How can I alter a sequence in dynamic SQL?

问题:

I'm trying to create a script to migrate data from one DB to another. One thing I'm not currently able to do is set the nextval of a sequence to the nextval of a sequence in another DB.

I got the difference in values from user_sequences and generated the following dynamic SQL statements:

execute immediate 'alter sequence myseq increment by 100';
execute immediate 'select myseq.nextval from dual';
execute immediate 'alter sequence myseq increment by 1';

commit;

But nothing happens. What am I missing? If I run the same statements outside the procedure, they work fine:

alter sequence myseq increment by 100;
select myseq.nextval from dual;
alter sequence myseq increment by 1;

commit;

EDIT: Apologies to all for not being clear. I'm actually altering the sequence in the same DB. I'm only getting the value to be set from a remote DB. Perhaps it was unnecessary to mention the remote DB as it doesn't affect things. I only mentioned it to explain what my goals were.

Step 1. I get the nextval of the sequence from a remote DB.

select (select last_number
        from dba_sequences@remoteDB
        where upper(sequence_name) = upper(v_sequence_name)) - (select last_number
                                                                from user_sequences
                                                                where upper(sequence_name) = upper(v_sequence_name)) increment_by
from dual;    

Step 2. I generate dynamic SQL statements with this value:

execute immediate 'alter sequence myseq increment by 100';
execute immediate 'select myseq.nextval from dual';
execute immediate 'alter sequence myseq increment by 1';

commit;

No error was raised, but nothing happened. When I wrote the SQL statements with DBMS_OUTPUT.PUT_LINE and ran them outside they worked.

回答1:

Now you have properly explained your requirements here is some code. I have written this so it will work for any sequence in your schema.

create or replace procedure resync_seq
    (p_seq_name in user_sequences.sequence_name%type)
is
    local_val pls_integer;
    remote_val pls_integer;
    diff pls_integer;
begin
    execute immediate 'select '|| p_seq_name ||'.nextval from dual'
           into local_val;
    select last_number into remote_val
    from user_sequences@remote_db
    where sequence_name = p_seq_name ;
    diff := remote_val - local_val;

    if diff > 0
    then
        execute immediate 'alter sequence  '|| p_seq_name ||' increment by ' ||to_char(diff);
        execute immediate 'select '|| p_seq_name ||'.nextval from dual'
           into local_val;
        execute immediate 'alter sequence  '|| p_seq_name ||' increment by 1';
    end if;

end;

The procedure doesn't need a COMMIT because DDL statements issue an implicit commit (two in fact).

You can execute it and see the synced value like this (in SQL*PLus):

exec resync_seq('MYSEQ')
select myseq.currval
from dual

回答2:

I wasn't quite able to understand what you mean, but here is a wild guess:

I don't see it in your code, but you're talking about executing DDL (CREATE, ALTER etc.) on another database, so I assume you are using Database Links. It is not possible to use Database Links to execute DDL on another database. You might want to consider that.

---

After the information you provided, this might be what you need. And if you want to set the current value of the sequence, you can't, according to this documentation, you need to drop/create:

declare
  ln_lastNumber DBA_SEQUENCES.LAST_NUMBER%type;
  lv_sequenceName DBA_SEQUENCES.SEQUENCE_NAME%type := 'MYSEQ';
begin
  select LAST_NUMBER
  into ln_lastNumber
  from DBA_SEQUENCES--or @remote_db;
  where
    --Your predicates;

  execute immediate 'drop sequence ' || lv_sequenceName;
  execute immediate 'create sequence ' || lv_sequenceName || ' starts with ' || ln_lastNumber;
exception
  when no_data_found then
    dbms_output.put_line('No sequence found!'); -- Or log somehow.
    raise;
  when others then
    raise;
end;

回答3:

Also, DDL in dynamic SQL pacakges requires

AUTHID CURRENT_USER

when declaring the package specification, if you want it to have the credentials of the current user

回答4:

I took the code provided by APC and modified as below:

create or replace procedure resync_seq
    (p_seq_name in user_sequences.sequence_name%type, 
     p_table_name in user_tables.table_name%type, 
     p_pk in user_cons_columns.column_name%type)
is
     local_val pls_integer;
     remote_val pls_integer;
     diff pls_integer;
begin
      execute immediate 'select '|| p_seq_name ||'.nextval from dual'
       into local_val;

       execute immediate 'select max('||p_pk||') from '||p_table_name ||' ' 
       into remote_val ;

       diff := remote_val - local_val;

       if diff > 0
          then
          execute immediate 'alter sequence  '|| p_seq_name ||' increment by ' ||to_char(diff);
          execute immediate 'select '|| p_seq_name ||'.nextval from dual'
       into local_val;
          execute immediate 'alter sequence  '|| p_seq_name ||' increment by 1';
       end if;

 end;