Creating Indexes to optimize query
Perhaps this will help.  If your data looks something like this:

create table mytab1 as (select level myid1 from dual connect by level &lt;=2000000);
create table mytab2 as (select level+20000 myid2 from dual connect by level &lt;=100);


No indexes:


  Cost: 881, Consistent Gets: 3140, CPU Used: 174


Index on MyTab1 Table:

create index myid1_1 on mytab1 (myid1);



  Cost: 104, Consistent Gets: 103, CPU Used: 14


Index on both MyTab1 and MyTab2:

create index myid2_1 on mytab2 (myid2);



  Cost: 53, Consistent Gets: 102, CPU Used: 12


Your mileage will vary depending on quite a few factors, but in general an index on myid1 would probably be beneficial.  

If you haven't already you should read through the Oracle Concepts Guide (pdf) particularly the section on indexing.