Subquery optimization forIN is not as effective as for=.

Even if you uselower_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 aFOREIGN 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:

If and only if the preceding queries have noORDER BY clause guaranteeing a deterministic order.

For example, forINSERT ... 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:

This problem may also affect database restoration usingmysqlbinlog|mysql.

The easiest way to avoid this problem is to add anORDER 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. SeeServer 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 aLOAD DATA statement. Seemysqlbinlog — Utility for Processing Binary Log Files.

RENAME does not work withTEMPORARY tables or tables used in aMERGE table.

When usingSET CHARACTER SET, you cannot use translated characters in database, table, and column names.

Prior to MySQL 8.0.17, you cannot use_ or% withESCAPE inLIKE ... ESCAPE.

The server uses only the firstmax_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 withBIGINT 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.

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

InMIN(),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 anUPDATE 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> UPDATEtbl_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 handleDISTINCT 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 hiddenid column may differ).

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

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

Creation of a table of typeMERGE does not check whether the underlying tables are compatible types.

If you useALTER 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.

AnUPDATE statement involving a temporary table with a join on a non-temporary table having a trigger defined on it can result in an error, even though the update statement reads only the non-temporary table, in the following cases: