What is the role format file (.frm file) ?

FRM stands for FoRMat and it is storing data dictionary information for tables in .frm files. It describes the table’s format like table definition, including fields and structure. The name of this file is same as the table name.

Specially InnoDB engine stores encoded information about the table and managing its own data dictionary inside the tablespace.

When MySQL drops a table or a database, it also deletes .frm file which is associated with it.

We can also take backup of .frm file, restore the table definition from .frm file, can move .frm file from one storage to another storage.

The .frm file associated with a table that can be found in the directory that represents the database to which the table belongs.

You can check the MySQL data directory using below SHOW command.

SHOW VARIABLES LIKE 'datadir';

Important Note:
There is also one limitation of .frm file structure.
The portion of the information stored in the .frm file cannot grow beyond the 64KB limit, so if the table definition reaches this size, no more columns can be added.

In the previous post, I had explained the basic theory on the Heap Table of Database Technology.
In this post, I am going to explain the Heap table terminology of MySQL.

MySQL Memory Storage Engine also called as Heap Storage structure.

Using Memory Storage Engine, you can store a table of data in server memory for fast retrieval.
This table of data also supports B-Tree index so you can get a result in a best-optimized way. Auto_Increment also supported by the Tables of Memory Engine.

There might be chances of loss data when any hardware crashes or power outage of Server Memory.

You can store and use only those data which are in temporary use and make sure that data in memory engine has always back up in physical stored table.

When to use Memory Engine of MySQL ?

If data are non-critical, need fast retrieval and only for read-only access with a minimum of updates.
The volume of data can be fit into Memory area because it internally uses the swap out of virtual memory pages.

Brief details of Memory Engine Mechanism:

The table of Memory Engine stores it’s table definition into one disk file, and extension of this file is .frm.
All other data stores in a specified area of memory.
The data in memory table is not only for one single connection, it is available for all running related connections and sessions of MySQL Server.

Create sample table using Memory Engine:

CREATE TABLE Test (TestNo INT,TestName VARCHAR(50)) ENGINE = MEMORY;

This all basic about MySQL Memory or Heap Engine.

Any confusion, like difference between Temporary table and Memory table.
If yes, then please visit next post.