-- 将得到的字符串转换成数字
select TO_NUMBER(to_char(CURRENT_DATE,'D'),'99')

-- 因为得到的星期一为2，所以要减去2
select TO_NUMBER(to_char(CURRENT_DATE,'D'),'99')-2

-- 将得到的数字乘以 -1 比如例子中：-1*3 就是 -3 ，也就是减去 3天
select cast(-1*3 || 'days' as interval) 

-- 就是将当天减去0天 得到了星期一的日期
select cast(-1*0 || 'days' as interval) + CURRENT_DATE

SELECT  to_char(CURRENT_DATE +cast(-1*(TO_NUMBER(to_char(CURRENT_DATE,'D'),'99')-2) ||' days' as interval),'yyyy-mm-dd');

select  to_char('2019-09-27'::date - (  case when EXTRACT(DOW FROM TIMESTAMP '2019-09-27') = 0 then 6 
     when EXTRACT(DOW FROM TIMESTAMP '2019-09-27') = 1 then 0 
     else EXTRACT(DOW FROM TIMESTAMP '2019-09-27') -1
     end )::integer,'yyyy-mm-dd') as 星期一日期 
     ,to_char('2019-09-27'::date + (   case when EXTRACT(DOW FROM TIMESTAMP '2019-09-27') = 0 then 0 
     else 7- EXTRACT(DOW FROM TIMESTAMP '2019-09-27')
     end )::integer,'yyyy-mm-dd') 星期日日期

select  to_char(cast(-1*(TO_NUMBER(to_char(to_date('2019'||'-01-01','yyyy-mm-dd'),'D'),'99')-2)|| 'days' as interval) + to_date('2019'||'-01-01','yyyy-mm-dd'),'yyyy-mm-dd') 在2019年1月1日所属星期的星期一日期
, to_char(to_date(cast(-1*(TO_NUMBER(to_char(to_date('2019'||'-01-01','yyyy-mm-dd'),'D'),'99')-2)|| 'days' as interval) + to_date('2019'||'-01-01','yyyy-mm-dd'),'yyyy-mm-dd')
+cast(38*7|| 'days' as interval),'yyyy-mm-dd') 第38周的开始时间
, to_char(to_date(cast(-1*(TO_NUMBER(to_char(to_date('2019'||'-01-01','yyyy-mm-dd'),'D'),'99')-2)|| 'days' as interval) + to_date('2019'||'-01-01','yyyy-mm-dd'),'yyyy-mm-dd')
+cast(38*7+6|| 'days' as interval),'yyyy-mm-dd') 第38周的结束时间