- 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 does not 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 does not work inside GROUP_CONCAT() if you do not use all and only those columns that are in the DISTINCT list.
- 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.
- With statement-based binary 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. However, 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).
- CREATE TABLE ... SELECT or INSERT ... SELECT statements that insert zero or NULL values into an AUTO_INCREMENT column.
- DELETE if you are deleting rows from a table that has foreign keys with ON DELETE CASCADE properties.
- REPLACE ... SELECT, INSERT IGNORE ... SELECT if you have duplicate key values in the inserted data.
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:
- The table is stored using a different storage engine on the master than on the slave. (It is possible to use different storage engines on the master and slave. For example, you can use InnoDB on the master, but MyISAM on the slave if the slave has less available disk space.)
- MySQL buffer sizes (key_buffer_size, and so on) are different on the master and slave.
- The master and slave run different MySQL versions, and the optimizer code differs between these versions.
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. Using row-based or mixed logging format also avoids the problem.
- Log file names are based on the server host name if you do not specify a file name with the startup option. To retain the same log file names if you change your host name to something else, you must explicitly use options such as --log-bin=old_host_name-bin. Alternatively, rename the old files to reflect your host name change. If these are binary logs, you must edit the binary log index file and fix the binary log file names there as well. (The same is true for the relay logs on a slave server.)
- mysqlbinlog does not delete temporary files left after a LOAD DATA INFILE statement.
- RENAME does not work with TEMPORARY tables or tables used in a MERGE table.
- When using SET CHARACTER SET, you cannot use translated characters in database, table, and column names.
- You cannot use _ or % with ESCAPE in LIKE ... ESCAPE.
- The server uses only the first max_sort_length bytes when comparing data values. This means that values cannot reliably be used in GROUP BY, ORDER BY, or DISTINCT if they differ only after the first max_sort_length bytes. To work around this, increase the variable value. 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.
- 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 does not 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 do not participate in the DISTINCT comparison.
An example of this is:
SELECT DISTINCT mp3id FROM band_downloads
WHERE userid = 9 ORDER BY id DESC;
SELECT DISTINCT band_downloads.mp3id
WHERE band_downloads.userid = 9
AND band_mp3.id = band_downloads.mp3id
ORDER BY band_downloads.id DESC;
- In the second case, 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 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 does not 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.