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MySQL 5.7 Reference Manual  /  ...  /  Limits on InnoDB Tables

15.8.8 Limits on InnoDB Tables


Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables. This is an unsupported operation. If you do this, MySQL does not restart until you restore the old system tables from a backup or regenerate them by reinitializing the data directory (see Section 2.10.1, “Initializing the Data Directory”).


It is not a good idea to configure InnoDB to use data files or log files on NFS volumes. Otherwise, the files might be locked by other processes and become unavailable for use by MySQL.

Maximums and Minimums

  • A table can contain a maximum of 1017 columns (raised in MySQL 5.6.9 from the earlier limit of 1000). Virtual generated columns are included in this limit.

  • A table can contain a maximum of 64 secondary indexes.

  • If innodb_large_prefix is enabled (the default in MySQL 5.7.7), the index key prefix limit is 3072 bytes for InnoDB tables that use DYNAMIC or COMPRESSED row format. If innodb_large_prefix is disabled, the index key prefix limit is 767 bytes for tables of any row format.

    innodb_large_prefix is deprecated in MySQL 5.7.7 and will be removed in a future release. innodb_large_prefix was introduced in MySQL 5.5 to disable large index key prefixes for compatibility with earlier versions of InnoDB that do not support large index key prefixes.

    The index key prefix length limit is 767 bytes for InnoDB tables that use the REDUNDANT or COMPACT row format. For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a utf8mb3 character set and the maximum of 3 bytes for each character.

    Attempting to use an index key prefix length that exceeds the limit returns an error. To avoid such errors in replication configurations, avoid enabling innodb_large_prefix on the master if it cannot also be enabled on slaves.

    The limits that apply to index key prefixes also apply to full-column index keys.

  • If you reduce the InnoDB page size to 8KB or 4KB by specifying the innodb_page_size option when creating the MySQL instance, the maximum length of the index key is lowered proportionally, based on the limit of 3072 bytes for a 16KB page size. That is, the maximum index key length is 1536 bytes when the page size is 8KB, and 768 bytes when the page size is 4KB.

  • A maximum of 16 columns is permitted for multicolumn indexes. Exceeding the limit returns an error.

    ERROR 1070 (42000): Too many key parts specified; max 16 parts allowed
  • The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a page for 4KB, 8KB, 16KB, and 32KB page sizes. For example, the maximum row length for the default innodb_page_size of 16KB is about 8000 bytes. For an InnoDB page size of 64KB, the maximum row length is about 16000 bytes. LONGBLOB and LONGTEXT columns must be less than 4GB, and the total row length, including BLOB and TEXT columns, must be less than 4GB.

    If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page, as described in Section 15.12.2, “File Space Management”.

  • Although InnoDB supports row sizes larger than 65,535 bytes internally, MySQL itself imposes a row-size limit of 65,535 for the combined size of all columns:

    mysql> CREATE TABLE t (a VARCHAR(8000), b VARCHAR(10000),
        -> c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
        -> f VARCHAR(10000), g VARCHAR(10000)) ENGINE=InnoDB;
    ERROR 1118 (42000): Row size too large. The maximum row size for the
    used table type, not counting BLOBs, is 65535. You have to change some
    columns to TEXT or BLOBs

    See Section C.10.4, “Limits on Table Column Count and Row Size”.

  • On some older operating systems, files must be less than 2GB. This is not a limitation of InnoDB itself, but if you require a large tablespace, configure it using several smaller data files rather than one large data file.

  • The combined size of the InnoDB log files can be up to 512GB.

  • The minimum tablespace size is slightly larger than 10MB. The maximum tablespace size is four billion pages (64TB). This is also the maximum size for a table.

  • The default page size in InnoDB is 16KB. You can increase or decrease the page size by configuring the innodb_page_size option when creating the MySQL instance.

    Prior to MySQL 5.7.6, increasing the page size is not a supported operation, and there is no guarantee that InnoDB functions normally with a page size greater than 16KB. Problems compiling or running InnoDB may occur. In particular, ROW_FORMAT=COMPRESSED in the Barracuda file format assumes that the page size is at most 16KB and uses 14-bit pointers.

    As of MySQL 5.7.6, 32KB and 64KB page sizes are supported, but ROW_FORMAT=COMPRESSED is unsupported for page sizes greater than 16KB. For both 32KB and 64KB page sizes, the maximum record size is 16KB. For innodb_page_size=32k, extent size is 2MB. For innodb_page_size=64k, extent size is 4MB.

    A MySQL instance using a particular InnoDB page size cannot use data files or log files from an instance that uses a different page size.

Restrictions on InnoDB Tables

  • ANALYZE TABLE determines index cardinality (as displayed in the Cardinality column of SHOW INDEX output) by doing random dives to each of the index trees and updating index cardinality estimates accordingly. Because these are only estimates, repeated runs of ANALYZE TABLE could produce different numbers. This makes ANALYZE TABLE fast on InnoDB tables but not 100% accurate because it does not take all rows into account.

    You can make the statistics collected by ANALYZE TABLE more precise and more stable by turning on the innodb_stats_persistent configuration option, as explained in Section, “Configuring Persistent Optimizer Statistics Parameters”. When that setting is enabled, it is important to run ANALYZE TABLE after major changes to indexed column data, because the statistics are not recalculated periodically (such as after a server restart) as they traditionally have been.

    You can change the number of random dives by modifying the innodb_stats_persistent_sample_pages system variable (if the persistent statistics setting is turned on), or the innodb_stats_transient_sample_pages system variable (if the persistent statistics setting is turned off).

    MySQL uses index cardinality estimates only in join optimization. If some join is not optimized in the right way, you can try using ANALYZE TABLE. In the few cases that ANALYZE TABLE does not produce values good enough for your particular tables, you can use FORCE INDEX with your queries to force the use of a particular index, or set the max_seeks_for_key system variable to ensure that MySQL prefers index lookups over table scans. See Section 6.1.5, “Server System Variables”, and Section B.5.5, “Optimizer-Related Issues”.

  • If statements or transactions are running on a table and ANALYZE TABLE is run on the same table followed by a second ANALYZE TABLE operation, the second ANALYZE TABLE operation is blocked until the statements or transactions are completed. This behavior occurs because ANALYZE TABLE marks the currently loaded table definition as obsolete when ANALYZE TABLE is finished running. New statements or transactions (including a second ANALYZE TABLE statement) must load the new table definition into the table cache, which cannot occur until currently running statements or transactions are completed and the old table definition is purged. Loading multiple concurrent table definitions is not supported.

  • SHOW TABLE STATUS does not give accurate statistics on InnoDB tables, except for the physical size reserved by the table. The row count is only a rough estimate used in SQL optimization.

  • InnoDB does not keep an internal count of rows in a table because concurrent transactions might see different numbers of rows at the same time. Consequently, SELECT COUNT(*) FROM t statements only count rows visible to the current transaction.

    To process a SELECT COUNT(*) FROM t statement, InnoDB scans the clustered index, which takes some time if clustered index records are not entirely in the buffer pool. For a faster count, you can create a counter table and let your application update it according to the inserts and deletes it does. However, this method may not scale well in situations where thousands of concurrent transactions are initiating updates to the same counter table. If an approximate row count is sufficient, SHOW TABLE STATUS can be used.

    As of MySQL 5.7.2, SELECT COUNT(*) FROM t statements invoke a single handler call to the storage engine to scan the clustered index and return the row count to the optimizer. This optimization is evident in EXPLAIN output for SELECT COUNT(*) FROM t, which reports Select tables optimized away:

    *************************** 1. row ***************************
               id: 1
      select_type: SIMPLE
            table: NULL
       partitions: NULL
             type: NULL
    possible_keys: NULL
              key: NULL
          key_len: NULL
              ref: NULL
             rows: NULL
         filtered: NULL
            Extra: Select tables optimized away 

    In previous releases, a row count was typically performed by traversing a smaller secondary index and invoking a handler call for each record. A single handler call to the storage engine to count rows in the clustered index generally improves SELECT COUNT(*) FROM t performance. However, in the case of a large clustered index and a significantly smaller secondary index, performance degradation is possible compared to performance using the previous, non-optimized implementation.

  • On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in a binary format from Unix to Windows or from Windows to Unix, create all databases and tables using lowercase names.

  • An AUTO_INCREMENT column ai_col must be defined as part of an index such that it is possible to perform the equivalent of an indexed SELECT MAX(ai_col) lookup on the table to obtain the maximum column value. Typically, this is achieved by making the column the first column of some table index.

  • InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column while initializing a previously specified AUTO_INCREMENT column on a table.

    With innodb_autoinc_lock_mode=0, InnoDB uses a special AUTO-INC table lock mode where the lock is obtained and held to the end of the current SQL statement while accessing the auto-increment counter. Other clients cannot insert into the table while the AUTO-INC table lock is held. The same behavior occurs for bulk inserts with innodb_autoinc_lock_mode=1. Table-level AUTO-INC locks are not used with innodb_autoinc_lock_mode=2. For more information, See Section 15.8.6, “AUTO_INCREMENT Handling in InnoDB”.

  • When you restart the MySQL server, InnoDB may reuse an old value that was generated for an AUTO_INCREMENT column but never stored (that is, a value that was generated during an old transaction that was rolled back).

  • When an AUTO_INCREMENT integer column runs out of values, a subsequent INSERT operation returns a duplicate-key error. This is general MySQL behavior, similar to how MyISAM works.

  • DELETE FROM tbl_name does not regenerate the table but instead deletes all rows, one by one.

  • Cascaded foreign key actions do not activate triggers.

  • You cannot create a table with a column name that matches the name of an internal InnoDB column (including DB_ROW_ID, DB_TRX_ID, DB_ROLL_PTR, and DB_MIX_ID). This restriction applies to use of the names in any letter case.

    mysql> CREATE TABLE t1 (c1 INT, db_row_id INT) ENGINE=INNODB;
    ERROR 1166 (42000): Incorrect column name 'db_row_id'

Locking and Transactions

  • LOCK TABLES acquires two locks on each table if innodb_table_locks=1 (the default). In addition to a table lock on the MySQL layer, it also acquires an InnoDB table lock. Versions of MySQL before 4.1.2 did not acquire InnoDB table locks; the old behavior can be selected by setting innodb_table_locks=0. If no InnoDB table lock is acquired, LOCK TABLES completes even if some records of the tables are being locked by other transactions.

    In MySQL 5.7, innodb_table_locks=0 has no effect for tables locked explicitly with LOCK TABLES ... WRITE. It does have an effect for tables locked for read or write by LOCK TABLES ... WRITE implicitly (for example, through triggers) or by LOCK TABLES ... READ.

  • All InnoDB locks held by a transaction are released when the transaction is committed or aborted. Thus, it does not make much sense to invoke LOCK TABLES on InnoDB tables in autocommit=1 mode because the acquired InnoDB table locks would be released immediately.

  • You cannot lock additional tables in the middle of a transaction because LOCK TABLES performs an implicit COMMIT and UNLOCK TABLES.

  • The limit on data-modifying transactions is now 96 * 1023 concurrent transactions that generate undo records. As of MySQL 5.7.2, 32 of 128 rollback segments are assigned to non-redo logs for transactions that modify temporary tables and related objects. This reduces the maximum number of concurrent data-modifying transactions from 128K to 96K. The 96K limit assumes that transactions do not modify temporary tables. If all data-modifying transactions also modify temporary tables, the limit is 32K concurrent transactions.

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