Troubleshooting

Jonathan Lewis's picture

Reverse Key

A question came up on the OTN database forum recently asking if you could have a partitioned index on a non-partitioned table.

(Aside: I’m not sure whether it would be quicker to read the manuals or try the experiment – either would probably be quicker than posing the question to the forum. As so often happens in these RTFM questions the OP didn’t bother to acknowledge any of the responses)

Jonathan Lewis's picture

Predicate Order

A recent OTN post demonstrated a very important point about looking at execution plans – especially when you don’t use the right data types. The question was:

We’ve this query which throws invalid number

SELECT * FROM table A
WHERE A.corporate_id IN (59375,54387) AND TRUNC(created_dt) BETWEEN '19-DEC-14' AND '25-DEC-14';

However it works fine if we use not in instead of in

SELECT * FROM table A  
WHERE A.corporate_id  NOT IN (59375,54387) AND TRUNC(created_dt) BETWEEN '19-DEC-14' AND '25-DEC-14';

Please assist.

Jonathan Lewis's picture

Understanding SQL

From time to time someone publishes a query on the OTN database forum and asks how to make it go faster, and you look at it and think it’s a nice example to explain a couple of principles because it’s short, easy to understand, obvious what sort of things might be wrong, and easy to fix. Then, after you’ve made a couple of suggestions and explained a couple of ideas the provider simply fades into the distance and doesn’t tell you any more about the query, or whether they’ve taken advantage of your advice, or found some other way to address the problem.

Such a query, with its execution plan, appeared a couple of weeks ago:

Jonathan Lewis's picture

Migrated rows

I received an email recently describing a problem with a query which was running a full tablescan but: “almost all the waits are on ‘db file sequential read’ and the disk read is 10 times the table blocks”.  Some further information supplied was that the tablespace was using ASSM and 16KB block size; the table had 272 columns (ouch!) and the Oracle version was 11.2.0.4.

Jonathan Lewis's picture

Not Exists

This whole thing about “not exists” subqueries can run and run. In the previous episode I walked through some ideas of how the following query might perform depending on the data, the indexes, and the transformation that the optimizer might apply:

Jonathan Lewis's picture

Not Exists

Another question on a seemingly simple “not exists” query has appeared on OTN just a few days after my last post about the construct. There are two little differences between the actual form of the two queries that make it worth repeating the analysis.

The first query was of the form:


select from big_table
where  not exists (select exact_matching_row from small table);

while the new query is of the form:

Jonathan Lewis's picture

ANSI expansion

Here’s a quirky little bug that appeared on the OTN database forum in the last 24 hours which (in 12c, at least) produces an issue which I can best demonstrate with the following cut-n-paste:

Jonathan Lewis's picture

ASH

There was a little conversation on Oracle-L about ASH (active session history) recently which I thought worth highlighting – partly because it raised a detail that I had got wrong until Tim Gorman corrected me a few years ago.

Once every second the dynamic performance view v$active_session_history copies information about active sessions from v$session. (There are a couple of exceptions to the this rule – for example if a session has called dbms_lock.sleep() it will appear in v$session as state = ‘ACTIVE’, but it will not be recorded in v$active_session_history.) Each of these snapshots is referred to as a “sample” and may hold zero, one, or many rows.

Jonathan Lewis's picture

Flashback Logging

One of the waits that is specific to ASSM (automatic segment space management) is the “enq: FB – contention” wait. You find that the “FB” enqueue has the following description and wait information when you query v$lock_type, and v$event_name:

randolf.geist's picture

New Version Of XPLAN_ASH Utility - In-Memory Support

A new version 4.21 of the XPLAN_ASH utility is available for download. I publish this version because it will be used in the recent video tutorials explaining the Active Session History functionality of the script.

As usual the latest version can be downloaded here.

This is mainly a maintenance release that fixes some incompatibilities of the 4.2 version with less recent versions (10.2 and 11.2.0.1).

As an extra however, this version now differentiates between general CPU usage and in-memory CPU usage (similar to 12.1.0.2 Real-Time SQL Monitoring). This is not done in all possible sections of the output yet, but the most important ones are already covered.

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