There are at least three different ways how the Oracle optimizer can come up with a so called TEMP table transformation, that is materializing an intermediate result set:- As part of a star transformation the repeated access to dimensions can be materialized- As part of evaluating GROUPING SETs intermediate result sets can be materialized- Common Subquery/Table Expressions (CTE, WITH clause)Probably the most common usage of the materialization is in conjunction with the WITH clause.This is nothing new but since I came across this issue several times recently, here's a short demonstration and a reminder that this so called "TEMP Table Transformation" - at least in the context of the WITH clause - isn't really cost-based, in contrast to most other optimizer transformations nowadays - although the unnest transformation of subqueries also has a "no-brainer" variant where costing isn't considered.The logic simp
A new version 4.1 of the XPLAN_ASH utility is available for download.
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
A minor update 4.01 to the XPLAN_ASH utility is available for download.
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 126.96.36.199+
- Since version 4.0 added from 188.8.131.52 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
A new version of the XPLAN_ASH tool (detailed analysis of a single SQL statement execution) is available for download. The previous post includes links to video tutorials explaining what the tool is about.
The new version comes with numerous improvements and new features. The most important ones are:
When the optimizer has to estimate the data volume (the BYTES column in the plan output), it usually bases this information on the column statistics, if applicable and available (think of complex expressions).However, whenever there is a VIEW operator in an execution plan, that represents an unmerged view, the optimizer obviously "loses" this information and starts applying defaults that are based on the column definition.Depending on the actual content of the columns this can lead to dramatic differences in data volume estimates.Both, under- and overestimates are possible, because for character based columns these defaults seem to be based on an assumed 50% fill grade, so a VARCHAR2(100 BYTE) column counts as 50 bytes data volume.For multi-byte character sets the same rule applies based on the maximum width of a column using the "char" semantics, so a VARCHAR2(1000 CHAR) column counts as 2000 byte
The values provided by the “parse count (total)” and “session cursor cache hits” statistics are subject to several bugs. And, what’s worse, for years Oracle didn’t care to fix it. This is my impression, at least.
Then, when few weeks ago I read in the Oracle Support note 13837105.8 (Bug 13837105 – statistics “parse count (total)” and “session cursor cache hits” miscounted) that the bug introduced in 184.108.40.206 was fixed, I hoped that others bugs in this area were fixed as well.
Unfortunately, it’s not the case. What a disappointment!
The TIMESTAMP WITH TIME ZONE data type that got introduced a long time ago is known for some oddities, for example Tony Hasler has a nice summary of some of them here.Here is another oddity that shows up when trying to aggregate on such a data type. Have a look at the following simple example:
create table t
rownum as id
, date '2000-01-01' + rownum - 1 as some_date
, cast(date '2000-01-01' + rownum - 1 as timestamp) as some_timestamp
, cast(date '2000-01-01' + rownum - 1 as timestamp with local time zone) as some_timestamp_with_local_tz
, cast(date '2000-01-01' + rownum - 1 as timestamp with time zone) as some_timestamp_with_timezone
A new major release (version 3.0) of my XPLAN_ASH tool is available for download.
In addition to many changes to the way the information is presented and many other smaller changes to functionality there is one major new feature: XPLAN_ASH now also supports S-ASH, the free ASH implementation.
If you run XPLAN_ASH in a S-ASH repository owner schema, it will automatically detect that and adjust accordingly.
XPLAN_ASH was tested against the latest stable version of S-ASH (2.3). There are some minor changes required to that S-ASH release in order to function properly with XPLAN_ASH. Most of them will be included in the next S-ASH release as they really are only minor and don't influence the general S-ASH functionality at all.
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.
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.