9.17 Restrictions on InnoDB Tables

• A table cannot contain more than 1000 columns .

• The maximum key length is 1024 bytes.

• The maximum row length, except for BLOB and TEXT columns, 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 also BLOB and TEXT columns, must be less than 4GB. InnoDB stores the first 512 bytes of a BLOB or TEXT column in the row, and the rest into separate pages.

• On some operating systems, data files must be less than 2GB.

• 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.

• InnoDB tables do not support FULLTEXT indexes.

• On Windows, InnoDB always stores database and table names internally in lowercase. To move databases in binary format from Unix to Windows or from Windows to Unix, you should have all database and table names in lowercase.

• Warning: 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 will not restart until you restore the old system tables from a backup or regenerate them with the mysql_install_db script.

• InnoDB does not keep an internal count of rows in a table. (This would actually be somewhat complicated because of multi-versioning .) To process a SELECT COUNT(*) FROM T statement, InnoDB must scan an index of the table, which will take some time if the table is not entirely in the buffer pool. 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 your table does not change often, using the MySQL query cache is a good solution. SHOW TABLE STATUS also can be used if an approximate row count is sufficient. See Section 9.12, "InnoDB Performance Tuning Tips."

• For an AUTO_INCREMENT column, you must always define an index for the table, and that index must contain just the AUTO_INCREMENT column. In MyISAM tables, the AUTO_INCREMENT column may be part of a multi-column index.

• InnoDB does not support the AUTO_INCREMENT table option for setting the initial sequence value in a CREATE TABLE or ALTER TABLE statement. To set the value with InnoDB , insert a dummy row with a value one less and delete that dummy row, or insert the first row with an explicit value specified.

• When you restart the MySQL server, InnoDB may reuse an old value for an AUTO_INCREMENT column (that is, a value that was assigned to an old transaction that was rolled back).

• When an AUTO_INCREMENT column runs out of values, InnoDB wraps a BIGINT to -9223372036854775808 and BIGINT UNSIGNED to 1 . However, BIGINT values have 64 bits, so do note that if you were to insert one million rows per second, it would still take about a million years before BIGINT reached its upper bound. With all other integer type columns, a duplicate-key error will result. This is similar to how MyISAM works, as it is mostly general MySQL behavior and not about any storage engine in particular.

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

• TRUNCATE tbl_name is mapped to DELETE FROM tbl_name for InnoDB and doesn't reset the AUTO_INCREMENT counter.

• 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.

• If you try to create a unique index on a prefix of a column you will get an error:

  
  

  CREATE TABLE T (A CHAR(20), B INT, UNIQUE (A(5))) TYPE = InnoDB;
  

  
  

  If you create a non-unique index on a prefix of a column, InnoDB will create an index over the whole column.

  These restrictions are removed starting from MySQL 4.0.14 and 4.1.1.

• INSERT DELAYED is not supported for InnoDB tables.

• The MySQL LOCK TABLES operation does not know about InnoDB row-level locks set by already completed SQL statements. This means that you can get a table lock on a table even if there still exist transactions by other users that have row-level locks on the same table. Thus your operations on the table may have to wait if they collide with these locks of other users. Also a deadlock is possible. However, this does not endanger transaction integrity, because the row-level locks set by InnoDB will always take care of the integrity. Also, a table lock prevents other transactions from acquiring more row-level locks (in a conflicting lock mode) on the table.

• Before MySQL 3.23.52, replication always ran with autocommit enabled. Therefore consistent reads in the slave would also see partially processed transactions, and thus the read would not be really consistent in the slave. This restriction was removed in MySQL 3.23.52.

• The LOAD TABLE FROM MASTER statement for setting up replication slave servers does not yet work for InnoDB tables. A workaround is to alter the table to MyISAM on the master, do then the load, and after that alter the master table back to InnoDB .

• The default database page size in InnoDB is 16KB. By recompiling the code, you can set it to values ranging from 8KB to 64KB. You have to update the values of UNIV_PAGE_SIZE and UNIV_PAGE_SIZE_SHIFT in the univ.i source file.