Bulk collect is the bulk binding syntax for select statements.(Performance tuning - PL/SQL)

Bulk Collect

Executing sql statements in plsql programs causes a context switch between the plsql engine and the sql engine.
Too many context switches may degrade performance dramatically. In order to reduce the number of these context
switches we can use a feature named bulk binding. Bulk binding lets us to transfer rows between the sql engine and the plsql engine as collections.
Bulk binding is available for select, insert, delete and update statements.

Bulk collect is the bulk binding syntax for select statements. All examples below are simplified versions of the live cases i have seen.

One of the things i usually come across is that developers usually tend to use cursor for loops to process data. They declare a cursor, open it, fetch from it row by row in a loop and process the row they fetch.


SQL> create table a_all_objects as select * from all_objects;

Table created.

SQL> select count(*) from a_all_objects;

  COUNT(*)
----------
     90154

SQL> insert into a_all_objects select * from a_all_objects;

90154 rows created.

SQL> /

180308 rows created.

SQL> commit;

Commit complete.

SQL> set timing on

SQL> declare
cursor c1 is select OBJECT_NAME from a_all_objects;
      rec1 c1%rowtype;
  begin
  open c1;
  loop
     fetch c1 into rec1;
     exit when c1%notfound;

     null;

  end loop;
  end;
  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:06.17
SQL>

SQL> declare
 cursor c1 is select OBJECT_NAME from a_all_objects;
  type c1_type is table of c1%rowtype;
  rec1 c1_type;
  begin
  open c1;

     fetch c1 bulk collect into rec1;


  end;
  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.37
SQL>
SQL>
SQL> alter system flush buffer_cache;

System altered.

As can be clearly seen, bulk collecting the rows shows a huge performance improvement over fetching row by row.

The above method (which fetched all the rows) may not be applicable to all cases. When there are many rows to process,
 we can limit the number of rows to bulk collect, process those rows and fetch again. Otherwise process memory gets bigger and bigger as you fetch the rows.

SQL> declare

cursor c1 is select object_name from a_all_objects;
  type c1_type is table of c1%rowtype;
  rec1 c1_type;
  begin
  open c1;
  loop
     fetch c1 bulk collect into rec1 limit 200;
     for i in 1..rec1.count loop
             null;
     end loop;
     exit when c1%notfound;
  end loop;
  end;
  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.35
SQL>