KEY Partitioning However, if the unique key column were not defined as  NOT NULL , then the previous statement would fail. In both of these cases, the partitioning key is the  id  column, even though it is not shown in the output of  SHOW CREATE TABLE  or in the  PARTITION_EXPRESSION  column of the INFORMATION_SCHEMA.PARTITIONS  table. Unlike the case with other partitioning types, columns used for partitioning by  KEY  are not restricted to integer or  NULL  values. For example, the following  CREATE TABLE  statement is valid: CREATE TABLE tm1 ( s1 CHAR(32) PRIMARY KEY ) PARTITION BY KEY(s1) PARTITIONS 10; The preceding statement would  not  be valid, were a different partitioning type to be specified. (In this case, simply using  PARTITION BY KEY()  would also be valid and have the same effect as PARTITION BY KEY(s1) , since  s1  is the table’s primary key.) For additional information about this issue, see Section 24.6, “Restrictions and Limitations on Partitioning” . Columns with index prefixes are not supported in partitioning keys. This means that  CHAR ,  VARCHAR , BINARY , and  VARBINARY  columns can be used in a partitioning key, as long as they do not employ prefixes; because a prefix must be specified for  BLOB  and  TEXT  columns in index definitions, it is not possible to use columns of these two types in partitioning keys. Prior to MySQL 8.0.21, columns using prefixes were permitted when creating, altering, or upgrading a partitioned table, even though they were not included in the table’s partitioning key; in MySQL 8.0.21 and later, this permissive behavior is deprecated, and the server displays appropriate warnings or errors when one or more such columns are used. See Column index prefixes not supported for key partitioning, for more information and examples. Tables using the  NDB  storage engine are implicitly partitioned by  KEY , using the table’s primary key as the partitioning key (as with other MySQL storage engines). In the event that the NDB Cluster table has no explicit primary key, the “hidden”  primary key generated by the  NDB  storage engine for each NDB Cluster table is used as the partitioning key. Note If you define an explicit partitioning scheme for an  NDB  table, the table must have an explicit primary key, and any columns used in the partitioning expression must be part of this key. However, if the table uses an “empty”  partitioning expression —that is,  PARTITION BY KEY()  with no column references—then no explicit primary key is required. You can observe this partitioning using the  ndb_desc  utility (with the  -p  option). Important For a key-partitioned table, you cannot execute an  ALTER TABLE DROP PRIMARY KEY , as doing so generates the error  ERROR 1466 (HY000): Field in list of fields for partition function not found in table . This is not an issue for NDB Cluster tables which are partitioned by  KEY ; in such cases, the table is reorganized using the “hidden”  primary key as the table’s new partitioning key. See Chapter 23,  MySQL NDB Cluster 8.0 . 4679