How do I use bulk collect and insert in Pl/SQl

Question

I want to fetch around 6 millions rows from one table and insert them all into another table. How do I do it using BULK COLLECT and FORALL ?

Answer 1

The SQL engine parse and executes the SQL Statements but in some cases ,returns data to the PL/SQL engine.

During execution a PL/SQL statement, every SQL statement cause a context switch between the two engine. When the PL/SQL engine find the SQL statement, it stop and pass the control to SQL engine. The SQL engine execute the statement and returns back to the data in to PL/SQL engine. This transfer of control is call Context switch. Generally switching is very fast between PL/SQL engine but the context switch performed large no of time hurt performance . SQL engine retrieves all the rows and load them into the collection and switch back to PL/SQL engine. Using bulk collect multiple row can be fetched with single context switch.

Example : 1

DECLARE

Type stcode_Tab IS TABLE OF demo_bulk_collect.storycode%TYPE;
Type category_Tab IS TABLE OF demo_bulk_collect.category%TYPE;
s_code stcode_Tab;
cat_tab category_Tab;
Start_Time NUMBER;
End_Time NUMBER;

CURSOR c1 IS 
select storycode,category from DEMO_BULK_COLLECT;
BEGIN
   Start_Time:= DBMS_UTILITY.GET_TIME;
   FOR rec in c1
   LOOP
     NULL;
     --insert into bulk_collect_a values(rec.storycode,rec.category);
   END LOOP;
    End_Time:= DBMS_UTILITY.GET_TIME;
    DBMS_OUTPUT.PUT_LINE('Time for Standard Fetch  :-' ||(End_Time-Start_Time) ||'  Sec');

    Start_Time:= DBMS_UTILITY.GET_TIME;    
    Open c1;
        FETCH c1 BULK COLLECT INTO s_code,cat_tab;
    Close c1;
 FOR x in s_code.FIRST..s_code.LAST
 LOOP
 null;        
 END LOOP;
End_Time:= DBMS_UTILITY.GET_TIME; 
DBMS_OUTPUT.PUT_LINE('Using Bulk collect fetch time :-' ||(End_Time-Start_Time) ||'  Sec');
END;