Bulk Update for multiple columns based on columns from other table - sql

Question

I am trying to do a bulk update in Oracle. The operation involves updating millions of records in target table based on column values from another table.The scenario goes like this.
I have 2 tables:
T1 (source)
t1_col1 t1_col2 t1_col3 t1_col4 t1_col5
T2 (target)
t2_col1 t2_col2 t2_col3 t2_col4 t2_col5 t2_col6
I need to do an update like this:
update t1
set t2_col1 = t1_col1,
t2_col2 = t1_col2,
t2_col3 = sysdate
where t2_col4 = t1_col4
and t2_col5 = t1_col5
and t2_col6 = null
How can I achieve the above update for multiple columns comprising millions of records. Going through the forum, i understand this should be done utilizing bulk collect, cursor, for all, limit etc. I am unable to come up with a query based on this though. Appreciate your help.

Answer 1

The update of millions of columns is always slow. No matter if you do this using UPDATE or BULK COLLECT. The fastest method is to CREATE AS SELECT.
Example, HR scheme:
Creating a target table, EMP, for example as a copy of employees table:
create table emp as select * from employees; update emp set salary = salary * 2;
Create the THIRD temporary table, with new values your update, here I change SALARY column
create table emp_new as SELECT
emp.employee_id,
emp.first_name,
emp.last_name,
emp.email,
emp.phone_number,
emp.hire_date,
emp.job_id,
e.salary,
emp.commission_pct,
emp.manager_id,
emp.department_id FROM
employees e, emp WHERE emp.employee_id = e.employee_id;
Drop the original target table:
create table emp_arch as select * from emp; -- you can want to archive data drop table emp;
Rename the third table to the target table_name
rename emp_new to emp;
That's all, it should be the fastest way. You have to remember that you should add constraints and indexes the to new table.
I did once comparison of how to load data using different methods so if you interested you can check this. At the and there are scripts.
https://how2ora-en.blogspot.com/2020/03/loading-data-sql-vs-forall-perf-tests.html
If you want to use bulk collect it could look like this:
declare
type tabEmp is table of employees%rowtype;
tEmp tabEmp;
bulk_errors exception;
pragma exception_init(bulk_errors, -24381);
nErrCnt number;
n_errcode number;
v_msg varchar2(4000);
n_idx number;
cursor cur is select * from employees;
begin
open cur;
loop
fetch cur bulk collect into tEmp;
exit when tEmp.count =0;
BEGIN
forall i in 1..tEmp.count
update emp set salary = tEmp(i).salary;
exception when bulk_errors then
nErrCnt:= sql%bulk_exceptions.count;
for i in 1 .. nErrCnt
loop
n_errcode := sql%bulk_exceptions(i).error_code;
v_msg := sqlerrm(-n_errcode);
n_idx := sql%bulk_exceptions(i).error_index;
dbms_output.put_line(n_errcode);
end loop;
end;
end loop;
close cur;
end;