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MySQL 9.4 Reference Manual

28.4.8 The INFORMATION_SCHEMA INNODB_CMP_PER_INDEX andINNODB_CMP_PER_INDEX_RESET Tables

TheINNODB_CMP_PER_INDEX andINNODB_CMP_PER_INDEX_RESET tables provide status information on operations related tocompressedInnoDB 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 compressedInnoDB table, both the table data and all thesecondary indexes are compressed. In this context, the table data is treated as just another index, one that happens to contain all the columns: theclustered index.

TheINNODB_CMP_PER_INDEX andINNODB_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 theCOMPRESS_OPS value to get the number ofcompression failures. Divide by theCOMPRESS_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. CompressedInnoDB pages are uncompressed whenever compressionfails, or the first time a compressed page is accessed in thebuffer 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: 0compress_ops_ok: 0  compress_time: 0 uncompress_ops: 23451uncompress_time: 4*************************** 2. row ***************************  database_name: employees     table_name: salaries     index_name: emp_no   compress_ops: 0compress_ops_ok: 0  compress_time: 0 uncompress_ops: 1597uncompress_time: 0

Notes

Use these tables to measure the effectiveness ofInnoDB tablecompression for specific tables, indexes, or both.
You must have thePROCESS privilege to query these tables.
Use theINFORMATION_SCHEMACOLUMNS table or theSHOW 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 andINNODB_CMP_PER_INDEX_RESET statistics are not gathered by default. You must enable theinnodb_cmp_per_index_enabled system variable before performing the operations on compressed tables that you want to monitor.
For usage information, seeSection 17.9.1.4, “Monitoring InnoDB Table Compression at Runtime” andSection 17.15.1.3, “Using the Compression Information Schema Tables”. For general information aboutInnoDB table compression, seeSection 17.9, “InnoDB Table and Page Compression”.

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