variables in PL/SQL are implicitly bind variables.
create or replace procedure dsal(p_empno in number)
as
begin
update emp set sal=sal*2 where empno = p_empno;
commit;
end;
/
Now you might be thinking that you've got to replace the p_empno with a bind variable. However, the good news is that every reference to a PL/SQL variable is in fact a bind variable.
Dynamic SQL
In fact, the only time you need to consciously decide to use bind variables when working with PL/SQL is when using Dynamic SQL.
Dynamic SQL, allows you to execute a string containing SQL using the EXECUTE IMMEDIATE command. For next example would always require a hard parse when it is submitted:
create or replace procedure dsal(p_empno in number)
as
begin
execute immediate
'update emp set sal = sal*2 where empno = '||p_empno;
commit;
end;
/
The way to use bind variables instead is to change the EXECUTE IMMEDIATE command as follows:
create or replace procedure dsal(p_empno in number)
as
begin
execute immediate
'update emp set
sal = sal*2 where empno = :x' using p_empno;
commit;
end;
/
And that's all there is to it. One thing to bear in mind, though, is that you can't substitute actual object names (tables, views, columns etc) with bind variables - you can only subsitute literals. If the object name is generated at runtime, you'll still need to string concatenate these parts, and the SQL will only match with those already in the shared pool when the same object name comes up. However, whenever you're using dynamic SQL to build up the predicate part of a statement, use bind variables instead and you'll reduce dramatically the amount of latch contention going on.
The Performance Killer
Just to give you a tiny idea of how huge of a difference this can make performance wise, you only need to run a very small test:
Here is the Performance Killer ....
SQL> alter system flush shared_pool;
SQL> set serveroutput on;
declare
type rc is ref cursor;
l_rc rc;
l_dummy all_objects.object_name%type;
l_start number default dbms_utility.get_time;
begin
for i in 1 .. 1000
loop
open l_rc for
'select object_name
from all_objects
where object_id = ' || i;
fetch l_rc into l_dummy;
close l_rc;
-- dbms_output.put_line(l_dummy);
end loop;
dbms_output.put_line
(round((dbms_utility.get_time-l_start)/100, 2) ||
' Seconds...' );
end;
/
Modified code
declare
type rc is ref cursor;
l_rc rc;
l_dummy all_objects.object_name%type;
l_start number default dbms_utility.get_time;
begin
for i in 1 .. 1000
loop
open l_rc for
'select object_name
from all_objects
where object_id = :x'
using i;
fetch l_rc into l_dummy;
close l_rc;
-- dbms_output.put_line(l_dummy);
end loop;
dbms_output.put_line
(round((dbms_utility.get_time-l_start)/100, 2) ||
' Seconds...' );
end;
/
Modified code runs faster than the earlier version. This is because it reduces the number of parses required. This is good for scalability as less cpu and memory is consumed.
