尝试在同一条select语句中使用Nextval以及“ Currval的替代项”

Question

目标是根据2个业务条件添加一个具有唯一ID的新列（x_event_code）。

Table : tmp_event_cd
ID  Member  First_Date  Last_date
1   123455678   8/23/2014   8/24/2014
2   123455678   8/23/2014   8/24/2014
3   123455678   8/24/2014   8/24/2014
4   123455678   8/24/2014   8/24/2014
5   123455678   8/25/2014   8/25/2014
6   123455678   10/25/2014  10/25/2014
1   567890124   10/25/2014  10/25/2014
2   567890124   10/25/2014  10/25/2014
3   567890124   12/27/2014  12/27/2014



Psql:
Drop table if exists tmp_event_cd_1 ;
Create table tmp_event_cd_1  as 
(
Select 
ID,Member,First_Date,Last_date,
(Case   
    WHEN    (ID = '1' )  then nextval('Seq_1')  
    WHEN    (ID != '1' and First_Date::Date between 
    -- Member_first_Date 
    (FIRST_VALUE(First_date) over (partition by Member order by First_date)::Text ::date)  AND
    -- Member_last_date
    (FIRST_VALUE(Last_date) over (partition by Member order by First_date)::Text ::date) ) 
    then Currval('Seq_1') 
        /*Currval() not supported*/
        /*Retain the value assigned above untill next member*/
    Else    nextval('Seq_1') 
    end ) as x_event_code 
from  tmp_event_cd 
order by Member,id,First_Date, Last_date
)distributed randomly;


Expected final table:
ID  Member  First_Date  Last_date   x_event_code
1   123455678   8/23/2014   8/24/2014   1
2   123455678   8/23/2014   8/24/2014   1
3   123455678   8/24/2014   8/24/2014   1
4   123455678   8/24/2014   8/24/2014   1
5   123455678   8/25/2014   8/25/2014   2
6   123455678   10/25/2014  10/25/2014  3
1   567890124   10/25/2014  10/25/2014  4
2   567890124   10/25/2014  10/25/2014  4
3   567890124   12/27/2014  12/27/2014  5

“ x_event_code”应保留序列“ Seq_1”中的当前值。 我的Postgres版本不支持。 我尝试在第二种情况下使用（从seq_1中选择last_value），但是即使nextval（）增加了，它也只保留了一个相同的值。

我不能使用基于变量的函数，因为该表具有数十亿条记录。

对此表示感谢。