The MEMORY Storage Engine •The counter is not changed during further updates. •When the last instance of a table is closed (because a  FLUSH TABLES  operation was performed or because there is no room in the table cache), the counter is decremented if the table has been updated at any point. •When you repair the table or check the table and it is found to be okay, the counter is reset to zero. •To avoid problems with interaction with other processes that might check the table, the counter is not decremented on close if it was zero. •A  MyISAM  table is copied without first issuing  LOCK TABLES  and  FLUSH TABLES . •MySQL has crashed between an update and the final close. (The table may still be okay because MySQL always issues writes for everything between each statement.) •A table was modified by  myisamchk --recover  or  myisamchk --update-state  at the same time that it was in use by  mysqld . •Multiple  mysqld  servers are using the table and one server performed a  REPAIR TABLE  or  CHECK TABLE  on the table while it was in use by another server. In this setup, it is safe to use  CHECK TABLE , although you might get the warning from other servers. However,  REPAIR TABLE  should be avoided because when one server replaces the data file with a new one, this is not known to the other servers. Feature Support B-tree indexes Yes Backup/point-in-time recovery  (Implemented in the server, rather than in the storage engine.) Yes Cluster database support No Clustered indexes No Compressed data No Data caches N/A Encrypted data Yes (Implemented in the server via encryption functions.) Foreign key support No Full-text search indexes No Geospatial data type support No Geospatial indexing support No 3410 Table 16.4 MEMORY Storage Engine Features 16.3 The MEMORY Storage Engine In other words, the counter can become incorrect only under these conditions: In general, it is a bad idea to share a data directory among multiple servers. See Section 5.8, “Running Multiple MySQL Instances on One Machine” , for additional discussion. The  MEMORY  storage engine (formerly known as  HEAP ) creates special-purpose tables with contents that are stored in memory. Because the data is vulnerable to crashes, hardware issues, or power outages, only use these tables as temporary work areas or read-only caches for data pulled from other tables.