Finding last number generated by a sequence before it got deleted - oracle

Question

We found that there is a sequence "_SEQUENCE" is not present in one of our client's UAT instance. We don't how it got deleted and when. It is a very crucial sequence because the numbers generated by this sequence are used as unique column values in many tables across DB. In other words, no two columns (of specific column types) in any two tables in the DB will have the same number as value. On few of these columns, we have unique index also.
We can create the sequence again but we don't know what should be the initial value of the sequence because we don't know what was the last number the old sequence generated. If we set a wrong number as the initial value and by chance, if it generates the same number for the same column of a table which is already present, we may end up getting unique key violation exception.
We can set the initial value to a very big number but that is the last option. Now;
Is it possible to find the last number the sequence "_SEQUENCE" generated before it got deleted?
Is it possible to find which process deleted the sequence "_SEQUENCE" and when?

Answer 1

The flashback operation is not available for a sequence, while it's available for tables. A mechanishm might be produced by the contribution of a DDL trigger mostly created in SYS or SYSTEM database users. As an example consider this procedure :
create or replace procedure pr_ddl_oper as
v_oty varchar2(75) := ora_dict_obj_type;
v_don varchar2(75) := ora_dict_obj_name;
v_evt varchar2(75) := ora_sysevent;
v_olu varchar2(75) := nvl(ora_login_user,'Unknown Schema');
v_sql ora_name_list_t;
v_stm clob;
v_sct owa.vc_arr;
n pls_integer;
n_max pls_integer := 10000; -- max number of rows for CLOB object
-- to hold object's source.
begin
v_sct(1) := 'SESSIONID';
v_sct(2) := 'IP_ADDRESS';
v_sct(3) := 'TERMINAL';
v_sct(4) := 'OS_USER';
v_sct(5) := 'AUTHENTICATION_TYPE';
v_sct(6) := 'CLIENT_INFO';
v_sct(7) := 'MODULE';
for i in 1..7
loop
v_sct(i) := sys_context('USERENV',v_sct(i));
end loop;
select decode(v_sct(1),0,null,v_sct(1)),decode(upper(v_sct(3)),
'UNKNOWN',null,v_sct(3))
into v_sct(1),v_sct(3)
from dual;
n := ora_sql_txt( v_sql );
if n > n_max then
n := n_max;
end if;
for i in 1..n
loop
v_stm := v_stm || v_sql(i);
end loop;
insert into log_ddl_oper(event_time,usr,evnt,stmt,sessionid,ip,terminal,os_user,
auth_type,object_type,object_name,client_info,module_info)
values(sysdate,v_olu,v_evt,v_stm,v_sct(1),v_sct(2),v_sct(3),v_sct(4),v_sct(5),
v_oty,v_don,v_sct(6),v_sct(7));
end;
which could be called by this trigger as to be a future reference :
--| Compiling this trigger, especially for Production Systems, should be handled with care |
create or replace trigger system.trg_admin_ddl before ddl on database
declare
begin
pr_ddl_oper;
end;
By connecting SYS user you can query the last_number column for your dropped Sequence :
select last_number
from dba_sequences
as of timestamp to_timestamp('2018-11-27 23:50:17', 'YYYY-MM-DD HH24:MI:SS') s
where s.sequence_name = 'MYSEQ';
where the timestamp value could be detected by
select l.event_time
--> returns to_timestamp('2018-11-27 23:50:17', 'YYYY-MM-DD HH24:MI:SS')
--> to use for the above SQL Select statement
from log_ddl_oper l
where l.object_name = 'MYSEQ'
and l.evnt = 'DROP'