rmoff

May 26, 2011

Data Warehousing and Statistics in Oracle 11g – Automatic Optimizer Statistics Collection

Filed under: dbms_stats, DWH, oracle, Statistics — rmoff @ 13:35

Chucking a stick in the spokes of your carefully-tested ETL/BI …

My opinion is that automated stats gathering for non-system objects should be disabled on Oracle Data Warehouses across all environments.

All it does it cover up poor design or implementation which has omitted to consider statistics management. Once you get into the realms of millions or billions of rows of data, the automated housekeeping may well not have time to stat all of your tables on each run. And then it becomes a quasi-lottery when your tables will get processed. Or what if you’re working with intra-day loads (eg. near real-time) – the housekeeping job only runs once a day by default.

Even if you have a suitable window and are happy that the job gathers all that it needs to all of the time, what if you want to run your batch at the same time as the task window defined? If you want to run your batch highly parallel (and why wouldn’t you?) then will the stats gather suffer? or affect your batch by running stats highly parallel too?

Suppose you are relying on the auto stats job, and don’t want to run it at the same time as your batch, so you come up with a suitable schedule for them to run at different times. What happens when your DW grows and you need to add a new batch process, and so have to move the window again? How do you know that moving it won’t affect the previous batch’s stats?

If you’re building on an existing system and want to test the performance of your new batch, how are you going to simulate the behaviour of your auto stats job? Even if you trigger it manually, are you going to accurately simulate the statistics that it may or may not need to gather each night? How do you factor in the magical 10% staleness to trigger a stats gather? That is one serious test rig if you want to reproduce all of that.

If you have stats management in place, then turning the auto stats off (for non-system objects) won’t hurt. And if you’re not, then the auto stats job will cover this up in your environments all the way from Dev through to Prod. The first time someone will ask about stats management is when you’re scratching your head over a report or ETL stage “that used to work fine”. And then the horrible truth will dawn that you screwed up, and should have built it into your design from the beginning.

As we say around here, if you want a job done properly, do it tha’ sen. Or rather, as Greg Rahn more articulately says:

I tend to advise people that for a DW the stats gathering should be part of the data flow (ETL/ELT) process and to disable the default job
[…]
If you wish to collect your statistics manually, then you should change the value of AUTOSTATS_TARGET to ORACLE instead of AUTO (DBMS_STATS.SET_PARAM(‘AUTOSTATS_TARGET’,’ORACLE’)). This will keep the dictionary stats up to date and allow you to manually gather stats on your schemas

Julian Dyke says something supporting this view too in his presentation here:

In complex databases do not rely on Auto job
– Unpredictable collection behaviour / duration
– Unpredictable execution plan changes

If you can’t disable the autostats job for whatever reason (maybe another application on the same DB would require changes to accommodate it), then you can shield yourself from its nefarious influences by using LOCK_SCHEMA_STATS to lock the stats on your schema(s). When you manually maintain the stats yourself, you either unlock them first, or use the FORCE option of the stats procedures.

Stabilisers on a high-performance motorbike

It’s easy enough to understand why Oracle built the Automated Stats job, and why it’s enabled by default. In an effort to move towards the Self Managing Database, it makes sense to automate whatever you can, so that the scope for poor performance is reduced. Abstracting things slightly, the optimizer is just part of the DB code, and stats reason for being is to support the optimizer, so why not keep it under the covers where possible?
The trouble with this is that it might be fine for the middle of the road. The bog standard, quick-win, fire it and run project doing nicely standard OLTP work. One fewer thing for the developer to worry about. It’s probably quite good for lots of things. But Oracle RDBMS is a big beast, and an expensive bit of kit. Do you really want to meander along in the slow lane all the time, content to be using a one-size-fits-all approach?
Kawasaki motorbike with stabilisers

If you’re serious about exploiting the power of Oracle for your data warehouse, then you need to understand what needs to be done to get it to perform. One of the big factors is accurate, representative statistics. And to get these you have to take the stabilisers off and learn how to do it properly yourself, because you’re the one that understands your data. Data loads are going to be different, data distribution is going to be different, reporting is going to be different. There’s a finite set of patterns that you’ll find in standard DW methodology, but it’s up to you to read about them (Greg Rahn, Doug Burns, et al) and understand how they apply to your system, and not rely on Oracle’s approximation of a stats method for an average system.

Why do I need to manage the stats myself? Doesn’t Oracle do it automagically when they’re stale?

Doesn’t Oracle gather stats automagically when they’re stale?
Yes, it does, BUT:

  • Only if the window allocated to it allows for time
  • not stale ≠ representative stats .
    Or to rearrange the equation: your stats can be unrepresentative of your data, and the stats not be ‘stale’.

So even whilst they’re not “stale”, that’s not to say the global statistics for your table are still representative. After one day, the statistics are already becoming unrepresentative of the data (think max value of date, transaction number, etc), but are still not “stale”.
Oracle will, by default, consider a table “stale” once 10% has changed. But most DWs are going to be loading many millions of rows a day, so the 10% (default) change for a table to be considered stale is going to be quite high. A table loading 20 million rows per day will hit c.1 billion rows in total after less than two months. But of a billion rows, a hundred million (10%) need to change before the table’s statistics are “stale”. 20 into 100 goes 5 … so your statistics would only become “stale” roughly every five days.

There’s a good presentation from OpenWorld 2008 by Oracle’s Real World Performance Group, entitled Real-World Database Performance Techniques and Methods. In it they discuss statistics management in detail, including the following “Six Challenges to the Cost Based Optimizer”:

1. Data skew
2. Bind peeking
3. Column low/high values
4. Data correlation between columns
5. Cardinality Approximations
6. The debugging process

At least two of these (data skew, and column low/high values – out-of-range) can occur (which is bad, mm’kay?) with statistics which are STALE=FALSE.

The point is, if you’re serious about getting the best explain plan from the CBO, you can’t rely on STALE as a sole indicator of how representative your statistics are of your data.

Let’s remember why we even care about good statistics. Some people seem to think that it’s optional. That it’s the geek equivalent of spending every weekend lovingly polishing the exterior of one’s favourite car – nice to have and ideally should be done, but ultimately just for show and won’t make it go any faster.
The DB is there to support the users of whatever application it is. And users being users, want their answers now. This gives us our starting point, and a logical flow of conclusions drawn from this:

  • Our requirement is for performance, and repeatable, consistent performance.
    • To get this we want Oracle to execute the query as efficiently as possible.
    • To do this, Oracle needs to understand the data that it’s being asked to query.
    • If it doesn’t understand the data, how can we expect to access it in the most efficient way?
    • This understanding is imparted to Oracle through statistics.
    • So statistics need to be representative of the data.

As soon as you are changing data (eg a DW batch load), you need to consider whether the stats are still going to give the CBO the best chance of getting the right plan. If they aren’t as representative of your data as they could be then you can’t expect the CBO to come up with the best plan.
If your data doesn’t change much and once a week works for you then great. But the point is you need to understand your data, so that you can plan your statistics strategy around it so that Oracle can understand it.

Reading & References

Thanks to Greg Rahn for reviewing my post and suggesting some changes

May 19, 2011

OBIEE performance – get your database sweating

Filed under: bi, DWH, obiee, oracle, performance, rant, rrdtool — rmoff @ 16:01

Just because something produces the correct numbers on the report, it doesn’t mean you can stop there.

How you are producing those numbers matters, and matters a lot if you have an interest in the long-term health of your system and its ability to scale.

OBIEE is the case in point here, but the principle applies to any architecture with >1 tiers or components.

Let me start with a rhetorical question. The user has got a report which has ten rows of data. Which of the following methods is going to be a more efficient way to generate the report?

  1. Transfer the ten rows of data from the database back to the rendering application, and the rendered report to the user
  2. Transfer fourteen million rows of data from the database back to the rendering application, get the application to crunch these to the required ten rows, transfer rendered report to the user

Obviously, it’s the first one. In the second one (and this is no hyperbolic example to prove a point, I’m sorry to say) we have the following overheads:

  • Network traffic of fourteen million rows from the database to the application server
  • Disk and/or Memory impact on the application server, depending on how it’s processing those fourteen millions rows
  • CPU impact on the application server when it processes those fourteen million rows

Considering OBIEE specifically, you should be aiming to be able to answer all of your questions on the database directly. As soon as you start using the database simply as a source for dumping big volumes of rows into your OBIEE server, you’re storing up problems.
An RDBMS is designed for crunching lots of data. You’ve already lifted all that data off the disk when you selected it out of the tables on Oracle DB – why not get Oracle DB to do the additional processing required too? Where’s the benefit in lifting & shifting all of that data to then reprocess it again?
In fact, consider why you’re crunching big volumes of data each time in the first place. If multiple people have the same big queries, are you missing some aggregates in your data source that would support these queries much better? Or do you need to be looking towards multi-dimensional sources such as Oracle OLAP or Essbase?

The flexibility and power of OBIEE to do its own calculations and aggregations on data is a double-edged sword, and one to be wielded with responsibility. It is a great feature and one which cannot be avoided if you’re using federated queries across disparate sources. But if you’re using it out of ignorance or laziness to brute-force a solution instead of doing the big work at the lowest level possible then you’re asking for trouble.

If you rely on OBIEE to do the heavy work of your reporting solution, then you need to factor this in to your hardware spec for the machine. As a reporting interface to well aggregated data with fairly simple reports, we’ve found that it runs at minimal CPU, and doesn’t stress the disk. You can predict fairly reliably that this should scale just fine so long as your database can. But if you put the workload on the OBIEE server too, you’re going to hit bottlenecks much sooner.

The principle of moving as little data around as possible is described by Cary Millsap as “Filter Early” and described well in his blog article. Thanks to @RonCrisco for helping me remember the name. @cdturri pointed out that it’s also just common sense :-).
Applying it to a specific system, Exadata uses it in its SmartScan technology where it applies filtering of data directly on the storage rather than bringing all the data back up to the database SQL processing layer (h/t @ocpdba).

Case-study

Here’s an example of a situation where all is not well in OBIEE-land.

We were alerted to it by unix complaining that a filesystem was running low on space. Checking the contents of the filesystem we spotted these monsters:

[user@server]/data/bi/tmp $ls -lrt
total 112914646
[...]
-rw-------   1 user   biadmin    29122976800 Apr  6 11:32 nQS_20117_465_33898735.TMP
-rw-------   1 user   biadmin    24816966976 Apr  6 11:12 nQS_20117_464_33792457.TMP
-rw-------   1 user   biadmin    3582054936 Apr  6 11:46 nQS_20117_469_37979712.TMP
[...]

These are temporary files generated by BI Server (nqsserver), and for those of you viewing in black and white, those files are getting on for 30GB a go!

When queried, NQQuery.log embarrassingly admits facts such as:

Rows 13,894,550, bytes 3,260,497,648 retrieved from database query id: xxxx

and Usage Tracking shows the kind of damage being inflicted:

                      Total
                       time     row num db     cum num
START_TS     END_TS     sec   count  query      db row 

06-APR 10:05 10:18      579       0      6   3,436,816 
06-APR 10:05 10:18      553       0      4   3,239,101 
06-APR 10:05 10:18      383       0      3   1,624,656 
06-APR 10:11 11:48     5694       0      1  13,894,550 
06-APR 10:11 11:48     4314       0      1  11,840,156 
06-APR 10:21 10:27      336     456      4   3,239,101 

A look at the machine’s vital statistics for this time period shows the impact:

Affected BI Server, showing impact of workload which quietens down c.12:00

For a bit more detail, here’s the time period at greater resolution:

Systems metric during the heavy report execution

Notice that the server starts paging out during the work, and there’s a lot of disk activity – both read and write. CPU goes from a normal c.5% up to 20-30%.

This was one user doing this — care to place bets on how the system would perform if it were rolled out to ten users running this kind of workload?

For reference, here’s one of our other BI Servers which serves at least 10 times as many users, where the reports do the bulk of their work in the database:

Another BI Server on the same day, with a greater number of users but reports which do their big work on the database

Monitoring for problems

There are a few easy ways to look out for this bad behaviour. The first is in Usage Tracking, where you can look at S_NQ_ACCT.CUM_NUM_DB_ROW and compare it to S_NQ_ACCT.ROW_COUNT, even writing an alert for when this ratio goes above a defined threshold.
You could also look at an alert on S_NQ_ACCT.CUM_NUM_DB_ROW going above a fixed number, since depending on your system’s purpose and architectural principles you might want to aim to never be pulling back more rows from the database than necessary for a report.

I’d also pay close attention to S_NQ_ACCT.NUM_DB_QUERY, as this will show you reports generating more than one query on the database and thus increasing the workload on the BI Server processing the combined results.

Another thing to check is the NQQuery.log, looking at the number of bytes returned from a database query. This could feed into some kind of generated alert for closer investigation if a query returns above a certain number.
OBI 10g:

grep retrieved NQQuery.log|awk '{print $5}'|sort -n

OBI 11g:

grep retrieved NQQuery.log|awk '{print $20}'|sort -n

(if you’re running on Windows then to run this command you’ll need either cygwin or google for a Win32 version of grep/awk/sort – or brew your own Powershell version)

I’d also strongly recommend monitoring your BI Server’s TEMP folder (defined as WORK_DIRECTORY_PATHS in NQSConfig.INI), as this is where it lands the data to disk when it’s got to crunch it. Monitor this for two reasons – to spot when bad stuff’s happening, but also in case it fills up and causes your BI Server to barf. For the latter, you can expect to get:

[nQSError: 10058] A general error has occurred. 
[nQSError: 46118] Out of disk space. 

Developing for the long term

How do you assess your development projects for quality?
You presumably are keen that the reports match the specifications and return the right numbers. You hopefully also have NFRs for how quickly these reports run.
But what about system impact of new developments? How do you quantify this?

Unless you are delivering a standalone project, fire-and-forget, then maybe you can performance test for load and concurrency up-front to validate how well your solution will scale to the required users.
But lots of BI projects are iterative and may well be across functions too. Just because your development project is first to land it doesn’t give you the right to dump a steaming pile on the servers and proclaim that it works with your user volumes and meets their NFRs, so all is well. What about all the projects that come along next, and are going to be hindered by your mess?

This is where another double-edged sword comes into play – “Best Practice”. For me, Best Practice is a way of doing something that multiple implementations, time and expertise has shown to be the best way of not screwing up. It is the starting point from which to work, understanding deviations as required. What it is not, and what gets it the bad reputation, is a fixed set of crude rules to be adhered to blindly and implemented without questioning or understanding.

If a system is not adhering to the kind of best practice I’m talking about here – filter early, in essence – then there may be a good reason. But that reason must be consciously and loudly acknowledged and documented.

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