SQL & PL/SQL

Hi

I recently faced an interview below are some of the questions I was unable to answer confidently.
Please can you answer these:

1. In an Explain plan, what does the columns "Time" and "Rows" suggest? Is it the actual time the query should take and the actual number of rows it should return? Or something else?

2. I have a Select, Insert, Update and Delete grants on a table. Which of these I can do:
a. Create a trigger for (Insert or Update or delete) on that table.
b. Create a trigger on view of that tabe.

3. There are 2 triggers on a table (before Insert on that table). The only difference between 2 triggers is - one is row level, other is statement level.
Now If i try to insert data in that table, which of these triggers will be fired?

4. I have a table with 90 million records. I want to delete some 60 million records randomly. What is the most efficient way to do this? [Can use PLSQL as well]



Thanks
AB