A table can contain a maximum of 1000 columns.

A table can contain a maximum of 64 secondary indexes.

An index key for a single-column index can be up to 767 bytes. The same length limit applies to any index key prefix. See Section 13.1.13, “CREATE INDEX Syntax”.

The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes. This limit applies to the length of the combined index key in a multi-column index.

The maximum row length, except for variable-length columns (VARBINARY, VARCHAR, BLOB and TEXT), is slightly less than half of a database page. That is, the maximum row length is about 8000 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 14.6.10.2, “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 E.7.4, “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, you will need to configure it using several smaller data files rather than one or a file large data files.

The combined size of the InnoDB log files must be less than 4GB.

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

The default database page size in InnoDB is 16KB.

InnoDB tables do not support FULLTEXT indexes.

InnoDB tables support spatial data types, but not indexes on them.

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

For InnoDB Plugin, you can change the number of random dives by modifying the innodb_stats_sample_pages system variable. For more information, see Changes for Flexibility, Ease of Use and Reliability.

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 5.1.4, “Server System Variables”, and Section C.5.6, “Optimizer-Related Issues”.

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. To process a SELECT COUNT(*) FROM t statement, InnoDB must scan an index of the table, which takes some time if the index is not entirely in the buffer pool. If your table does not change often, using the MySQL query cache is a good solution. To get a fast count, you have to use a counter table you create yourself and let your application update it according to the inserts and deletes it does. If an approximate row count is sufficient, SHOW TABLE STATUS can be used. See Section 14.6.12.1, “InnoDB Performance Tuning Tips”.

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.

While initializing a previously specified AUTO_INCREMENT column on a table, InnoDB sets an exclusive lock on the end of the index associated with the AUTO_INCREMENT column. While accessing the auto-increment counter, InnoDB uses a specific AUTO-INC table lock mode where the lock lasts only to the end of the current SQL statement, not to the end of the entire transaction. Other clients cannot insert into the table while the AUTO-INC table lock is held. See Section 14.6.3.3, “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.

Under some conditions, TRUNCATE tbl_name for an InnoDB table is mapped to DELETE FROM tbl_name. See Section 13.1.34, “TRUNCATE TABLE Syntax”.

The LOAD TABLE FROM MASTER statement for setting up replication slave servers does not work for InnoDB tables. A workaround is to alter the table to MyISAM on the master, then do the load, and after that alter the master table back to InnoDB. Do not do this if the tables use InnoDB-specific features such as foreign keys.

Currently, 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). In versions of MySQL before 5.1.10 this would cause a crash, since 5.1.10 the server will report error 1005 and refers to error –1 in the error message. This restriction applies only to use of the names in uppercase.

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.

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.

InnoDB has a limit of 1023 concurrent transactions that have created undo records by modifying data. Workarounds include keeping transactions as small and fast as possible, delaying changes until near the end of the transaction, and using stored routines to reduce client/server latency delays. Applications should commit transactions before doing time-consuming client-side operations.