Troubleshooting

Jonathan Lewis's picture

Quantum Space

Here’s a not very serious note that makes a serious point.  I’ve got a small tablespace made up of 4 files, and here’s a little report I can run against the data dictionary for that tablespace:


select 'File space' What, nvl(sum(user_bytes)/1048576,0) MB from dba_data_files where tablespace_name = 'LOB_TEST'
union all
select 'Free space',      nvl(sum(bytes/1048576),0)         from dba_free_space where tablespace_name = 'LOB_TEST'
union all
select 'Extents',         nvl(sum(bytes/1048576),0)         from dba_extents    where tablespace_name = 'LOB_TEST'
union all
select 'Segments',        nvl(sum(bytes/1048576),0)         from dba_segments   where tablespace_name = 'LOB_TEST'
;

The name of the tablespace isn’t significant – it happens to be a tablespace I created to do some tests relating to space allocation with securefile LOBs, and it’s been hanging around ever since.

Jonathan Lewis's picture

Aliases

Here’s a performance problem that came up on OTN recently. The following query (reformatted) takes “ages” to run – how do you address the problem:

SELECT
	COUNT(*) 
FROM
	smp_dbuser2.workflow_step_report
WHERE
	report_ID IN (
		SELECT	report_id
		FROM	smp_dbuser2.workflow_report
		WHERE	trunc(start_time) = '28-Apr-2017'
		AND	user_id = 'nbi_ssc'
	)
;


Various pieces of relevant information were supplied (the workflow_report table holds 1.4M rows the workflow_step_report table holds 740M rows and some indexes were described), but most significantly we were given the execution plan:

Jonathan Lewis's picture

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 ?

Jonathan Lewis's picture

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.

Jonathan Lewis's picture

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.

Jonathan Lewis's picture

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.

Jonathan Lewis's picture

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:

Jonathan Lewis's picture

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 ?

Jonathan Lewis's picture

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).

Jonathan Lewis's picture

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:

To prevent automated spam submissions leave this field empty.
Syndicate content