A memory storage engine primarily contains data that resides directly in the computer's volatile random-access memory (RAM).
The MEMORY storage engine, formerly known as HEAP, creates special-purpose tables with contents that are stored in memory. Unlike traditional database engines that store data on disk (like hard drives or SSDs), a memory engine keeps all table data in RAM.
Core Components and Contents
Based on the nature of the MEMORY storage engine, its contents revolve around:
- Data: The actual rows and columns of information for the tables created using this engine. This data is held entirely in RAM.
- Table Structures: The definitions (schema) of the special-purpose tables, including column names, data types, and indexes. While the schema might persist, the data itself is volatile.
- Indexes: Indexes created on MEMORY tables are also stored in RAM to facilitate fast data retrieval.
Purpose and Usage
Because the data in a memory storage engine is held in RAM, it offers extremely fast read and write speeds compared to disk-based storage engines. However, this speed comes with a significant trade-off in terms of data durability.
As the reference notes: 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.
This highlights the specific use cases for memory tables:
- Temporary Work Areas: Ideal for storing intermediate results of complex queries or for holding data that is being processed and does not need long-term persistence.
- Read-Only Caches: Useful for caching frequently accessed data from slower, disk-based tables to improve application performance. Since the data is a copy (a cache), its loss is not catastrophic as it can be reloaded from the source.
Limitations
The primary limitation of a memory storage engine's contents is their volatility. When the database server is shut down (intentionally or due to a crash) or the system loses power, all data stored in MEMORY tables is lost. This makes them unsuitable for storing critical or persistent data.
In summary, a memory storage engine holds table data and indexes in RAM within designated tables, making it fast but vulnerable to data loss upon system interruption.
