Partitioning •Optional disk-backed operation for data durability. •Shared-nothing architecture and multiple-host operation with no single point of failure, enabling 99.999% availability. •Automatic data distribution across nodes; application developers need not craft custom sharding or partitioning solutions. •Support for variable-length data types (including  BLOB  and  TEXT ) not supported by  MEMORY . Partitioning Performance Characteristics MEMORY  tables cannot be partitioned. MEMORY  performance is constrained by contention resulting from single-thread execution and table lock overhead when processing updates. This limits scalability when load increases, particularly for statement mixes that include writes. Despite the in-memory processing for  MEMORY  tables, they are not necessarily faster than  InnoDB  tables on a busy server, for general-purpose queries, or under a read/write workload. In particular, the table locking involved with performing updates can slow down concurrent usage of  MEMORY  tables from multiple sessions. Depending on the kinds of queries performed on a  MEMORY  table, you might create indexes as either the default hash data structure (for looking up single values based on a unique key), or a general-purpose B- tree data structure (for all kinds of queries involving equality, inequality, or range operators such as less than or greater than). The following sections illustrate the syntax for creating both kinds of indexes. A common performance issue is using the default hash indexes in workloads where B-tree indexes are more efficient. The  MEMORY  storage engine does not create any files on disk. The table definition is stored in the MySQL data dictionary. MEMORY  tables have the following characteristics: •Space for  MEMORY  tables is allocated in small blocks. Tables use 100% dynamic hashing for inserts. No overflow area or extra key space is needed. No extra space is needed for free lists. Deleted rows are put in a linked list and are reused when you insert new data into the table.  MEMORY  tables also have none of the problems commonly associated with deletes plus inserts in hashed tables. • MEMORY  tables use a fixed-length row-storage format. Variable-length types such as  VARCHAR  are stored using a fixed length. • MEMORY  tables cannot contain  BLOB  or  TEXT  columns. • MEMORY  includes support for  AUTO_INCREMENT  columns. •Non- TEMPORARY MEMORY  tables are shared among all clients, just like any other non- TEMPORARY  table. To create a  MEMORY  table, specify the clause  ENGINE=MEMORY  on the  CREATE TABLE  statement. Characteristics of MEMORY Tables DDL Operations for MEMORY Tables 3412