Troubleshooting

Jonathan Lewis's picture

NVL() change

One of the problems of functions is that the optimizer generally doesn’t have any idea on how a predicate based on function(col) might affect the cardinality. However,  the optimizer group are constantly refining the algorithms to cover an increasing number of special cases more accurately. This is a good thing, of course – but it does mean that you might be unlucky on an upgrade where a better cardinality estimate leads to a less efficient execution plan. Consider for example the simple query (where d1 is column of type date):

select	*
from	t1
where	nvl(d1,to_date('01-01-1900','dd-mm-yyyy')) < sysdate

Now, there are many cases in many versions of Oracle, where the optimizer will appear to calculate the cardinality of

nvl(columnX,{constant}) operator {constant}

as if it were:

Jonathan Lewis's picture

NVL() change

One of the problems of functions is that the optimizer generally doesn’t have any idea on how a predicate based on function(col) might affect the cardinality. However,  the optimizer group are constantly refining the algorithms to cover an increasing number of special cases more accurately. This is a good thing, of course – but it does mean that you might be unlucky on an upgrade where a better cardinality estimate leads to a less efficient execution plan. Consider for example the simple query (where d1 is column of type date):

select	*
from	t1
where	nvl(d1,to_date('01-01-1900','dd-mm-yyyy')) < sysdate

Now, there are many cases in many versions of Oracle, where the optimizer will appear to calculate the cardinality of

nvl(columnX,{constant}) operator {constant}

as if it were:

randolf.geist's picture

New Version Of XPLAN_ASH Utility

A minor update 4.01 to the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

These are the notes from the change log:

- More info for RAC Cross Instance Parallel Execution: Many sections now show a GLOBAL aggregate info in addition to instance-specific data

- The Parallel Execution Server Set detection and ASSUMED_DEGREE info now makes use of the undocumented PX_STEP_ID and PX_STEPS_ARG info (bit mask part of the PX_FLAGS column) on 11.2.0.2+

- Since version 4.0 added from 11.2.0.2 on the PX *MAX* DOP in the "SQL statement execution ASH Summary" based on the new PX_FLAGS column of ASH it makes sense to add a PX *MIN* DOP in the summary to see at one glance if different DOPs were used or not

Jonathan Lewis's picture

Juggernaut

One of the problems of “knowing” so much about Oracle is that the more you know the more you have to check on each new release of the software. An incoming ping on my posting “Lock Horror” reminded me that I was writing about 11.2.0.1, and the terminal release is 11.2.0.4, and the whole thing may have changed in 12.1.0.1 – so I ought to re-run some tests to make sure that the articel is up to date if it’s likely to be read a few times in the next few days.

Unfortunately, although I often add a URL to scripts I’ve used to confirm results published in the blog, I don’t usually include a script name in my blog postings  to remind me where to go if I want to re-run the tests. So how do I find the right script(s) ? Typically I list all the likely scripts and compare dates with the date on the blog; so here’s what I got for “lock”.

Jonathan Lewis's picture

Juggernaut

One of the problems of “knowing” so much about Oracle is that the more you know the more you have to check on each new release of the software. An incoming ping on my posting “Lock Horror” reminded me that I was writing about 11.2.0.1, and the terminal release is 11.2.0.4, and the whole thing may have changed in 12.1.0.1 – so I ought to re-run some tests to make sure that the articel is up to date if it’s likely to be read a few times in the next few days.

Unfortunately, although I often add a URL to scripts I’ve used to confirm results published in the blog, I don’t usually include a script name in my blog postings  to remind me where to go if I want to re-run the tests. So how do I find the right script(s) ? Typically I list all the likely scripts and compare dates with the date on the blog; so here’s what I got for “lock”.

Jonathan Lewis's picture

Diagnostics

Here’s a little test you might want to try. Examine the following script, and decide what sort of symptoms you would see in the AWR report.


create global temporary table gtt1(n1 number);

execute dbms_workload_repository.create_snapshot;

insert into gtt1 values(1);
truncate table gtt1;

-- repeat insert/truncate for a total of 100 cycles

execute dbms_workload_repository.create_snapshot;

-- generate an AWR report across the interval.

I don’t need anyone to tell me their results – but if your predictions and the actual results match then you can give yourself a pat on the head.
You might also like to enable SQL trace for all the inserts/truncate to see if that shows you anything interesting.

This is one of the simpler scripts of the 3,500 I have on my laptop that help me interpret the symptoms I see in client systems.

Jonathan Lewis's picture

Diagnostics

Here’s a little test you might want to try. Examine the following script, and decide what sort of symptoms you would see in the AWR report.


create global temporary table gtt1(n1 number);

execute dbms_workload_repository.create_snapshot;

insert into gtt1 values(1);
truncate table gtt1;

-- repeat insert/truncate for a total of 100 cycles

execute dbms_workload_repository.create_snapshot;

-- generate an AWR report across the interval.

I don’t need anyone to tell me their results – but if your predictions and the actual results match then you can give yourself a pat on the head.
You might also like to enable SQL trace for all the inserts/truncate to see if that shows you anything interesting.

This is one of the simpler scripts of the 3,500 I have on my laptop that help me interpret the symptoms I see in client systems.

Jonathan Lewis's picture

Flashback Fail ?

Sitting in an airport, waiting for a plane, I decided to read a note (pdf) about Flashback data archive written by Beat Ramseier from Trivadis.  I’d got about three quarters of the way through it when I paused for thought and figured out that on the typical database implementation something nasty is going to happen after approximately 3 years and 9 months.  Can you guess why ?

Jonathan Lewis's picture

Row Migration

At one of the presentations I attended at RMOUG this year the presenter claimed that if a row kept increasing in size and had to migrate from block to block as a consequence then each migration of that row would leave a pointer in the previous block so that an indexed access to the row would start at the original table block and have to follow an ever growing chain of pointers to reach the data.

This is not correct, and it’s worth making a little fuss about the error since it’s the sort of thing that can easily become an urban legend that results in people rebuilding tables “for performance” when they don’t need to.

Jonathan Lewis's picture

RAC Plans

Recently appeared on Mos – “Bug 18219084 : DIFFERENT EXECUTION PLAN ACROSS RAC INSTANCES”

Now, I’m not going to claim that the following applies to this particular case – but it’s perfectly reasonable to expect to see different plans for the same query on RAC, and it’s perfectly possible for the two different plans to have amazingly different performance characteristics; and in this particular case I can see an obvious reason why the two nodes could have different plans.

Here’s the query reported in the bug:

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