How to make the mysql MEMORY ENGINE store more data?

Question

I want to alter a table from INNODB to MEMORY ENGINE.

So I typed this command:

alter table sns ENGINE=MEMORY;

Then the MySQL shows

Answer 1

If you're still having a problem, remember that the disk space a table occupies is often less than the memory requirement. Using VARCHAR (256) with a string of length 8 will consume 8 bytes on disk but because STORAGE doesn't support dynamic rows, it reserves the full 256 bytes in memory for every instance.

Answer 2

Increase max_heap_table_size.

Answer 3

The max size for the memory table is set on creation and altering and based on the max_heap_table_size value.

So when you want to raise the max size for an existing memory table you can change the max_heap_table_size and then apply it by altering the table to the same storage engine.

Example:

# Raise max size to 4GB
SET max_heap_table_size = 1024 * 1024 * 1024 * 4;

# If already a memory table, the alter will not change anything.
# Only apply the new max size.
ALTER TABLE table_name ENGINE=MEMORY;

Misunderstanding regarding tmp_table_size

The tmp_table_size variable only determines the max size for internal memory tables. Not user-defined.

As stated in the MySQL docs:

The maximum size of internal in-memory temporary tables. This variable does not apply to user-created MEMORY tables.

Answer 4

You should adjust the way you make and load the table

CREATE TABLE sns_memory SELECT * FROM sns WHERE 1=2;
ALTER TABLE sns_memory ENGINE=MEMORY;
INSERT INTO sns_memory SELECT * FROM sns;
DROP TABLE sns;
ALTER TABLE sns_memory RENAME sns;

This will get around any imposed limits by tmp_table_size and max_heap_table_size.

Just the same, you need to do two things:

Add this to /etc/my.cnf

[mysqld]
tmp_table_size=2G
max_heap_table_size=2G

this will cover mysql restarts. To set these values in mysqld right now without restarting run this:

SET GLOBAL tmp_table_size = 1024 * 1024 * 1024 * 2;
SET GLOBAL max_heap_table_size = 1024 * 1024 * 1024 * 2;

If you are checking the above variables with

SELECT @@max_heap_table_size;

or

SHOW VARIABLES LIKE 'max_heap_table_size';

you may notice that they don't seem to change following the SET GLOBAL... statements. This is because the settings only apply to new connections to the server. Make a new connection, and you'll see the values update or you could change it within your session by running:

SET tmp_table_size = 1024 * 1024 * 1024 * 2;
SET max_heap_table_size = 1024 * 1024 * 1024 * 2;

Answer 5

max_heap_table_size is what you are looking for