tables is  MEDIUM . This has the same result as running  myisamchk --medium-check  tbl_name  on the table. The default check type also is  MEDIUM  for static-format MyISAM  tables, unless  CHANGED  or  FAST  is specified. In that case, the default is  QUICK . The row scan is skipped for  CHANGED  and  FAST  because the rows are very seldom corrupted. By default, the entire table is read row by row and the checksum is calculated. For large tables, this could take a long time, thus you would only perform this operation occasionally. This row-by-row calculation is During the checksum operation, the table is locked with a read lock for  InnoDB  and  MyISAM . For a nonexistent table,  CHECKSUM TABLE  returns  NULL  and generates a warning. This statement is not supported for views. If you run  CHECKSUM TABLE  against a view, the  Checksum value is always  NULL , and a warning is returned. This statement requires the  SELECT  privilege for the table. CHECKSUM TABLE  reports a checksum for the contents of a table. You can use this statement to verify that the contents are the same before and after a backup, rollback, or other operation that is intended to put the data back to a known state. The following notes apply to  MyISAM  tables: CHECK TABLE Usage Notes for MyISAM Tables 13.7.3.3 CHECKSUM TABLE Statement Performance Considerations CHECKSUM TABLE  tbl_name  [,  tbl_name ] ... [QUICK | EXTENDED] 2894