Estimate execution time for CTAS

Question

Hi,
I am searching for long to find a way to estimate the execution time for CTAS commands. I am a DBA. Our users run CTAS commands to load millions of rows. The commands fetch data from 4-5 very big tables each with millions of records and process them using where clause and group by clause and finally create the table. All these things are coded in the CTAS command. These CTAS sometime takes long time like 5 , 8 Hrs. Users frequently ask me to find how long it's going to take. I use both OEM and TOAD. But I couldn't find the time estimated from these tools. I feel that there must be some way, but I don't know the method.
Can any body please help me in this regard?
Thanks & Regards
Ananda Basak

Answer 1

It depends on a number of factors chief among them how accurate your estimate needs to be but also including things like what version of Oracle you're using, how accurate your database statistics are, etc.
One option is to look at the TIME column in the plan. For example, if I wanted to do a CTAS to create a copy of the EMP table, the optimizer expects that to take on the order of a second. Of course, the optimizer's estimates are only estimates and are only as accurate as the database statistics that are in place. If the optimizer generates a bad plan, it's likely because the optimizer expects some operation to take much more or much less time than it does in reality in which case the optimizer's runtime estimate is likely to be way off.
SQL> explain plan for create table emp_copy as select * from emp;
Explained.
SQL> ed
Wrote file afiedt.buf
  1  select *
  2*   from table( dbms_xplan.display() )
SQL> /
PLAN_TABLE_OUTPUT
Plan hash value: 2748781111
| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | CREATE TABLE STATEMENT |          |    14 |   546 |     4   (0)| 00:00:01 |
|   1 |  LOAD AS SELECT        | EMP_COPY |       |       |            |          |
|   2 |   TABLE ACCESS FULL    | EMP      |    14 |   546 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------Depending on the query plan, you may be able to query the GV$SESSION_LONGOPS table to track the progress of any long-running operations in your session. If your query plan involves a lot of full table scans, sorts that take more than a few seconds, hash joins, etc. then it is likely that you'll be able to chart the progress of a query over time by watching GV$SESSION_LONGOPS change. Of course, if your query is going to need to do many long-running operations, you'll need to a human to interpret the data a bit in order to figure out where in the plan Oracle currently is and how far along that means the entire query is.
SELECT *
  FROM gv$session_longops
WHERE time_remaining > 0If you're using 11g and you have the performance and tuning pack licensed, you could also potentially use the V$SQL_PLAN_MONITOR view.
Justin