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11gR2

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ITL Deadlocks (script)

A reader of this blog, VijayS, asked me to share the script I use to demo ITL deadlocks that I mentioned in this comment. Since other readers might be interested, here is the script.

ASM AU Size And LMT AUTOALLOCATE

When using Locally Managed Tablespaces (LMT) with variable, system managed extent sizes (AUTOALLOCATE) and data files residing in ASM the Allocation Unit (AU) size can make a significant difference to the algorithm that searches for free extents.The corresponding free extent search algorithm when searching for free extents >= the AU size seems to only search for free extents on AU boundaries in order to avoid I/O splitting.Furthermore the algorithm seems to use two extent sizes when searching for free extents: A "desired" (for example 8MB) and a "minimum acceptable" (for example 1MB) extent size - however when performing the search the "desired" size seems to be relevant when limiting the search to free extents on AU boundaries.This can lead to some surprising side effects, in particular when using 4MB AUs.It effectively means that although you might have plenty o

Oracle Forms and Reports Services 11gR2 on Oracle Linux 6…

Last year I wrote an article about the installation of Oracle Forms and Reports Services 11gR2 on Oracle Linux 5. I’ve now written the article for Oracle Forms and Reports Services 11gR2 on Oracle Linux 6. The latest patch of F&RS is certified for OL6, along with JDK6 and JDK7.

In addition to the installation articles, I’ve compiled a collection of random notes about post-installation configuration into a separate article. I keep adding to it every time I come across a new (for me) issue.

"Cost-free" joins - 2

In the previous post I've demonstrated an unexpected Nested Loop Join caused by an extreme data distribution. Although unexpected at first sight, the performance of the execution plan selected by the optimizer is decent - provided the estimates are in the right ballpark.Here is another case of an unexpected execution plan, this time about Merge Joins.

Merge Joins

In order to appreciate why the execution plan encountered is unexpected, first a quick summary about how Merge Joins work:A Merge Join is essentially a Nested Loop operation from one sorted row source into another sorted row source.

"Cost-free" joins - 1

Recently I came across some interesting edge cases regarding the costing of joins. They all have in common that they result in (at first sight) unexpected execution plans, but only some of them are actual threats to performance.

Outer Joins

The first one is about outer joins with an extreme data distribution. Consider the following data setup:


create table t1
as
select
rownum as id
, rpad('x', 100) as filler
, case when rownum > 1e6 then rownum end as null_fk
from
dual
connect by
level <= 1e6
;

exec dbms_stats.gather_table_stats(null, 't1')

create table t2
as
select
rownum as id
, rpad('x', 100) as filler
from
dual
connect by
level <= 1e6
;

QB_NAME hint query block name length limitation

Oracle 10g introduced the QB_NAME hint that can come handy in case hints need to be applied to more complex statements, in particular when possibly multiple layers of views / subqueries are involved.Jonathan Lewis has a older blog post that describes more details.Just in case you wonder why sometimes apparently the QB_NAME hint - along with all other hints that refer to the assigned query block name - seems to be ignored: One possible reason is that it looks like there is an undocumented length limitation of the query block names that can be assigned - 20 characters seem to be the maximum possible (I haven't checked the effect of multi-byte database character sets).Consider this simple example:


drop table t1;

purge table t1;

Exadata Smart Scan Projection Limitation

Here is an interesting limitation to Exadata Smart Scans - if more than 254 columns from a table (not HCC compressed, more on that in moment) need to be projected, Smart Scans for that particular segment will be disabled and Exadata will fall back to conventional I/O.

How dNFS database clone works – part 1

There is new feature in Oracle 11.2.0.2 called dNFS clone. It has been described by Kevin Closson on his blog post - Oracle Database 11g Direct NFS Clonedb Feature  and very good configuration description has been posted by Tim Hall on his blog - Direct NFS (DNFS) Clonedb in Oracle Database 11g Release 2 (Patchset 11.2.0.2). I have played with it just after I found both blogs but never think how Oracle implemented that feature. I came back to it when my colleague asked me if I ever use that in production environment and what are performance implications of using it. 

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How dNFS database clone works – part 1

There is new feature in Oracle 11.2.0.2 called dNFS clone. It has been described by Kevin Closson on his blog post - Oracle Database 11g Direct NFS Clonedb Feature  and very good configuration description has been posted by Tim Hall on his blog - Direct NFS (DNFS) Clonedb in Oracle Database 11g Release 2 (Patchset 11.2.0.2). I have played with it just after I found both blogs but never think how Oracle implemented that feature. I came back to it when my colleague asked me if I ever use that in production environment and what are performance implications of using it.