Can someone explain this scenario?

Hi List,

I have encountered a performance problem (and since solved it) but I was hoping someone could explain exactly why it happened. Let me explain:

Effectively, insert 1 row into a table and then insert that same row into a "history" table. This process was completed as a single transaction (along with some other irrelevant statements) and then the results were commited before looping around and doing the process for the next item of data. The two key steps were:
1) Insert a row into a ~1million row table using "insert into table1... values..."
2) Insert a rows into a ~100million row table using "insert into table2... select... from table1 where col1 = constant", col1 has a unique index defined.

The tables and indexes were all analyzed and accurate. The second insert was taking around ~15 seconds to complete, where in a smaller environment the performance was fine. I added a hint to the select component and this reduced execution to ~3seconds. Eventually I duplicated the first statement and reduced execution time to a fraction of a second - what I was originally expecting.

We often perform this type of task, however the first statement is usually an update of an existing row. The performance has never been noticed as a problem before - we typically run in the "hundreds or thousands of records per seconds" range performing these updates depending on the complexity.

My best guess is that I was getting some kind of wait, perhaps the indexes of table1 were still being updated when I was attempting the scan for the second insert? This would explain why having the first statement as an update doesn't cause the problem - the unique index columns are never updated.

Unfortunately I don't have any time currently to investigate the problem further - I simply changed the script as mentioned, but I would love to understand if this is expected behaviour or not. Do any guru's out there have some insight? For the record, we are running 8.1.7.4 on a Sun E4500.

Thanks in advance,

Mark
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Author: Mark Richard
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