mysql count(0) count(*) count(主键) count(非空字段) 效率比较
写代码的时候经理在背后说了一句count(0)的效率高于count(*) ，索性全部测试了一下
结论：1、count(0)效率等于count(*)等于count(主键)等于count(非空字段)
但是 唯一的区别就是count只会统计非空字段
系统配置
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 85
model name : Intel(R) Xeon(R) Platinum 8163 CPU @ 2.50GHz
stepping : 4
microcode : 0x1
cpu MHz : 2499.986
cache size : 33792 KB
physical id : 0
siblings : 1
core id : 0
cpu cores : 1
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl eagerfpu pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm mpx avx512f rdseed adx smap avx512cd xsaveopt xsavec xgetbv1
bugs :
bogomips : 4999.97
clflush size : 64
cache_alignment : 64
address sizes : 46 bits physical, 48 bits virtual
power management:
MemTotal: 2048212 kB
MemFree: 74828 kB
MemAvailable: 1449236 kB
Buffers: 11572 kB
Cached: 1472124 kB
SwapCached: 0 kB
Active: 1460916 kB
Inactive: 432892 kB
Active(anon): 410588 kB
Inactive(anon): 2200 kB
Active(file): 1050328 kB
Inactive(file): 430692 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 410164 kB
Mapped: 37160 kB
Shmem: 2672 kB
Slab: 61228 kB
SReclaimable: 50132 kB
SUnreclaim: 11096 kB
KernelStack: 2272 kB
PageTables: 3200 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 1024104 kB
Committed_AS: 705104 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 0 kB
VmallocChunk: 0 kB
HardwareCorrupted: 0 kB
AnonHugePages: 374784 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 53120 kB
DirectMap2M: 2043904 kB
DirectMap1G: 0 kB
表结构
CREATE TABLE `test` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` char(50) NOT NULL,
`text` varchar(50) DEFAULT NULL,
`userid` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20000001 DEFAULT CHARSET=latin1
创建模拟数据
DELIMITER $$
USE `test`$$
DROP PROCEDURE IF EXISTS `quickInsert`$$
CREATE DEFINER=`root`@`%` PROCEDURE `quickInsert`()
BEGIN
DECLARE i INT;
SET i =0;
START TRANSACTION;
WHILE i < 10000000 DO
IF i MOD 2 = 0 THEN
INSERT INTO test (NAME,TEXT,userid) VALUES (MD5(RAND()*1000),NULL,RAND()*500);
ELSE
INSERT INTO test (NAME,TEXT,userid) VALUES (MD5(RAND()*1000),MD5(RAND()*3000),RAND()*500);
END IF;
SET i = i+1;
END WHILE;
COMMIT;
END$$
DELIMITER ;
关闭缓存
query_cache_size = 0
mysql vserion
5.7.21-0ubuntu0.16.04.1-log
数据大小 1千万条 25~30G
CALL quickInsert(); #插入数据
count(0)测试
- 14.030s
- 17.094s
- 17.391s
- 17.398s
- 17.081s
count(*)测试
- 17.297s
- 17.403s
- 17.296s
- 16.965s
- 17.389s
排除系统波动，基本上没有太大差别
count(主键)
- 16.980s
- 16.982s
- 17.405s
- 17.229s
- 17.095s
count(非空字段) 开始出现大量的不确定的时间，所以多测试了多次
- 17.009s
- 16.902s
- 16.993s
- 14.607s
- 12.843s
- 17.404s
- 17.413s
- 17.397s
- 16.898s
可以看出基本上没有太大的区别，出现的波动，我琢磨了一下可能是阿里云上的这台服务器为突发性能的，可能不是太稳定