SET @def_var= @@session.transaction_prealloc_size;
SET SESSION transaction_prealloc_size=1024*1024*1024*1;
SHOW PROCESSLIST;
Id	User	Host	db	Command	Time	State	Info
<Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
SET SESSION transaction_prealloc_size=1024*1024*1024*2;
SHOW PROCESSLIST;
Id	User	Host	db	Command	Time	State	Info
<Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
SET SESSION transaction_prealloc_size=1024*1024*1024*3;
SHOW PROCESSLIST;
Id	User	Host	db	Command	Time	State	Info
<Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
SET SESSION transaction_prealloc_size=1024*1024*1024*4;
SHOW PROCESSLIST;
Id	User	Host	db	Command	Time	State	Info
<Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
SET SESSION transaction_prealloc_size=1024*1024*1024*5;
SHOW PROCESSLIST;
Id	User	Host	db	Command	Time	State	Info
<Id>	root	<Host>	test	Query	<Time>	NULL	SHOW PROCESSLIST
SET @@session.transaction_prealloc_size= @def_var;