G Known errors and design deficiencies in MySQL

The following problems are known and have a very high priority to get fixed:

• ANALYZE TABLE on a BDB table may in some case make the table unusable until one has restarted mysqld. When this happens you will see errors like the following in the MySQL error file:

  001207 22:07:56  bdb:  log_flush: LSN past current end-of-log
  

• Don't execute ALTER TABLE on a BDB table on which you are running not completed multi-statement transactions. (The transaction will probably be ignored).
• Doing a LOCK TABLE .. and FLUSH TABLES .. doesn't guarantee that there isn't a half-finished transaction in progress on the table.
• BDB tables are a bit slow to open from this. If you have many BDB tables in a database, it will take a long time to use the mysql client on the database if you are not using the -A option or if you are using rehash. This is especially notable when you have a big table cache.

The following problems are known and will be fixed in due time:

• mysqldump on a MERGE table doesn't include the current mapped tables.
• For the moment MATCH only works with SELECT statements.
• When using SET CHARACTER SET, one can't use translated characters in database, table and column names.
• DELETE FROM merge_table used without a WHERE will only clear the mapping for the table, not delete everything in the mapped tables
• You cannot build in another directory when using MIT-pthreads. Because this requires changes to MIT-pthreads, we are not likely to fix this.
• BLOB values 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 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 in MySQL, VARCHAR columns are treated the same way.
• You can only have up to 255 ENUM and SET columns in one table.
• 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.
• In the UPDATE statement, columns are updated from left to right. If you refer to a updated column, you will get the updated value instead of the original value. For example:

  mysql> UPDATE tbl_name SET KEY=KEY+1,KEY=KEY+1
  

  will update KEY with 2 instead of with 1.
• You can't use temporary tables more than once in the same query.

  select * from temporary_table, temporary_table as t2;
  

• The optimizer may handle DISTINCT differently if you are using 'hidden' columns in a join or not. In a join, hidden columns are counted as part of the result (even if they are not shown) while in normal queries hidden columns doesn't participate in the DISTINCT comparison. We will probably change this in the future to never compare the hidden columns when executing DISTINCT 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 in MySQL 3.23.x get two identical rows in the result set (because the hidden 'id' column may differ). Note that the this only happens for queries where you don't have the ORDER BY columns in the result, something that is you are not allowed to do in ANSI SQL.
• Because MySQL allows you to work with table types that don't support transactions, and thus can't rollback data, some things behave a little differently in MySQL than in other SQL servers. This is just to ensure that MySQL never need to do a rollback for a SQL command. This may be a little akward at times as column values must be checked in the application, but this will actually give you a nice speed increase as it allows MySQL to do some optimizations that otherwise would be very hard to do. If you set a column to a wrong value, MySQL will, instead of doing a rollback, store the best possible value in the column:
  • If you try to store a value outside the range in a numerical column, MySQL will instead store the smallest or biggest possible value in the column.
  • If you try to store a string that doesn't start with a number into a numerical column, MySQL will store 0 into it.
  • If you try to store NULL into a column that doesn't take NULL values, MySQL will store 0 or '' (empty string) in it instead. (This behavior can, however, be changed with the -DDONT_USE_DEFAULT_FIELDS compile option).
  • MySQL allows you to store some wrong date values into DATE and DATETIME columns. (Like 2000-02-31 or 2000-02-00). If the date is totally wrong, MySQL will store the special 0000-00-00 date value in the column.
  • If you set an enum to an unsupported value, it will be set to the error value 'empty string', with numeric value 0.
• If you execute a PROCEDURE on a query that returns an empty set, in some cases the PROCEDURE will not transform the columns.
• Creation of a table of type MERGE doesn't check if the underlying tables are of compatible types.

The following are known bugs in earlier versions of MySQL:

• You can get a hung thread if you do a DROP TABLE on a table that is one among many tables that is locked with LOCK TABLES.
• In the following case you can get a core dump:
  1. Delayed insert handler has pending inserts to a table.
  2. LOCK table with WRITE
  3. FLUSH TABLES
• Before MySQL Version 3.23.2 an UPDATE that updated a key with a WHERE on the same key may have failed because the key was used to search for records and the same row may have been found multiple times:

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

  A workaround is to use:

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

  This will work because MySQL will not use index on expressions in the WHERE clause.
• Before MySQL Version 3.23, all numeric types where treated as fixed-point fields. That means you had to specify how many decimals a floating-point field shall have. All results were returned with the correct number of decimals.

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

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