13.2.15. Restrictions on InnoDB Tables

from:http://dev.mysql.com/doc/refman/4.1/en/innodb-restrictions.html

innodb的一些限制。比如字段个数不得超过1000个。

13.2.15. Restrictions on InnoDB Tables

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 does not restart until you restore the old system tables from a backup or re-generate them with the mysql_install_db script.

Warning

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.

• A table cannot contain more than 1000 columns.

• The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 1024 bytes.

• Index key prefixes can be up to 767 bytes (255 bytes before MySQL 4.1.2). See Section 12.1.4, “CREATE INDEXSyntax”.

• 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 LONGTEXTcolumns 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 inSection 13.2.12.2, “File Space Management”.

• 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.

• InnoDB tables do not support FULLTEXT indexes.

• InnoDB tables do not support spatial data types.

• 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.

  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 themax_seeks_for_key system variable to ensure that MySQL prefers index lookups over table scans. SeeSection 5.1.3, “Server System Variables”, and Section B.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. (In practice, this would be somewhat complicated due to multi-versioning.) 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. SHOW TABLE STATUS also can be used if an approximate row count is sufficient. See Section 13.2.14.1, “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, you should create all databases and tables using lowercase names.

• 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.

• Before MySQL 4.1.12, InnoDB does not support the AUTO_INCREMENT table option for setting the initial sequence value in an ALTER TABLE statement. Before MySQL 4.1.14, the same is true for CREATE TABLE. 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.

• 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. In accessing the auto-increment counter, InnoDBuses a specific table lock mode AUTO-INC 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 13.2.5.3, “AUTO_INCREMENT Handling in InnoDB”.

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

• When an AUTO_INCREMENT column runs out of values, InnoDB wraps a BIGINT to -9223372036854775808 andBIGINT UNSIGNED to 1. However, BIGINT values have 64 bits, so if you were to insert one million rows per second, it would still take nearly three hundred thousand years before BIGINT reached its upper bound. With all other integer type columns, a duplicate-key error results. This is similar to how MyISAM works, because 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.

• Under some conditions, TRUNCATE tbl_name for an InnoDB table is mapped to DELETE FROM tbl_name and does not reset the AUTO_INCREMENT counter. See Section 12.1.10, “TRUNCATE TABLE Syntax”.

• Before MySQL 4.0.14 or 4.1.0, if you tried to create a unique index on a prefix of a column you got an error:

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

  If you created a nonunique index on a prefix of a column, InnoDB created an index over the whole column. These restrictions were removed in MySQL 4.0.14.

• Before MySQL 4.0.20 or 4.1.2, the MySQL LOCK TABLES operation does not know about InnoDB row-level locks set by completed SQL statements. This means that you can get a table lock on a table even if there still exist transactions by other users who 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 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.

• Beginning with MySQL 4.0.20 and 4.1.2, the MySQL LOCK TABLES operation acquires two locks on each table ifinnodb_table_locks=1 (the default). In addition to a table lock on the MySQL layer, it also acquires an InnoDBtable lock. Older versions of MySQL do not acquire InnoDB table locks. Beginning with MySQL 4.0.22 and 4.1.7, 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 acquiredInnoDB table locks would be released immediately.

• Sometimes it would be useful to lock further tables in the course of a transaction. Unfortunately, LOCK TABLES in MySQL performs an implicit COMMIT and UNLOCK TABLES. An InnoDB variant of LOCK TABLES has been planned that can be executed in the middle of a transaction.

• 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 work for InnoDBtables. 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.

•  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 must update the values of UNIV_PAGE_SIZE and UNIV_PAGE_SIZE_SHIFT in the univ.isource file.

  Note

  Changing the page size is not a supported operation and there is no guarantee that InnoDB will function normally with a page size other than 16KB. Problems compiling or running InnoDB may occur.

  A version of InnoDB built for one page size cannot use data files or log files from a version built for a different page size.

• You cannot create a table with a column name that matches the name of an internal InnoDB column (includingDB_ROW_ID, DB_TRX_ID, DB_ROLL_PTR, and DB_MIX_ID). In versions of MySQL before 4.1.19 this would cause a crash, since 4.1.19 the server will report error 1005 and refers to error –1 in the error message. This limitation applies only to use of the names in uppercase.

• 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 and delaying changes until near the end of the transaction. Applications should commit transactions before doing time-consuming client-side operations.