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Parallel Execution Analysis Using ASH - The XPLAN_ASH Tool


Note: This blog post actually serves three purposes:

  1. It introduces and describes my latest contribution to the Oracle Community,  the "XPLAN_ASH" tool

  • It accompanies a future OTN article on Parallel Execution that will be published some time in the future

  • It is supposed to act as a teaser for my upcoming "Parallel Execution Masterclass" that will be organized by Oracle University and can be booked later this year
  • Table Of Contents


    Real-Time SQL Monitoring Overview

    Real-Time SQL Monitoring Shortcomings

    Adaptive STAT lines in SQL trace

    Lately I’ve been using SQL runtime execution statistics combined with SQL monitor for performance diagnostics and, honestly, almost forgot about SQL trace. So this note is not very useful to me but it might be to someone: along with ALL_EXECUTIONS option appeared in (I believe) 11gR2, there’s a new option starting with which can significantly decrease amount of data in the trace files for STAT lines compared to ALL_EXECUTIONS, still providing some of them from time to time.

    Here is a case to demonstrate:

    Report Generators And Query Transformations

    Usually the Cost-Based Optimizer arrives at a reasonable execution plan if it gets the estimates regarding cardinality and data scattering / clustering right (if you want to learn more about that why not watch my Webinar available at ""?).

    Here is an example I've recently come across where this wasn't case - the optimizer obviously preferred plans with a significantly higher cost.

    The setup to reproduce the issue is simple:

    Column Groups - Edge Cases

    Oracle 11g added Extended Statistics support for column groups in order to detect correlated columns for filter predicates using an equal comparison.

    Note that Oracle 11g also added the ability to use the number of distinct keys of a composite index as an upper limit for the cardinality estimates for matching column predicates, which means that the optimizer is now capable of detecting correlated columns without the explicit addition of Extended Statistics / Column Groups.

    Coalesce Subquery Transformation - COALESCE_SQ

    Oracle 11.2 introduced a set of new Query Transformations, among others the ability to coalesce subqueries which means that multiple correlated subqueries can be merged into a number of less subqueries.

    Timur Akhmadeev already demonstrated the basic principles in a blog entry, but when I was recently involved into supporting a TPC-H benchmark for a particular storage vendor I saw a quite impressive application of this optimization that I would like to share here.

    Join Views, ROWIDs And Query Transformations

    Here is an odd little bug that was discussed a couple of weeks ago on the OTN forums.

    It's about queries on join views by ROWID that fail with "ORA-01410: invalid ROWID" under certain circumstances. The bug can only be reproduced when using the 11.2 code base. In fact the same setup will cause an internal error in 11.1, but 10.2 will return correct results.

    It's probably not a very common scenario but it is an interesting example of how features that work fine by themselves can cause problems when used together.

    First of all (hopefully) some of you may ask: How is it possible to query from a join view by ROWID, since the view is based on multiple objects and hence doesn't have a simple one-to-one mapping to a ROWID of a single table?

    Dynamic Sampling And Indexes

    There is a lot more to say about Dynamic Sampling and indexes, and I'll try to cover these basics in my Dynamic Sampling series on, but two recent discussions on the OTN forums and on Charles Hooper's blog prompted me to publish this blog post.

    These discussions revolved around the following issues with Dynamic Sampling and indexes:

    1. CREATE INDEX On Empty Table

    Nice Additions For Troubleshooting

    This is just a short note that Oracle has added several nice details to and respectively that can be helpful for troubleshooting.

    ASH, PGA Memory And TEMP Consumption

    Since the V$ACTIVE_SESSION_HISTORY view (that requires Enterprise Edition plus Diagnostic License) contains the PGA_ALLOCATED and TEMP_SPACE_ALLOCATED columns.

    In particular the latter closes an instrumentation gap that always bothered me in the past: So far it wasn't easy to answer the question which session used to allocate TEMP space in the past. Of course it is easy to answer while the TEMP allocation was still held by a session by looking at the corresponding V$ views like V$SORT_USAGE, but once the allocation was released answering questions like why was my TEMP space exhausted three hours ago was something that couldn't be told by looking at the information provided by Oracle.

    Incremental Partition Statistics Review


    Here is a summary of the findings while evaluating Incremental Partition Statistics that have been introduced in Oracle 11g.

    The most important point to understand is that Incremental Partition Statistics are not "cost-free", so anyone who is telling you that you can gather statistics on the lowest level (partition or sub-partition in case of composite partitioning) without any noticeable overhead in comparison to non-incremental statistics (on the lowest level) is not telling you the truth.

    Although this might be obvious I've already personally heard someone making such claims so it's probably worth to mention.

    In principle you need to test on your individual system whether the overhead that is added to each statistics update on the lowest level outweighs the overhead of actually gathering statistics on higher levels, of course in particular on global level.

    Beware of ACFS when upgrading to

    This post is about a potential pitfall when migrating from 11.2.0.x to the next point release. I stumbled over problem this one on a two node cluster.

    The operating system is Oracle Linux 5.5 running and I wanted to go to As you know, Grid Infrastructure upgrades are out-of-place, in other words require a separate Oracle home. This is also one of the reasons I wouldn’t want less than 20G on a non-lab like environment for the Grid Infrastructure mount points …

    Now when you are upgrading from 11.2.0.x to you need to apply a one-off patch, but the correct one! Search for patch number 12539000 (11203:ASM UPGRADE FAILED ON FIRST NODE WITH ORA-03113) and apply the one that matches your version-and pay attention to these PSUs! There is the obvious required opatch update to be performed before again as well.