A question on bulk collect

Question

My Version
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit ProductionPls see the code below
declare
type empno_arr_type is table of int;
empno_arr empno_arr_type;
begin
select empno  into empno_arr
from emp;
end;
Output
PLS-00642: local collection types not allowed in SQL statementsNow same code with bulk collect executes fine.
declare
type empno_arr_type is table of int;
empno_arr empno_arr_type;
begin
select empno bulk collect into empno_arr
from emp;
end;Just wanted to undestand what exactly is the difference other than reduced context switch with bulk bind.
What could have made the second code work since both are local collection types only?
Thanks,
CJ

Answer 1

910555 wrote:
Just wanted to undestand what exactly is the difference other than reduced context switch with bulk bind.There is basically a single context switch with an implicit SQL cursor in PL/SQL. So whether or not bulk fetching/binding is used, is immaterial - it does not reduce context switching as there is already the minimal possible context switching for executing that implicit cursor.
E.g.
declare
begin
   --// single context switch (1 row fetched)
  select salary into salaryAmount from emp where empno = 1234;
   --// single context switch (20 rows fetched)
  select salary bulk collect into salaryAmounts from emp where deptno = 1234;
end;Context switching happens each time you use a SQL cursor interface. E.g.
declare
  cursor c is ...
begin
  --// context switch
  open c;
  loop
    --// context switch
    fetch from c  ....
  end loop;
  --// context switch
  close c;
end;Bulk processing and binding reduce context switching when using cursor processing loops like in the above example. If you can fetch more rows per loop iterations, then there will be less context switching.
For example, the cursor outputs a 1000 rows. The above sample code using single row fetches in the loop, will have a 1000 loop iterations and therefore a 1000 context switches. A bulk fetch of a 100, will fetch a 100 rows per loop iteration for a total cost of 10 iterations and 10 context switches to consume the 1000 rows output of that cursor.
What could have made the second code work since both are local collection types only?You are confusing the receive structure (in PL/SQL) with context switching.
When you fetch from a cursor, you need to supply a PL/SQL structure to receive the output (called a SQL projection) from the cursor.
If the fetch is a single row fetch, a scalar (single value) structure is needed.
If the fetch is a multi-row (bulk) fetch, the structure needs to be an array (aka collection or nested table).