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This variable sets the maximum size to which user-created MEMORY  tables are permitted to grow. The value of the variable is used to calculate MEMORY  table MAX_ROWS  values. Setting this variable has no effect on any existing MEMORY  table, unless the table is re-created with a statement such as CREATE TABLE  or altered with ALTER TABLE  or TRUNCATE TABLE . A server restart also sets the maximum size of existing MEMORY  tables to the global max_heap_table_size  value. 

This variable is also used in conjunction with tmp_table_size  to limit the size of internal in-memory tables. See Section 8.4.4, “Internal Temporary Table Use in MySQL” . 

max_heap_table_size  is not replicated. See Section 17.5.1.21, “Replication and MEMORY Tables” , and Section 17.5.1.39, “Replication and Variables” , for more information. 

As of MySQL 8.0.26, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges” . 

• max_insert_delayed_threads 

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This variable is a synonym for max_delayed_threads . Like max_delayed_threads , it is deprecated (because DELAYED  inserts are not supported) and subject to removal in a future MySQL release. 

As of MySQL 8.0.26, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges” . 

• max_join_size 

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