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job 和schedule中repeat_interval参数都是用来控制执行的频率或周期，虽然说周期是一个时间性概念，不过repeat_interval指定的时候并不是一个时间值，而是由一组关键字描述的时间。
除了前面介绍job和schedule的repeat_interval参数时，提到该参数拥有freq以及interval两个关键字，其实除此之外，还有如bymonth、byweekno、byyearday、bydate等等参数，可以用来进行更精确的定义，比如通过bymonth关键字指定调度运行的月份，byday指定调度在哪天运行等等。
repeat_interval 参数的详细语法如下：
repeat_interval = regular_schedule | combined_schedule
==============================
regular_schedule = frequency_clause
[";" interval_clause] [";" bymonth_clause] [";" byweekno_clause]
[";" byyearday_clause] [";" bydate_clause] [";" bymonthday_clause]
[";" byday_clause] [";" byhour_clause] [";" byminute_clause]
[";" bysecond_clause] [";" bysetpos_clause] [";" include_clause]
[";" exclude_clause] [";" intersect_clause][";" periods_clause]
[";" byperiod_clause]
==============================
combined_schedule = schedule_list [";" include_clause]
[";" exclude_clause] [";" intersect_clause]
frequency_clause = "freq" "=" ( predefined_frequency | user_defined_frequency )
predefined_frequency = "yearly" | "monthly" | "weekly" | "daily" |
"hourly" | "minutely" | "secondly"
user_defined_frequency = named_schedule
==============================
interval_clause = "interval" "=" intervalnum
intervalnum = 1 through 99
bymonth_clause = "bymonth" "=" monthlist
monthlist = monthday ( "," monthday)*
month = numeric_month | char_month
numeric_month = 1 | 2 | 3 ... 12
char_month = "jan" | "feb" | "mar" | "apr" | "may" | "jun" |
"jul" | "aug" | "sep" | "oct" | "nov" | "dec"
byweekno_clause = "byweekno" "=" weeknumber_list
weeknumber_list = weeknumber ( "," weeknumber)*
weeknumber = [minus] weekno
weekno = 1 through 53
byyearday_clause = "byyearday" "=" yearday_list
yearday_list = yearday ( "," yearday)*
yearday = [minus] yeardaynum
yeardaynum = 1 through 366
bydate_clause = "bydate" "=" date_list
date_list = date ( "," date)*
date = [yyyy]mmdd [ offset | span ]
bymonthday_clause = "bymonthday" "=" monthday_list
monthday_list = monthday ( "," monthday)*
monthday = [minus] monthdaynum
monthdaynum = 1 through 31
byday_clause = "byday" "=" byday_list
byday_list = byday ( "," byday)*
byday = [weekdaynum] day
weekdaynum = [minus] daynum
daynum = 1 through 53 /* if frequency is yearly */
daynum = 1 through 5 /* if frequency is monthly */
day = "mon" | "tue" | "wed" | "thu" | "fri" | "sat" | "sun"
byhour_clause = "byhour" "=" hour_list
hour_list = hour ( "," hour)*
hour = 0 through 23
byminute_clause = "byminute" "=" minute_list
minute_list = minute ( "," minute)*
minute = 0 through 59
bysecond_clause = "bysecond" "=" second_list
second_list = second ( "," second)*
second = 0 through 59
bysetpos_clause = "bysetpos" "=" setpos_list
setpos_list = setpos ("," setpos)*
setpos = [minus] setpos_num
setpos_num = 1 through 9999
==============================
include_clause = "include" "=" schedule_list
exclude_clause = "exclude" "=" schedule_list
intersect_clause = "intersect" "=" schedule_list
schedule_list = schedule_clause ("," schedule_clause)*
schedule_clause = named_schedule [ offset ]
named_schedule = [schema "."] schedule
periods_clause = "periods" "=" periodnum
byperiod_clause = "byperiod" "=" period_list
period_list = periodnum ("," periodnum)*
periodnum = 1 through 100
==============================
offset = ("+" | "-") ["offset:"] duration_val
span = ("+" | "-" | "^") "span:" duration_val
duration_val = dur-weeks | dur_days
dur_weeks = numofweeks "w"
dur_days = numofdays "d"
numofweeks = 1 through 53
numofdays = 1 through 376
minus = "-"
这个语法形式看起来复杂无比，其实实用起来很简单，之所以看起来复杂，是因为其功能太过灵活(之前的三思系列笔记中，已经阐述过灵活与复杂的关系)，这里不准备逐条解释每一个语法细节，下面将着重通过一些常用设置，希望能够更有助于广大同仁的理解。
例如：设置任务仅在周5的时候运行：
repeat_interval => 'freq=daily; byday=fri';
repeat_interval => 'freq=weekly; byday=fri';
repeat_interval => 'freq=yearly; byday=fri';
上述三条语句虽然指定的关键字小有差异，不过功能相同。
设置任务隔一周运行一次，并且仅在周5运行：
repeat_interval => 'freq=weekly; interval=2; byday=fri';
设置任务在当月最后一天运行：
repeat_interval => 'freq=monthly; bymonthday=-1';
设置任务在3月10日运行：
repeat_interval => 'freq=yearly; bymonth=mar; bymonthday=10';
repeat_interval => 'freq=yearly; bydate=0310';
上述两条语句功能相同。
设置任务每10隔天运行：
repeat_interval => 'freq=daily; interval=10';
设置任务在每天的下午4、5、6点时运行：
repeat_interval => 'freq=daily; byhour=16,17,18';
设置任务在每月29日运行：
repeat_interval => 'freq=monthly; bymonthday=29';
设置任务在每年的最后一个周5运行：
repeat_interval => 'freq=yearly; byday=-1fri';
设置任务每隔50个小时运行：
repeat_interval => 'freq=hourly; interval=50';
另外，你是否在怀念常规job中设置interval的简便，虽然功能较弱，但是设置操作非常简单，无须懊恼，其实scheduler中的repeat_interval也完全可以按照那种方式设置，前面都说了，repeat_interval实际上是指定周期，直接指定一个时间值，当然也是周期喽。
比如说，设置任务每天执行一次，也可以设置repeat_interval参数值如下：
repeat_interval => 'trunc(sysdate)+1'
又比如设置任务每周执行一次：
repeat_interval => 'trunc(sysdate)+7'
不过需要注意，这种方式仅用于创建scheduler中jobs时使用，不能用于schedule。
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