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MEMORY
The MEMORY storage engine (formerly known as HEAP) creates 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.MEMORY performance is limited by contention due to single-thread execution and table lock overhead when processing updates. This limits scalability when load increases, especially 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. Particularly, the table locking involved with performing updates can slow down concurrent usage of MEMORY tables from multiple sessions.
The maximum size of MEMORY tables is limited by the max_heap_table_size system variable, which has a default value of 16MB.
Storage Limits |
RAM |
| Transactions | No |
| Locking granularity | Table |
| MVCC | No |
| Geospatial data type support | No |
| Geospatial indexing support | No |
| B-tree indexes | Yes |
| T-tree indexes | No |
| Hash indexes | Yes |
| Full-text search indexes | No |
| Clustered indexes | No |
| Data caches | N/A |
| Index caches | N/A |
| Compressed data | No |
| Encrypted data | Yes |
| Cluster database support | No |
| Replication support | Yes |
| Foreign key support | No |
| Backup/ Point-in-time recovery | Yes |
| Query cache support | Yes |
| Update statistics for data dictionary | Yes |