This section lists known issues in recent versions of MySQL.
For information about platform-specific issues, see the installation and debugging instructions in General Installation Guidance, and Debugging MySQL.
The following problems are known:
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 functionDATABASE()
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 replica.REPLACE
(andLOAD DATA
with theREPLACE
option) does not triggerON DELETE CASCADE
.DISTINCT
withORDER BY
does not work insideGROUP_CONCAT()
if you do not use all and only those columns that are in theDISTINCT
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 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).
For example:
CREATE TABLE ... SELECT
orINSERT ... SELECT
statements that insert zero orNULL
values into anAUTO_INCREMENT
column.DELETE
if you are deleting rows from a table that has foreign keys withON 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 noORDER BY
, theSELECT
may return rows in a different order (which results in a row having different ranks, hence getting a different number in theAUTO_INCREMENT
column), 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
InnoDB
on the source, butMyISAM
on 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 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=
. 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.)old_host_name
-binmysqlbinlog does not delete temporary files left after a
LOAD DATA
statement. See mysqlbinlog — Utility for Processing Binary Log Files.RENAME
does not work withTEMPORARY
tables or tables used in aMERGE
table.When using
SET CHARACTER SET
, you cannot use translated characters in database, table, and column names.You cannot use
_
or%
withESCAPE
inLIKE ... ESCAPE
.The server uses only the first
max_sort_length
bytes when comparing data values. This means that values cannot reliably be used inGROUP BY
,ORDER BY
, orDISTINCT
if they differ only after the firstmax_sort_length
bytes. To work around this, increase the variable value. The default value ofmax_sort_length
is 1024 and can be changed at server startup time or at runtime.Numeric calculations are done with
BIGINT
orDOUBLE
(both are normally 64 bits long). Which precision you get depends on the function. The general rule is that bit functions are performed withBIGINT
precision,IF()
andELT()
withBIGINT
orDOUBLE
precision, and the rest withDOUBLE
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.In
MIN()
,MAX()
, and other aggregate functions, MySQL currently comparesENUM
andSET
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 incrementsKEY
by2
, not1
: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 theDISTINCT
comparison.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, 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 thePROCEDURE
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 aUNIQUE
index to a table used in aMERGE
table and then add a normal index on theMERGE
table, the key order is different for the tables if there was an old, non-UNIQUE
key in the table. This is becauseALTER TABLE
putsUNIQUE
indexes before normal indexes to be able to detect duplicate keys as early as possible.