This section lists known issues in recent versions of MySQL.
The following problems are known:
Subquery optimization for
INis 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).
FOREIGN KEYconstraint does not work in replication because the constraint may have another name on the replica.
ORDER BYdoes not work inside
GROUP_CONCAT()if you do not use all and only those columns that are in the
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 signed integer context.
With statement-based binary logging, the source server 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 source and replica 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).
DELETEif you are deleting rows from a table that has foreign keys with
ON DELETE CASCADEproperties.
REPLACE ... SELECT,
INSERT IGNORE ... SELECTif you have duplicate key values in the inserted data.
If and only if the preceding queries have no
ORDER BYclause guaranteeing a deterministic order.
For example, for
INSERT ... SELECTwith no
ORDER BY, the
SELECTmay return rows in a different order (which results in a row having different ranks, hence getting a different number in the
AUTO_INCREMENTcolumn), depending on the choices made by the optimizers on the source and replica.
A query is optimized differently on the source and replica only if:
The table is stored using a different storage engine on the source than on the replica. (It is possible to use different storage engines on the source and replica. For example, you can use
InnoDBon the source, but
MyISAMon the replica if the replica has less available disk space.)
MySQL buffer sizes (
key_buffer_size, and so on) are different on the source and replica.
The source and replica 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 BYclause 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=. See Server Command Options. 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 replica.)
mysqlbinlog does not delete temporary files left after a
LOAD DATAstatement. See mysqlbinlog — Utility for Processing Binary Log Files.
RENAMEdoes not work with
TEMPORARYtables or tables used in a
SET CHARACTER SET, you cannot use translated characters in database, table, and column names.
The server uses only the first
max_sort_lengthbytes when comparing data values. This means that values cannot reliably be used in
ORDER BY, or
DISTINCTif they differ only after the first
max_sort_lengthbytes. To work around this, increase the variable value. The default value of
max_sort_lengthis 1024 and can be changed at server startup time or at runtime.
Numeric calculations are done with
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
DOUBLEprecision, and the rest with
DOUBLEprecision. 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.
UPDATEstatement, 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
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
DISTINCTdifferently 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
An example of this is:
SELECT DISTINCT mp3id FROM band_downloads WHERE userid = 9 ORDER BY id DESC;
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, you may get two identical rows in the result set (because the values in the hidden
idcolumn may differ).
Note that this happens only for queries that do not have the
ORDER BYcolumns in the result.
If you execute a
PROCEDUREon a query that returns an empty set, in some cases the
PROCEDUREdoes not transform the columns.
Creation of a table of type
MERGEdoes not check whether the underlying tables are compatible types.
If you use
ALTER TABLEto add a
UNIQUEindex to a table used in a
MERGEtable and then add a normal index on the
MERGEtable, the key order is different for the tables if there was an old, non-
UNIQUEkey in the table. This is because
UNIQUEindexes before normal indexes to be able to detect duplicate keys as early as possible.