E Known errors and design deficiencies in MySQL

• You cannot build in another directory when using MIT-pthreads. Since this requires changes to MIT-pthreads, we are not likely to fix this.
• BLOBs can't "reliably" be used in GROUP BY or ORDER BY or DISTINCT. Only the first max_sort_length bytes (default 1024) are used when comparing BLOBbs in these cases. This can be changed with the -O max_sort_length option to mysqld. A workaround for most cases is to use a substring: SELECT DISTINCT LEFT(blob,2048) FROM tbl_name.
• Calculation is done with BIGINT or DOUBLE (both are normally 64 bits long). It depends on the function which precision one gets. The general rule is that bit functions are done with BIGINT precision, IF, and ELT() with BIGINT or DOUBLE precision and the rest with DOUBLE precision. One should try to avoid using bigger unsigned long long values than 63 bits (9223372036854775807) for anything else than bit fields!
• All numeric types are treated as fixed-point fields. That means you must specify how many decimals a floating-point field shall have. All results will be returned with the correct number of decimals.
• All string columns, except BLOB and TEXT columns, automatically have all trailing spaces removed when retrieved. For CHAR types this is okay, and may be regarded as a feature according to ANSI SQL92. The bug is that VARCHAR columns are treated the same way.
• You can only have up to 255 ENUM and SET columns in one table.
• An UPDATE that updates a key with a WHERE on the same key may fail because the key is used to search for records and will be found multiple times:

  UPDATE SET KEY=KEY+1 WHERE KEY > 100;
  

  This will be fixed in newer MySQL versions by not using keys that contain fields that are going to be updated. Until this fixed you can use the current workaround:

  mysql> UPDATE SET KEY=KEY+1 WHERE KEY+0 > 100;
  

  This will work because MySQL will not use index on expressions in the WHERE clause.
• safe_mysqld re-directs 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 --log extensively, you should edit safe_mysqld to log to `'hostname'.err' instead of `'hostname'.log' so you can easily reclaim the space for the old log by deleting the old one and executing mysqladmin refresh.

For platform-specific bugs, see the sections about compiling and porting.

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