Subquery optimization for IN is not as effective as for =.

Even if you use lower_case_table_names=2 (which enables MySQL to remember the case used for databases and table names), MySQL does not remember the case used for database names for the function DATABASE() or within the various logs (on case-insensitive systems).

Dropping a FOREIGN KEY constraint doesn't work in replication because the constraint may have another name on the slave.

REPLACE (and LOAD DATA with the REPLACE option) does not trigger ON DELETE CASCADE.

DISTINCT with ORDER BY doesn't work inside GROUP_CONCAT() if you don't use all and only those columns that are in the DISTINCT list.

If one user has a long-running transaction and another user drops a table that is updated in the transaction, there is small chance that the binary log may contain the DROP TABLE command before the table is used in the transaction itself. We plan to fix this by having the DROP TABLE command wait until the table is not being used in any transaction.

When inserting a big integer value (between 2⁶³ and 2⁶⁴–1) into a decimal or string column, it is inserted as a negative value because the number is evaluated in a signed integer context.

FLUSH TABLES WITH READ LOCK does not block COMMIT if the server is running without binary logging, which may cause a problem (of consistency between tables) when doing a full backup.

ANALYZE TABLE on a BDB table may in some cases make the table unusable until you restart mysqld. If this happens, look for errors of the following form in the MySQL error file:

001207 22:07:56  bdb:  log_flush: LSN past current end-of-log

Don't execute ALTER TABLE on a BDB table on which you are running multiple-statement transactions until all those transactions complete. (The transaction might be ignored.)

ANALYZE TABLE, OPTIMIZE TABLE, and REPAIR TABLE may cause problems on tables for which you are using INSERT DELAYED.

Performing LOCK TABLE ... and FLUSH TABLES ... doesn't guarantee that there isn't a half-finished transaction in progress on the table.

BDB tables are relatively slow to open. If you have many BDB tables in a database, it takes a long time to use the mysql client on the database if you are not using the -A option or if you are using rehash. This is especially noticeable when you have a large table cache.

Replication uses query-level logging: The master writes the executed queries to the binary log. This is a very fast, compact, and efficient logging method that works perfectly in most cases.

It is possible for the data on the master and slave to become different if a query is designed in such a way that the data modification is nondeterministic (generally not a recommended practice, even outside of replication).

For example:

If and only if the preceding queries have no ORDER BY clause guaranteeing a deterministic order.

For example, for INSERT ... SELECT with no ORDER BY, the SELECT may return rows in a different order (which results in a row having different ranks, hence getting a different number in the AUTO_INCREMENT column), depending on the choices made by the optimizers on the master and slave.

A query is optimized differently on the master and slave only if:

This problem may also affect database restoration using mysqlbinlog|mysql.

The easiest way to avoid this problem is to add an ORDER BY clause to the aforementioned nondeterministic queries to ensure that the rows are always stored or modified in the same order.

In future MySQL versions, we will automatically add an ORDER BY clause when needed.

Log file names are based on the server host name (if you don't specify a file name with the startup option). You have to use options such as --log-bin=old_host_name-bin if you change your host name to something else. Another option is to rename the old files to reflect your host name change (if these are binary logs, you need to edit the binary log index file and fix the binary log file names there as well). See Section 5.1.2, “Server Command Options”.

mysqlbinlog does not delete temporary files left after a LOAD DATA INFILE command. See Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”.

RENAME doesn't work with TEMPORARY tables or tables used in a MERGE table.

Due to the way table format (.frm) files are stored, you cannot use character 255 (CHAR(255)) in table names, column names, or enumerations. This is scheduled to be fixed in version 5.1 when we implement new table definition format files.

When using SET CHARACTER SET, you can't use translated characters in database, table, and column names.

You can't use “_” or “%” with ESCAPE in LIKE ... ESCAPE.

You cannot build the server in another directory when using MIT-pthreads. Because this requires changes to MIT-pthreads, we are not likely to fix this. See Section 2.16.5, “MIT-pthreads Notes”.

BLOB and TEXT values can't reliably be used in GROUP BY, ORDER BY or DISTINCT. Only the first max_sort_length bytes are used when comparing BLOB values in these cases. The default value of max_sort_length is 1024 and can be changed at server startup time or at runtime.

Numeric calculations are done with BIGINT or DOUBLE (both are normally 64 bits long). Which precision you get depends on the function. The general rule is that bit functions are performed with BIGINT precision, IF() and ELT() with BIGINT or DOUBLE precision, and the rest with DOUBLE precision. You should try to avoid using unsigned long long values if they resolve to be larger than 63 bits (9223372036854775807) for anything other than bit fields.

You can have up to 255 ENUM and SET columns in one table.

In MIN(), MAX(), and other aggregate functions, MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set.

mysqld_safe redirects all messages from mysqld to the mysqld log. One problem with this is that if you execute mysqladmin refresh to close and reopen the log, stdout and stderr are still redirected to the old log. If you use the general query log extensively, you should edit mysqld_safe to log to host_name.err instead of host_name.log so that you can easily reclaim the space for the old log by deleting it and executing mysqladmin refresh.

In an UPDATE statement, columns are updated from left to right. If you refer to an updated column, you get the updated value instead of the original value. For example, the following statement increments KEY by 2, not 1:

mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1;

You can refer to multiple temporary tables in the same query, but you cannot refer to any given temporary table more than once. For example, the following doesn't work:

mysql> SELECT * FROM temp_table, temp_table AS t2;
ERROR 1137: Can't reopen table: 'temp_table'

The optimizer may handle DISTINCT differently when you are using “hidden” columns in a join than when you are not. In a join, hidden columns are counted as part of the result (even if they are not shown), whereas in normal queries, hidden columns don't participate in the DISTINCT comparison. We will probably change this in the future to never compare the hidden columns when executing DISTINCT.

An example of this is:

SELECT DISTINCT mp3id FROM band_downloads
       WHERE userid = 9 ORDER BY id DESC;

and

SELECT DISTINCT band_downloads.mp3id
       FROM band_downloads,band_mp3
       WHERE band_downloads.userid = 9
       AND band_mp3.id = band_downloads.mp3id
       ORDER BY band_downloads.id DESC;

In the second case, using MySQL Server 3.23.x, you may get two identical rows in the result set (because the values in the hidden id column may differ).

Note that this happens only for queries where that do not have the ORDER BY columns in the result.

If you execute a PROCEDURE on a query that returns an empty set, in some cases the PROCEDURE does not transform the columns.

Creation of a table of type MERGE doesn't check whether the underlying tables are compatible types.

If you use ALTER TABLE to add a UNIQUE index to a table used in a MERGE table and then add a normal index on the MERGE table, the key order is different for the tables if there was an old, non-UNIQUE key in the table. This is because ALTER TABLE puts UNIQUE indexes before normal indexes to be able to detect duplicate keys as early as possible.