Question

Is it possible to lock the table restricting reads from other sessions? I would more appreciate suggestions like “you can workaround it with …” than “Why do you need that?”.

Answer 1

At least you acknowledge this is a odd request.

There are some things you could do to implement this. All of them require some redirection, which means the table must be owned by a separate schema, not the one users connect with. This is because Oracle does not have a mechanism for blocking reads, and a schema owner always has access to their tables; all we can do is deny other users access to our tables.

One option is to use a view. Most of the time it points to your table.

create or replace view public_user.your_table 
    as select * from private_user.your_table;

But when you want to deny access you switch it to a different, empty table (which obviously must have the same projection as YOUR_TABLE).

create or replace view public_user.your_table 
    as select * from private_user.empty_table;

Then when you’ve finished you can run the first statement again.

This approach is relatively straightforward but requires issuing DDL. This means the account working on YOUR_TABLE also must have rights on the PUBLIC_USER.YOUR_TABLE. Also, issuing DDL will invalidate objects which have a dependency on the table (view).

So here is an alternative approach.

create table lock_table (col1 number not null);
insert into lock_table values (1);

create procedure lock_your_table as
    pragma autonomous_transaction;
begin
     update lock_table set col1 = 0;
     commit;
end;
create procedure unlock_your_table as
    pragma autonomous_transaction;
begin
     update lock_table set col1 = 1;
     commit;
end;

We have a different version of the view:

create or replace view public_user.your_table 
    as select * from private_user.your_table
                cross join lock_table
        where col1 = 1;

This will return all the rows in YOUR_TABLE or none or them, depending on the value of LOCK_TABLE.COL1. So you withdraw access to the table’s data you run lock_the_table() and to grant it you run unlock_the_table().

---

Why does Oracle make it so hard to be this? Because it is confusing to the application users. Oracle has implemented a multi-version concurrency model, which means we can always read tables, regardless of whether anybody else is working with the table. What you want to do flies in the face of that approach.