Troubleshooting

Jonathan Lewis's picture

Deferrable RI – 2

A question came up on Oracle-L recently about possible locking anomalies with deferrable referential integrity constraints.

randolf.geist's picture

New Version Of XPLAN_ASH Utility

A new version 4.1 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

This version in particular supports now the new 12c "Adaptive" plan feature - previous versions don't cope very well with those if you don't add the "ADAPTIVE" formatting option manually.

Here are the notes from the change log:

- GV$SQL_MONITOR and GV$SQL_PLAN_MONITOR can now be customized in the
settings as table names in case you want to use your own custom monitoring repository that copies data from GV$SQL_MONITOR and GV$SQL_PLAN_MONITOR in order to keep/persist monitoring data. The tables need to have at least those columns that are used by XPLAN_ASH from the original views

Jonathan Lewis's picture

10053 trace

I published a note yesterday about enabling SQL trace system-wide for a single statement – and got a response on twitter from Bertrand Drouvot referencing a blog post he’d done a few months ago about using a similar method to dump the optimizer trace (10053) for a statement whenever it was optimized.

Jonathan Lewis's picture

10053 trace

I published a note yesterday about enabling SQL trace system-wide for a single statement – and got a response on twitter from Bertrand Drouvot referencing a blog post he’d done a few months ago about using a similar method to dump the optimizer trace (10053) for a statement whenever it was optimized.

Jonathan Lewis's picture

sql_trace

Here’s a convenient aid to trouble-shooting that appeared in 11g with its enhancements to setting events. It’s a feature that may help you to work out (among other things) why a given statement seems to have a highly variable performance profile. If you can find the SQL_ID for a parent cursor you can enable tracing for just that cursor whenever it executes, whoever executes it.

Jonathan Lewis's picture

Feature Bypass

Here’s a little tip that might be helpful occasionally when you’re trying to work out why the optimizer transformation you were expecting isn’t appearing

If you’ve ever checked the 10053 trace (and who wants to do that for a complex query) you may have noticed lines like:

SU: SU bypassed: Remote table referenced.

So now you know that SU – Subquery Unnesting – has limitations in distributed queries.

When I first saw a line like this, it crossed my mind that it would be useful to keep a reference list of features that could be reported as bypassed, which I do through a simple unix line:

strings -a oracle | grep -i bypassed > bypassed.txt

If you need a reference for the various short codes for transformations you can find it near the top of the 10053 trace, looking like this:

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”.

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