Oracle Count() 性能问题
sn_id字段为主键,以下是 count(sn_id) 和 count(*)
执行的时间, 想问, 为何 count(*) 会比count(sn_id) 快?
SELECT
count(sn_id)
FROM
sn {executed in 235 msec}
SELECT
count(sn_id)
FROM
sn {executed in 187 msec}
SELECT
count(sn_id)
FROM
sn {executed in 219 msec}
SELECT
count(sn_id)
FROM
sn {executed in 266 msec}
SELECT
count(sn_id)
FROM
sn {executed in 203 msec}
=====================
SELECT
count(*)
FROM
sn {executed in 188 msec}
SELECT
count(*)
FROM
sn {executed in 172 msec}
SELECT
count(*)
FROM
sn {executed in 172 msec}
SELECT
count(*)
FROM
sn {executed in 172 msec}
SELECT
count(*)
FROM
sn {executed in 156 msec}
SELECT
count(*)
FROM
sn {executed in 172 msec}