Logging the results of a MERGE statement

Question

I have 2 tables: A temporary table with raw data. Rows in it may be repeating (more then 1 time). The second is the target table with actual data (every row is unique).

I'm transfering rows using a cursor. Inside the cursor I use a MERGE statement. How can I print to the console using DBMS_OUTPUT.PUT_LINE which rows are updated and which are deleted?

According to the official documentation there is no such feature for this statement.

Are there any workaround?

Answer 1

I don't understand why you would want to do this. The output of dbms_output requires someone to be there to look at it. Not only that it requires someone to look through all of the output otherwise it's pointless. If there are more than say 20 rows then no one will be bothered to do so. If no one looks through all the output to verify but you need to actually log it then you are actively harming yourself by doing it this way.

If you really need to log which rows are updated or deleted there are a couple of options; both involve performance hits though.

1. You could switch to a BULK COLLECT, which enables you to create a cursor with the ROWID of the temporary table. You BULK COLLECT a JOIN of your two tables into this. Update / delete from the target table based on rowid and according to your business logic then you update the temporary table with a flag of some kind to indicate the operation performed.

2. You create a trigger on your target table which logs what's happening to another table.

In reality unless it is important that the number of updates / deletes is known then you should not do anything. Create your MERGE statement in a manner that ensures that it errors if anything goes wrong and use the error logging clause to log any errors that you receive. Those are more likely to be the things you should be paying attention to.

Answer 2

Previous posters already said that this approach is suspicious, both because of the cursor/loop and the output log for review.

On SQL Server, there is an OUTPUT clause in the MERGE statement that allows you to insert a row in another table with the $action taken (insert,update,delete) and any columns from the inserted or deleted/overwritten data you want. This lets you summarize exactly as you asked.

The equivalent Oracle RETURNING clause may not work for MERGE but does for UPDATE and DELETE.