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dbakevlar's picture

Power BI 101 – Log Files and Tracing

Knowing where log files are and how to turn on debugging is an essential part of any technical job and this goes for Power BI, too.  Remember, as I learn, so does everyone else….Come on, pretty please?

Power BI Desktop

Log files and traces can be accessed one of two ways-

  • Via the Power BI Application
  • Via File Explorer

In the Power BI application, go to File –> Options and Settings –> Options –> Diagnostics.

Crash and dump files are automatically stored with an option to disable them from this screen, but unsure why you’d ever want to do this.  If Power BI does crash, you would lose any valuable data on what the cause was.

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To debug with a trace, you’ll need to enable it from this screen as well, as it’s not turned on by default.  Remember that tracing can be both resource and storage intensive, so only enable it when you actually need to diagnose something.  You can also choose to bypass tracing the geo code cache, as this is used to help map coordinates and it can be very chatty.

To view files, you can click on the open crash/dump file folder and this will open up a File Explorer to the traces directory on your pc.

Directly From File Explorer:

Ensure that File Explorer has viewing set to display hidden items.

C:\Users\\AppData\Local\Microsoft\Power BI Desktop\Traces

Log Files

These are all retained inside the Performance folder under the Traces directory

The file’s will be named with the following naming convention:


Locating the files that you need for your current process is easiest if you sort by Date Modified.  Verify that you’re working with the file that is being written to and not the file used to keep track of startup and shutdown log tracking, (0 KB):

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The third log file in the list above, and also the one started before the executable for Power BI Desktop, (PID 13396) is the Microsoft Mashup Container, (Microsoft.Mashup.Container.NetFX40.exe) with its own PID of 16692.  It’s contains valuable information about calculations,  measures and other caching processes.  Take care to ensure the PID of the one used by Power BI in the logs matches the one you’re inspecting in the Task Manager-  Excel and other programs are also known to have a version of this executable, so there may be more than one listed for Power BI, as well as others for different Microsoft applications.

Log File Breakdown

Each file will contain entries providing information on high level processing, including start time, total size of cache allocated for the task, process information, Process ID, (PID), Transaction ID, (TID) and duration.

An example of an entry can be seen below:

ObjectCacheSessions/CacheStats/Size {"Start":"2018-07-19T01:42:24.9707127Z","Action":"ObjectCacheSessions/CacheStats/Size","entryCount":"1","totalSize":"24","ProductVersion":"2.59.5135.781 (PBIDesktop)","ActivityId":"00000000-0000-0000-0000-000000000000","Process":"PBIDesktop","Pid":13396,"Tid":8,"Duration":"00:00:00.0046865"}

We can easily match up the Process name and the PID with what is displayed in our Task Manager detail view:

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We’ve now identified the process, the amount of memory allocated to perform a task captured in the log, start time and the duration.  The information in these log files can assist when diagnosing if Power BI desktop crashes, but the data collected is quite rudimentary.

If you shut down Power BI Desktop, the PBIDesktop* log file writes to the startup file, which was once empty and it then empties and saves off the timestamp of the exit of the program.

The Microsoft Mashup file has much of the same information, but includes deeper level processing work by Power BI, such as work done in the Query Editor or when we create a measure or new column/table.

In the three examples from the file below, you can see a compile, a save and then an evaluate task.  Note that the Host Process ID is my Power BI Desktop we’ve seen earlier, but the interaction with the Microsoft Mashup Container is demonstrated as well:

SimpleDocumentEvaluator/GetResult/Compile {"Start":"2018-07-19T01:48:46.1441843Z","Action":"SimpleDocumentEvaluator/GetResult/Compile","HostProcessId":"13396","ProductVersion":"2.59.5135.781 (PBIDesktop)","ActivityId":"04248470-07e1-4862-b184-a32f186f26fd","Process":"Microsoft.Mashup.Container.NetFX40","Pid":16692,"Tid":1,"Duration":"00:00:00.4302569"}

ObjectCache/CacheStats/Size {"Start":"2018-07-19T01:48:47.3504705Z","Action":"ObjectCache/CacheStats/Size","HostProcessId":"13396","entryCount":"5","totalSize":"14564","ProductVersion":"2.59.5135.781 (PBIDesktop)","ActivityId":"04248470-07e1-4862-b184-a32f186f26fd","Process":"Microsoft.Mashup.Container.NetFX40","Pid":16692,"Tid":1,"Duration":"00:00:00.0000170"}

SimpleDocumentEvaluator/GetResult/Evaluate {"Start":"2018-07-19T01:48:46.5744678Z","Action":"SimpleDocumentEvaluator/GetResult/Evaluate","HostProcessId":"13396","ProductVersion":"2.59.5135.781 (PBIDesktop)","ActivityId":"04248470-07e1-4862-b184-a32f186f26fd","Process":"Microsoft.Mashup.Container.NetFX40","Pid":16692,"Tid":1,"Duration":"00:00:00.7780750"}

Another common file in the Performance directory will contain the msmdsrv* naming convention, which collect log information on the data source loader.  Duration information and cache/memory allocation could offer valuable information on poor performance during data loading processes.  First stop is always to check the settings for the desktop to see what has been set for memory allocation vs. assuming it’s the default.

If I just start the program and don’t open anything, only the high level processing of starting, basic memory allocation and stopping will be tracked in the PBIDesktop* file until I open up a PBIX file.  Then anything that needs to be updated and refreshed for the visuals, etc. will begin to write log data to the Microsoft Mashup log file and if a data refresh must be performed, the msmdsrv file.

Trace files

When you do turn on debugging, tracing, as shown in the beginning of this post, a file is created in the parent directory, TRACES.

When enabled and after a restart of the Power BI Desktop, you will receive not only similar information about PID, TID, the process and the duration, but also encounter granule information about Power BI and what’s going on behind the scenes:

  • Application graphics info
  • Settings
  • Parameters
  • Background processes
  • Caching
  • Extensions
  • Query edits
  • Changes applied

You’ll even see entries similar to the following:


A mutex is a small, efficient allocation of memory.  As mutexes have thread affinity, it means the mutex can only be released by the thread in Power BI that owns it.  If it’s released by another thread, an application exception will be thrown in the application and trapped in the trace file.

The interesting aspect of tracing in Power BI Desktop, the options are put back to default, with granule level tracing disabled when you restart the application.














Copyright © DBA Kevlar [Power BI 101 - Log Files and Tracing], All Right Reserved. 2018.

randolf.geist's picture

Speaking At DOAG 2018 Conference And IT Tage 2018

#333333; font-family: Verdana, Arial, sans-serif; font-size: 13px;">I will be speaking at the yearly DOAG#333333; font-family: Verdana, Arial, sans-serif;"> conference in December as well as at the IT Tage in November. My talk will be "Oracle Optimizer System Statistics Update 2018" where I summarize the history and current state of affairs regarding System Statistics and I/O calibration in recent Oracle versions like 12c and 18c.

Uwe Hesse's picture

Easy way to create large demo-tables in #Exasol and #Oracle

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If you need a large set of data to test or demonstrate something, this does the trick:

Create demo table in Oracle

create table t as
rownum as id, -- growing unique number
'Just some text' as textcol,
mod(rownum,5) as numcol1, -- numbers 0-4 round-robin
mod(rownum,1000) as numcol2 , -- numbers 0-999 round robin
5000 as numcol3, -- constant
to_date ('01.' || lpad(to_char(mod(rownum,12)+1),2,'0') || '.2018' ,'dd.mm.yyyy') as time_id -- dates for every month 
from dual connect by level<=1e4 -- 10000 rows 

Create demo table in Exasol

create table t as
rownum as id, -- growing unique number
'Just some text' as textcol,
mod(rownum,5) as numcol1, -- numbers 0-4 round-robin
mod(rownum,1000) as numcol2 , -- numbers 0-999 round robin
5000 as numcol3, -- constant
to_date ('01.' || lpad(to_char(mod(rownum,12)+1),2,'0') || '.2018' ,'dd.mm.yyyy') as time_id -- dates for every month 
from (select 1 from dual connect by level<=1e4) -- 10000 rows 

In Oracle you may get this error message for a high number of rows (like 10 Mio): ORA-30009: Not enough memory for CONNECT BY operation.
This way it works in spite of it:

SQL> alter session set workarea_size_policy=manual;

Session altered.

SQL> alter session set sort_area_size=1000000000;

Session altered.

SQL> create table t as
     rownum as id, 
     'Just some text' as textcol,
     mod(rownum,5) as numcol1, 
     mod(rownum,1000) as numcol2 , 
     5000 as numcol3, 
     to_date ('01.' || lpad(to_char(mod(rownum,12)+1),2,'0') || '.2018' ,'dd.mm.yyyy') as time_id  
     from dual connect by level<=1e7 ; 
Table created.
SQL> select count(*) from t;


SQL> select bytes/1024/1024 as mb from user_segments where segment_name='T';


In Exasol, this problem doesn’t surface:

SQL_EXA> create table t as
         rownum as id,
         'Just some text' as textcol,
         mod(rownum,5) as numcol1,
         mod(rownum,1000) as numcol2 ,
         5000 as numcol3,
         to_date ('01.' || lpad(to_char(mod(rownum,12)+1),2,'0') || '.2018' ,'dd.mm.yyyy') as time_id
         from (select 1 from dual connect by level<=1e7) ; 
EXA: create table t as... 
Rows affected: 10000000 
SQL_EXA> SELECT raw_object_size/1024/1024 as mb FROM SYS.EXA_ALL_OBJECT_SIZES where object_name='T';
EXA: SELECT raw_object_size/1024/1024 as mb FROM SYS.EXA_ALL_OBJECT_SIZES w...


1 row in resultset.

And the resulting table is smaller because Exasol stores it in columnar compressed format </p />
    <div class=»

Richard Foote's picture

Announcement: Venue Confirmed For Upcoming Brussels “Oracle Indexing Internals and Best Practices” Seminar

I can finally confirm the venue for my upcoming “Oracle Indexing Internals and Best Practices” seminar in beautiful Brussels, Belgium running on 27-28 September 2018. The venue will be the Regus Brussels City Centre Training Rooms Facility, Avenue Louise / Louizalaan 65, Stephanie Square, 1050, Brussels. Note: This will be the last public seminar I’ll run […]

connor_mc_d's picture

Searching in Oracle Database documentation

Just a quick heads up with something I see from time to time in Chrome (but not in Firefox or any other browser).

Occasionally when doing a search, the results are not limited as per my criteria.  For example, if I am searching for information about Spatial in the Licensing Guide:


then when I click the Search button, the results might come back with a far broader search range:


If you experience this, there is an easy workaround – simply re-run the search. It only seems to occur (for me at least) on the first execution of the search.  Clicking the Search button again on the page, yielded the correct result second time around:


Let me know if you’ve seen similar behaviour – if it is not just me, I’ll pass it along to the Documentation team.

Happy searching!

Chris Antognini's picture

Which Privileges Are Required to Use the ADWC Service Console?

The Autonomous Data Warehouse Cloud (ADWC) service provides a Service Console that can be used to monitor the service activity and to carry out a small number of administration tasks (e.g. changing some resource management rules).

The documentation specifically says to use the ADMIN user to login. But, actually, any unlocked database user having the CREATE SESSION privilege can be used to login. This is not only useful, but, in my opinion, necessary. In fact, developers should be able to see the service activity without knowing the password of the ADMIN user.

The documentation provides no information about what privileges are needed to take advantage of the provided functionalities. If you login with a user having only the CREATE SESSION privilege you get either empty charts or errors when trying to use some of the functionality. The following figure illustrates:

The Service Console displays no data when a user without the necessary privileges login.

In other words, the UI is static and privileges are only checked when a specific functionality is used. I would expect something dynamic…

By looking at the roles owned by the ADMIN user, I noticed three roles with a name that could match what I was looking for: CONSOLE_MONITOR, CONSOLE_OPERATOR and CONSOLE_ADMIN. Then, with some trial and error, I came up with the following table that summarizes which role provides which functionality:

Overview No data Data visible Data visible
Activity No data Data visible Data visible
Activity – Cancel execution Unauthorized Authorized Authorized
Administration – Download Client Credentials Unauthorized Authorized Authorized
Administration – Set Resource Management Rules Unauthorized Authorized Authorized
Administration – Set Administrator Password Unauthorized Unauthorized Authorized
Administration – Manage Oracle ML Users Unauthorized Unauthorized Authorized
Administration – Download Oracle Instance Client Authorized Authorized Authorized
Administration – Send Feedback to Oracle Authorized Authorized Authorized


  • A user with the CONSOLE_MONITOR role has the same privileges than a user without any role. To me this state of affairs do not look correct. I would expect that the CONSOLE_MONITOR role makes visible the Overview and Activity data.
  • With these roles it is not possible to provide only the privileges to see data without being able to modify something or kill sessions.
  • The “Cancel execution” functionality is actually a “kill session”. My guess is that as soon as the service is upgrade to 18c then new functionality to cancel executions will be used instead.
Richard Foote's picture

Rebuilding Indexes: Danger With Clustering Factor Calculation (Chilly Down)

Let me start by saying if you don’t already following Jonathan Lewis’s excellent Oracle blog, do yourself a favour. In a recent article, Jonathan highlighted a danger with rebuilding indexes (or indeed creating an index) when used in relation to collecting index statistics with the TABLE_CACHED_BLOCKS preference. I’ve discussed the importance of the TABLE_CACHED_BLOCKS statistics […]

Uwe Hesse's picture

#Exasol Cluster Architecture

This article gives a more detailed view on the Exasol Cluster Architecture. A high level view is provided here.

Exasol Cluster Nodes: Hardware

An Exasol Cluster is built with commodity Intel servers without any particular expensive components. SAS hard drives and Ethernet Cards are sufficient. Especially there is no need for an additional storage layer like a SAN.

See here for a list of Exasol Certified Servers.

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Disk layout

As a best practice the hard drives of Exasol Cluster nodes are configured as RAID 1 pairs. Each cluster node holds four different areas on disk:

1.OS with 50 GB size containing CentOS Linux, EXAClusterOS and the Exasol database executables

2.Swap with 4 GB size

3.Data with 50 GB size containing Logfiles, Coredumps and BucketFS

4.Storage consuming the remaining capacity for the hard drives for the Data Volumes and Archive Volumes

The first three areas can be stored on dedicated disks in which case these disks are also configured in RAID 1 pairs, usually with a smaller size than those that contain the volumes. More common than having dedicated disks is having servers with only one type of disk. These are configured as hardware RAID 1 pairs. On top of that software RAID 0 partitions are being striped across all disks to contain OS, Swap and Data partition.

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Exasol 4+1 Cluster: Software Layers

This popular multi-node cluster serves as example to illustrate the concepts explained. It is called 4+1 cluster because it has 4 Active nodes and 1 Reserve node. Active and Reserve nodes have the same layers of software available. The purpose of the Reserve node is explained here. Upon cluster installation, the License Server copies these layers as tar-balls across the private network to the other nodes. The License Server is the only node in the cluster that boots from disk. Upon cluster startup, it provides the required SW layers to the other cluster nodes.

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Exasol License Essentials

There are three types of licenses available:

Database RAM License: This most commonly used model specifies the total amount of RAM that can be assigned to databases in the cluster.

Raw Data License: Specifies the maximum size of the raw data you can store across databases in the cluster.

Memory Data License: Specifies the maximum size of the compressed data you can store across all databases.

For licenses based on RAM, Exasol checks the RAM assignment at the start of the database. If the RAM in use exceeds the maximum RAM specified by the license, the database will not start.

For licenses based on data size (raw data license and memory data license), a periodic check is done by Exasol on the size of the data. If the size limit exceeds the value specified in the license, the database does not permit any further data insertion until the usage drops below the specified value.

Customers receive their license as a separate file. To activate the license, these license files are uploaded to the License Server using EXAoperation.

EXAStorage volumes

Storage Volumes are created with EXAoperation on specified nodes.

EXAStorage provides two kinds of volumes:

Data volumes:

Each database needs one volume for persistent data and one temporary volume for temporary data.

While the temporary volume is automatically created by a database process, the persistent data volume has to be created by an Exasol Administrator upon database creation.

Archive volumes:

Archive volumes are used to store backup files of an Exasol database.

Exasol 4+1 Cluster: Data & Archive Volume distribution

Data Volumes and Archive Volumes are hosted on  the hard drives of the active nodes of a cluster.

They consume the major capacity of these drives. The license server usually hosts EXAoperation.

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EXAoperation Essentials

EXAoperation is the major management GUI for Exasol Clusters, consisting of an Application Server and a small Configuration Database, both located on the License Server under normal circumstances. EXAoperation can be accessed from all Cluster Nodes via HTTPS. Should the License Server go down, EXAoperation will failover to another node while the availability of the Exasol database is not affected at all.

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Shared-nothing architecture (MPP processing)

Exasol was developed as a parallel system and is constructed according to the shared-nothing principle. Data is distributed across all nodes in a cluster. When responding to queries, all nodes co-operate and special parallel algorithms ensure that most data is processed locally in each individual node’s main memory.

When a query is sent to the system, it is first accepted by the node the client is connected to. The query is then distributed to all nodes. Intelligent algorithms optimize the query, determine the best plan of action and generate needed indexes on the fly. The system then processes the partial results based the local datasets. This processing paradigm is also known as SPMD (single program multiple data). All cluster nodes operate on an equal basis, there is no Master Node. The global query result is delivered back to the user through the original connection.

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Above picture shows a Cluster with 4 data nodes and one reserve node. The license server is the only server that boots from disk. It provides the OS used by the other nodes over the network.

Exasol uses a shared nothing architecture. The data stored in this database is symbolized with A,B,C,D to indicate that each node contains a different part of the database data. The active nodes n11-n14 each host database instances that operate on their part of the database locally in an MPP way. These instances communicate and coordinate over the private network.

Exasol Network Essentials

Each Cluster node needs at least two network connections: One for the Public Network and one for the Private Network. The Public Network is used for client connections. 1 Gb Ethernet is sufficient usually. The Private Network is used for the Cluster Interconnect of the nodes. 10 GB Ethernet or higher is recommended for the Private Network. Optionally, the Private Network can be separated into one Database Network (Database Instances communicate over it) and one Storage Network (Mirrored Segments are synchronized over this network).

Exasol Redundancy Essentials

Redundancy is an attribute that can be set upon EXAStorage Volume creation. It specifies the number of copies of the data that is hosted on Active Cluster nodes. In practice this is either Redundancy 1 or Redundancy 2. Redundancy 1 means there is no redundancy, so if one node fails, the volume with that redundancy is no longer available. Typically that is only seen with one-node Clusters. Redundancy 2 means that each node holds a copy of data that is operated on by a neighbor node, so the volume remains available if one node fails.

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Exasol 4+1 Cluster: Redundancy 2

If volumes are configured with redundancy 2 – which is a best practice – then each node holds a mirror of data that is operated on by a neighbor node. If e.g. n11 modifies A the mirror A‘ on n12 is synchronized over the private network. Should an active node fail, the reserve node will step in starting an instance.

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Franck Pachot's picture

Google Cloud Spanner – no decimal numeric data types

By Franck Pachot

Google Cloud Spanner is a distributed relational database focused on scalability without compromising consistency and integrity. It is available only as a managed service in Google Cloud. Basically, the idea is to keep the scalability advantages of NoSQL database (like Bigtable) but adding transactions, relational tables, SQL, structured data,… as in the relational databases we love for decades.
The commercial pitch includes all the NoSQL buzzwords, with the addition of the legacy properties of SQL databases:
Cloud Spanner is a fully managed, mission-critical, relational database service that offers transactional consistency at global scale, schemas, SQL (ANSI 2011 with extensions), and automatic, synchronous replication for high availability.
Here I’m testing something that is not mentioned, but is taken for granted with all SQL databases: the ability to add numbers without erroneous arithmetic results.

It is easy to test on the Google Cloud (which offers 1 year trials) by creating an instance:

Then create a Spanner database:

And create a table:

The table creation can also use the SQL create table statement. Here I’m testing one of the most important features of SQL databases: the numeric datatypes. This is where humans and computers do not speak the same language: Humans have full hands of 10 fingers, where computers deal only with binary digits. Humans numbers are decimal. Computer numbers are binary.

It seems that Google Spanner is binary only. According to the documentation, the only numeric types are:

  • INT64 for signed integers up to 9E18
  • FLOAT64 for floating point IEEE-754

So, there are no decimal datatypes and decimal values will be approximated by binary values. This is ok to store computer numbers, but not human numbers such as prices, salaries,…

In order to show the problem I’ve created a table with FLOAT64:


The SQL Query interface do not allow for DML other than SELECT:
DML not supported

So we can use the API or this simple from from the ‘data’ tab:

I’ve added 10 rows with ‘0.1’ which is easy to represent in decimal arithmetic, but not in binary arithmetic. Look at the sum:

This is binary arithmetic applied to decimal numbers: approximation. You can select each rows and see ‘0.1’ but when you sum all the 10 rows together, you get less than 1. That’s probably close enough for some ‘BigData’ usage, accountants will not like it.

If you wonder why it takes 100 milliseconds for this 10 rows table, remember that this is a distributed database across 3 continents. Here is the execution plan:

So what?

Do not forget that all the new trends for databases, in the ‘micro-services’ era, are focused at specific use-cases. They do not compete with the ‘old’ relational databases which are general purpose and have integrated, version after version, all the different ways to store and process data shared by multiple applications. Those NoSQL and NewSQL can be considered as an alternative only within the scope of what they are designed for. Spanner was desgined for Google internal use in Google AdWords and then provided as a service for similar use. It was developed to solve a specific problem: the lack of transactions in Bigtable.

Note that the Open Source alternative that is close to Google Spanner is CockroachDB which has a DECIMAL datatype to store fixed-point decimal numbers.


Cet article Google Cloud Spanner – no decimal numeric data types est apparu en premier sur Blog dbi services.

Jonathan Lewis's picture

Direct IOT

A recent (automatic ?) tweet from Connor McDonald highlighted an article he’d written a couple of years ago about an enhancement introduced in 12c that allowed for direct path inserts to index organized tables (IOTs). The article included a demonstration seemed to suggest that direct path loads to IOTs were of no benefit, and ended with the comment (which could be applied to any Oracle feature): “Direct mode insert is a very cool facility, but it doesn’t mean that it’s going to be the best option in every situation.”

Clearly it’s necessary to pose the question – “so when would direct mode insert be a good option for IOTs?” – because if it’s never a good option you have to wonder why it has been implemented. This naturally leads on to thinking about which tests have not yet been done – what aspects of IOTs did Connor not get round to examining in his article. (That’s a standard principle of trouble-shooting, or testing, or investigation: when someone shows you a test case (or when you think you’ve finished testing) one thing you should do before taking the results as gospel is to ask yourself what possible scenarios have not been covered by the test.)

So if you say IOT what are the obvious tests once you’ve got past the initial step of loading the IOT and seeing what happens. First, I think, would be “What if the IOT weren’t empty before the test started”; second would be “IOTs can have overflow segments, what impact might one have?”; third would be “Do secondary indexes have any effects?”; finally “What happens with bitmap indexes and the requirement for a mapping table?” (Then, of course, you can worry about mixing all the different possibilities together – but for the purposes of this note I’m just going to play with two simple examples: non-empty starting tables, and overflow segments.)

Here’s some code to define a suitable table:

create table t2 
with generator as (
                rownum id
        from dual 
        connect by 
                level <= 1e4 -- > comment to avoid WordPress format issue
	3 * rownum			id,
	lpad(rownum,10,'0')		v1,
	lpad('x',50,'x')		padding
        generator       v1,
        generator       v2
        rownum <= 1e5 -- > comment to avoid WordPress format issue
order by

		ownname     => null,
		tabname     => 'T2',
		method_opt  => 'for all columns size 1'

create table t1(
	constraint t1_pk primary key(id)
organization index
-- including v1
-- overflow
select * from t2

		ownname     => null,
		tabname     => 'T1',
		method_opt  => 'for all columns size 1'

I’ve created a heap table t2 that holds 100,000 rows with an id column that is randomly ordered; then I’ve used this ta1ble as a source to create an IOT, with the option to have an overflow segment that contains just the 100 character padding columns. I’ve used 3 * rownum to define the id column for t2 so that when I insert another copy of t2 into t1 I can add 1 (or 2) to the id and interleave the new data with the old data. (That’s another thought about IOT testing – are you loading your data in a pre-existing order that suits the IOTs or is it arriving in a way that’s badly out of order with respect to the IOT ordering; and does your data go in above the current high value, or spread across the whole range, or have a partial overlap with the top end of the range and then run on above it.)

Have created the starting data set, here’s the test:

execute snap_my_stats.start_snap
execute snap_events.start_snap

	/*  append */
into t1
	id + 1, v1, padding

execute snap_events.end_snap
execute snap_my_stats.end_snap

All I’m doing is using a couple of my snapshot packages to check the work done and time spent while insert 100,000 interleaved rows – which are supplied out of order – into the existing table. As shown the “append” is a comment, not a hint, so I’ll be running the test case a total of 4 times: no overflow, with and without the hint – then with the overflow, with and without the hint. (Then, of course, I could run the test without the overflow but an index on v1).

Here are some summary figures from the tests – first from the test without an overflow segment:

                                      Unhinted       With Append
                                  ============      ============
CPU used when call started                 153               102
CPU used by this session                   153               102
DB time                                    166               139

redo entries                           130,603            42,209
redo size                           78,315,064        65,055,376

sorts (rows)                                30           100,031

You’ll notice that with the /*+ append */ hint in place there’s a noticeable reduction in redo entries and CPU time, but this has been achieved at a cost of sorting the incoming data into order. The reduction in redo (entries and size) is due to an “array insert” effect that Oracle can take advantage of with the delayed index maintenance that takes place when the append hint is legal (See the section labelled Option 4 in this note). So even with an IOT with no overflow there’s a potential benefit to gain from direct path loading that depends on how much the new data overlaps the old data, and there’s a penalty that depends on the amount of sorting you’d have to do.

What happens in my case when I move the big padding column out to an overflow segment – here are the equivalent results:

Headline figures                      Unhinted       With Append
================                  ============      ============
CPU used when call started                 158                52
CPU used by this session                   158                52
DB time                                    163                94
redo entries                           116,669            16,690
redo size                           51,392,748        26,741,868
sorts (memory)                               4                 5
sorts (rows)                                33           100,032

Interestingly, comparing the unhinted results with the previous unhinted results, there’s little difference in the CPU usage between having the padding column in the “TOP” section of the IOT compared to having it in the overflow segment, though there is a significant reduction in redo (the index entries are still going all over the place one by one, but the overflow blocks are being pinned and packed much more efficiently). The difference between having the append hint or not, though, is damatic. One third of the CPU time (despited still having 100,000 rows to sort), and half the redo. One of the side effects of the overflow, of course, is that the things being sorted are much shorted (only the id and v1 columns that go into the TOP section, and not the whole IOT row.

So, if you already have an overflow segment that caters for a significant percentage of the row, it looks as if the benefit you could get from using the /*+ append */ hint would far outweigh the penalty of sorting you have to pay. Of course, an IOT with a large overflow doesn’t look much different from a heap table with index – so perhaps that result isn’t very surprising.

I’ll close by re-iterating Connor’s closing comment:

Direct mode insert is a very cool facility, but it doesn’t mean that it’s going to be the best option in every situation.

Before you dive in and embrace it, or ruthlessly push it to one side, make sure you do some testing that reflects the situations you have to handle.

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