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Troubleshooting

Quiz Night

If this is the closing section of thetkprof output from the trace file of a single end-user session that has a performance problem, what’s the most obvious deduction you can make about the cause of the problem, and what sort of action would you take next ?

Undo Understood

It’s hard to understand all the ramifications of Oracle’s undo handling, and it’s not hard to find cases where the resulting effects are very confusing. In a recent post on the OTN database forum resulted in one response insisting that the OP was obviously updating a table with frequent commits from one session while querying it from another thereby generating a large number of undo reads in the querying session.

Index out of range

I’ve waxed lyrical in the past about creating suitable column group statistics whenever you drop an index because even when the optimizer doesn’t use an index in its execution path it might have used the number of distinct keys of the index (user_indexes.distinct_keys) in its estimates of cardinality.

min/max Upgrade

A question came up on the OTN database forum a little while ago about a very simple query that was taking different execution paths on two databases with the same table and index definitions and similar data. In one database the plan used the “index full scan (min/max)” operation while the other database used a brute force “index fast full scan” operation.

Deception

One of the difficulties with trouble-shooting is that’s it very easy to overlook, or forget to go hunting for, the little details that turn a puzzle into a simple problem. Here’s an example showing how you can read a bit of an AWR report and think you’ve found an unpleasant anomaly. I’ve created a little model and taken a couple of AWR snapshots a few seconds apart so the numbers involved are going to be very small, but all I’m trying to demonstrate is a principle. So here’s a few lines of one of the more popular sections of an AWR report:

Quiz Night

The following is a straight, continuous, untouched, cut-n-paste from an SQL*Plus session on 12.1.0.2. How come the update doesn’t execute in parallel – noting that parallel DML has been enabled and the tablescan to identify rows to be updated does execute in parallel ?

Guesswork

A recent posting on the OTN database forum described a problem with an insert (as select) statement that sometimes ran extremely slowly: nothing interesting yet, there could be plenty of boring reasons for that to happen. The same SQL statement (by SQL_ID) might take 6 hours to insert 300K rows one night while taking just a few minutes to insert 900K another night (still nothing terribly interesting).

ASSM Help

I’ve written a couple of articles in the past about the problems of ASSM spending a lot of time trying to find blocks with usable free space. Without doing a bit of rocket science with some x$ objects, or O/S tracing for the relevant calls, or enabling a couple of nasty events, it’s not easy proving that ASSM might be a significant factor in a performance problem – until you get to 12c Release 2 where a staggering number of related statistics appear in v$sysstat.

I’ve published the full list of statistics (without explanation) at the end of this note, but here’s just a short extract showing the changes in my session’s ASSM stats due to a little PL/SQL loop inserting 10,000 rows, one row at a time into an empty table with a single index:

Delete/Insert #2

In the previous post I threw out a couple of options for addressing the requirement to transfer data from one table to another (“cut and paste” rather than just “copy”) without running into odd inconsistency errors. This triggered of a wonderful comment trail of alternatives based on how large the volume might be, how relaxed the concurrency requirements might be, and so on.

A comment by SydOracle1 picked up on my failure to get Oracle working with the “as of SCN” syntax because it kept reporting ORA-08187 and suggested a straightforward use of the VERSIONS strategy. I thought it was quite surprising that this could work given that “as of SCN” didn’t, so I whipped up a quick test to check it – adding a couple of little refinements to the supplied sample – and it worked.

Anniversary OICA

Happy anniversary to me!

On this day 10 years ago I published the first article in my blog. It was about the parameter optimizer_index_cost_adj (hence OICA), a parameter that has been a  source of many performance problems and baffled DBAs over the years and, if you read my first blog posting and follow the links, a parameter that should almost certainly be left untouched.