Bulk Binds and Collects in PL/SQL - Part - 1 - Page 2

Bulk Binds (Writing data in bulk)

Bulk binds improve performance of DML statements by minimizing the number of switches between the PL/SQL and SQL engines.

You may have a piece of code, which has multiple update, delete or insert statements on the same table. This results in multiple calls to the SQL engine for carrying out the transaction. By using bulk binds, you can carry out mass scale data manipulation at one shot. The altered data has to be stored in a PL/SQL collection in the code.

The FORALL statement is used for doing the bulk-processing job at one go. This statement is similar to the FOR-LOOP statement except that LOOP/END LOOP key words are not used. The FORALL statement needs a range to work on, along with whatever DML activity is to be carried.

Syntax:

  forall <counter> in <range begin> .. <range end>
    update/insert/delete

Below is a simple example without Bulk Binds. Notice the number of times the update is performed. Every SQL statement present in the PL/SQL code results in a call to the SQL engine for processing.

create or replace procedure updsal is
   cursor cr_emp is
     select empno, job, sal
     from   amemp
     where  job in ('MANAGER', 'PRESIDENT', 'DBA');
begin
   for rec in cr_emp loop
     ... some checks on the employee
     ...
     if rec.job = 'MANAGER' then
        update amemp
        set    sal = sal * 1.1
        where  empno = rec.empno;
     else
        update amemp
        set    sal = sal * 1.2
        where  empno = rec.empno;
     end if;
   end loop;
end;

The above code is changed as shown below, using the bulk binding option. Notice the single update call to the backend.

create or replace procedure updsal is
   cursor cr_emp is
     select empno, job, sal
     from   amemp
     where  job in ('MANAGER', 'PRESIDENT', 'DBA');

   type amemp_tab1 is table of amemp.empno%type
        index by binary_integer;

   type amemp_tab2 is table of amemp.sal%type
        index by binary_integer;

   empnum amemp_tab1;
   empsal amemp_tab2;
   cnt   number := 0;
begin
   for rec in cr_emp loop
     ... some checks on the employee
     ...

     cnt := cnt + 1;
     empnum(cnt) := rec.empno;

     if rec.job = 'MANAGER' then
        empsal(cnt) := rec.sal * 1.1;
     else
        empsal(cnt) := rec.sal * 1.2;
     end if;
   end loop;

   forall i in 1 .. cnt
     update amemp
     set    sal = empsal(i)
     where  empno = empnum(i);
end;
/

Below is an example that uses both bulk binding and collection together.

declare
  type emp_t is table of amemp.empno%type;
  lst  emp_t;
begin
  select empno bulk collect 
  into   lst
  from   amemp
  where  sal < 2000;

  ..some checks here...

  forall i in lst.first .. lst.last
    update amemp
    set    sal = sal * 1.1
    where  empno = lst(i);
end;
/

Bulk collects updated value return

You can even use bulk collects to return a value to the calling procedure using the RETURNING clause, without any additional fetch. Below is an example that updates the salary as and stores the updated information in a collection for further processing.

declare
  type no_t  is table of amemp.empno%type;
  type sal_t is table of amemp.sal%type;
  lno  no_t;
  lsal sal_t;

begin
  select empno bulk collect
  into   lno
  from   amemp
  where  job = 'DBA';
  ..
  ..
  forall i in lno.first .. lno.last
    update amemp
    set    sal =  sal * 1.1
    where  empno = lno(i)
    returning sal bulk collect into lsal;
 ..
  for i in lno.first .. lno.last loop
    dbms_output.put_line(lno(i) || '/' || lsal(i));
  end loop;
end;
/

In the next installment, we will cover the above features with more collection examples and performance issues that may be encountered in heavy processing.

» See All Articles by Columnist Amar Kumar Padhi