5.8 The INFORMATION_SCHEMA INNODB_CMP_PER_INDEX and INNODB_CMP_PER_INDEX_RESET Tables

The INNODB_CMP_PER_INDEX and INNODB_CMP_PER_INDEX_RESET tables provide status information on operations related to compressed InnoDB tables and indexes, with separate statistics for each combination of database, table, and index, to help you evaluate the performance and usefulness of compression for specific tables.

For a compressed InnoDB table, both the table data and all the secondary indexes are compressed. In this context, the table data is treated as just another index, one that happens to contain all the columns: the clustered index.

The INNODB_CMP_PER_INDEX and INNODB_CMP_PER_INDEX_RESET tables have these columns:

• DATABASE_NAME

  The schema (database) containing the applicable table.

• TABLE_NAME

  The table to monitor for compression statistics.

• INDEX_NAME

  The index to monitor for compression statistics.

• COMPRESS_OPS

  The number of compression operations attempted. Pages are compressed whenever an empty page is created or the space for the uncompressed modification log runs out.

• COMPRESS_OPS_OK

  The number of successful compression operations. Subtract from the COMPRESS_OPS value to get the number of compression failures. Divide by the COMPRESS_OPS value to get the percentage of compression failures.

• COMPRESS_TIME

  The total time in seconds used for compressing data in this index.

• UNCOMPRESS_OPS

  The number of uncompression operations performed. Compressed InnoDB pages are uncompressed whenever compression fails, or the first time a compressed page is accessed in the buffer pool and the uncompressed page does not exist.

• UNCOMPRESS_TIME

  The total time in seconds used for uncompressing data in this index.

Example

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_CMP_PER_INDEX\G
*************************** 1. row ***************************
  database_name: employees
     table_name: salaries
     index_name: PRIMARY
   compress_ops: 0
compress_ops_ok: 0
  compress_time: 0
 uncompress_ops: 23451
uncompress_time: 4
*************************** 2. row ***************************
  database_name: employees
     table_name: salaries
     index_name: emp_no
   compress_ops: 0
compress_ops_ok: 0
  compress_time: 0
 uncompress_ops: 1597
uncompress_time: 0

Notes

• Use these tables to measure the effectiveness of InnoDB table compression for specific tables, indexes, or both.

• You must have the PROCESS privilege to query these tables.

• Use the INFORMATION_SCHEMA COLUMNS table or the SHOW COLUMNS statement to view additional information about the columns of these tables, including data types and default values.

• Because collecting separate measurements for every index imposes substantial performance overhead, INNODB_CMP_PER_INDEX and INNODB_CMP_PER_INDEX_RESET statistics are not gathered by default. You must enable the innodb_cmp_per_index_enabled system variable before performing the operations on compressed tables that you want to monitor.

• For usage information, see Monitoring InnoDB Table Compression at Runtime and Using the Compression Information Schema Tables. For general information about InnoDB table compression, see InnoDB Table and Page Compression.