CREATE TABLE as SELECT - using MEMORY ENGINE (in RAM memory)

Question

I was reading about tables using MEMORY ENGINE (tables stored in ram).

• Is it possible to use CREATE TABLE AS SELECT syntax, but have the created table us MEMORY ENGINE?

• Is there a limit to the size of a MEMORY table? Could create a MEMORY table that is a copy of a pysical 1.5 GB table?

Answer 1

• Yes, it can be done, but not with simple syntax CREATE TABLE t AS SELECT ... - you will need to specify table columns in full statement, Sample:

  CREATE TABLE t (col1 INT(11), col2 INT(11)) 
  ENGINE=MEMORY 
  AS 
  SELECT * FROM another_t
  

• Maximum size of table by default if 16Mb, but it can be adjusted with max_heap_table_size server system variable. But please, note, that this limit is per engine - not per table. I.e. all your memory tables will share it. To restrict size of individual table, you'll need to operate on session value for max_heap_table_size, like

   mysql> SET max_heap_table_size = 24*1024*1024;