select与show 从内存分配的情况看，select仅查询需要的变量
而show 需要把所有的变量都查询后，在根据条件做过滤
select @@read_only;
malloc 内存的情况
my_malloc_58 thread= 7f78224c8700 malloc key= 152 size= 64 ptr= 7f77f400e230
my_malloc_58 thread= 7f78224c8700 malloc key= 152 size= 64 ptr= 7f77f400e280
show global variables like ‘read_only’;
malloc 内存的情况
my_malloc_58 thread= 7f78224fa700 malloc key= 8 size= 8192 ptr= 7f77f0330f00
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 2440 ptr= 7f77f0768dd0
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 49 ptr= 7f77f07a9ec0
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 1024 ptr= 7f77f04772d0
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 6592 ptr= 7f77f0738710
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 1024 ptr= 7f77f0769760
allocate_354 thread= 7f78224fa700 malloc key= 226 size= 80 ptr= 7f77f06b0e10
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 1024 ptr= 7f77f009a990
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 4032 ptr= 7f77f00a6de0
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 1024 ptr= 7f77f0765260
my_malloc_58 thread= 7f78224fa700 malloc key= 100 size= 9888 ptr= 7f77f05b7360
my_malloc_58 thread= 7f78224fa700 malloc key= 8 size= 8192 ptr= 7f77f051fd50
my_malloc_58 thread= 7f78224fa700 malloc key= 73 size= 80 ptr= 7f77f00b7070
my_malloc_58 thread= 7f78224fa700 malloc key= 295 size= 32 ptr= 7f77f089bbd0
my_malloc_58 thread= 7f78224fa700 malloc key= 292 size= 536 ptr= 7f77f058bd10
my_malloc_58 thread= 7f78224fa700 malloc key= 292 size= 53 ptr= 7f77f07a7870
my_malloc_58 thread= 7f78224fa700 malloc key= 293 size= 752 ptr= 7f77f008df00
my_malloc_58 thread= 7f78224fa700 malloc key= 0 size= 12680 ptr= 7f77f0461470
my_malloc_58 thread= 7f78224fa700 malloc key= 0 size= 535712 ptr= 7f77f00d8b00
my_malloc_58 thread= 7f78224fa700 malloc key= 294 size= 520280 ptr= 7f77f08ad160
my_malloc_58 thread= 7f78224fa700 malloc key= 294 size= 521304 ptr= 7f77f0950b10
my_malloc_58 thread= 7f78224fa700 malloc key= 294 size= 521304 ptr= 7f77f07ab700
my_malloc_58 thread= 7f78224fa700 malloc key= 294 size= 521304 ptr= 7f77f0497860