A new version 4.21 of the XPLAN_ASH utility is available for download. I publish this version because it will be used in the recent video tutorials explaining the Active Session History functionality of the script.
This is mainly a maintenance release that fixes some incompatibilities of the 4.2 version with less recent versions (10.2 and 220.127.116.11).
As an extra however, this version now differentiates between general CPU usage and in-memory CPU usage (similar to 18.104.22.168 Real-Time SQL Monitoring). This is not done in all possible sections of the output yet, but the most important ones are already covered.
I finally got around preparing another part of the XPLAN_ASH video tutorial.
This part is about the main funcationality of XPLAN_ASH: SQL statement execution analysis using Active Session History and Real-Time SQL Monitoring.
In this video tutorial I'll explain what the output of XPLAN_ASH is supposed to mean when using the Active Session History functionality of the script. Before diving into the details of the script output using sample reports I provide some overview and introduction in this part that hopefully makes it simpler to understand how the output is organized and what it is supposed to mean.
This is the initial, general introduction part. More parts to follow.
When using Parallel Execution, depending on the plan shape and the operations used, Oracle sometimes needs to turn non-blocking operations into blocking operations, which means in this case that the row source no longer passes its output data directly to the parent operation but buffers some data temporarily in PGA memory / TEMP. This is either accomplished via the special HASH JOIN BUFFERED operation, or simply by adding BUFFER SORT operations to the plan.The reason for such a behaviour in parallel plans is the limitation of Oracle Parallel Execution that allows only a single data redistribution to be active concurrently.
A new version 4.2 of the XPLAN_ASH utility is available for download.
There were no too significant changes in this release, mainly some new sections related to I/O figures were added.
One thing to note is that some of the sections in recent releases may require a linesize larger than 700, so the script's settings have been changed to 800. If you use corresponding settings for CMD.EXE under Windows for example you might have to adjust accordingly to prevent ugly line wrapping.
Here are the notes from the change log:
- New sections "Concurrent activity I/O Summary based on ASH" and "Concurrent activity I/O Summary per Instance based on ASH" to see the I/O activity summary for concurrent activity
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 22.214.171.124+
- Since version 4.0 added from 126.96.36.199 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 188.8.131.52 was fixed, I hoped that others bugs in this area were fixed as well.
Unfortunately, it’s not the case. What a disappointment!