Working with auto update stats

Posted: 31 May, 2013 | Author: | Filed under: Instance and operating system tuning, SQL query tuning | Tags: , , , , , , , , , , , , , , | 1 Comment

This article is superseded by my more comprehensive post, Experience with Auto Update Statistics (AUS).

This article has been written based on experience with version 11.70.FC5W1 and assumes some knowledge of stats and distributions and how they affect query optimisation. I appreciate any feedback in the comments section on my WordPress blog.

Auto update stats was introduced in 11.50.xC1 and, while being partly aimed at the embedded market, meant for the first time there was a complete solution to gathering database statistics bundled inside the product.

Several other tools exist to help with gathering statistics, for example AGS’s Server Studio can produce a set of UPDATE STATISTICS commands in a script to run against your database. Anecdotally, most DBAs use Art Kagel’s dostats utility, packaged up in the utils2_ak package available from the IIUG software repository. Dostats is pretty damn good although it’s not a complete solution as some scripts are needed to control it. It comes with a partner utility, drive_dostats, to do this but many DBAs, including myself, have written their own. Because dostats is the de-facto standard, I’ll refer to it a fair bit in this article. Also version 11.70 has a number of enhancements that don’t require you to use auto update stats; I’ll cover these as well.

So if you’re happily using dostats or another method to manage statistics, should you consider changing to auto update stats? Should it be your method of choice for a new-build instance? Well maybe: this article will go through some of the advantages and things to be aware of.

Here are some of its advantages:

  • The whole solution is part of the database engine and supported by IBM support.
  • It provides a complete framework, working within defined maintenance windows and is highly configurable.
  • It can be managed through OAT, although this is not needed.
  • Auto update stats does less work and takes less time than many solutions because it does not (by default) gather distributions on non-indexed columns or do separate low stats for each entire index key.
  • It’s fully integrated with the enhancements to update stats introduced in version 11.70.
  • It works on all your databases, including the system ones.
  • Perhaps my favourite feature: if you make manual adjustments, like increasing the resolution of the distributions on a column, auto update stats notices this and maintains the distributions at the new resolution. Similarly, if you manually create a distribution on a column it will maintain this.

One reason some DBAs don’t use auto update stats is because it involves using the job scheduler, which I’m told had issues in early releases with high CPU usage. For this reason, many DBAs touch a file called $INFORMIXDIR/etc/sysadmin/stop to stop it starting when the engine comes online. With 11.70.FC5W1 we run the job scheduler without any issues. (As an aside, if you’re not using it, it’s worth looking at the jobs to see what you’re missing: post_alarm_message is particularly useful.)

So can you just enable the job scheduler and let auto update stats do its thing? Not really. The first thing to look at is these onconfig parameters, which are in 11.70 and take effect regardless of the statistics method used:

  • AUTO_STAT_MODE
  • STATCHANGE
  • USTLOW_SAMPLE

Using auto stat mode and a non-zero value for STATCHANGE is something you need to consider very carefully. Internally the engine keeps a count of the number of inserts, updates and deletes that occur on each table, something that Keshava Murthy covered in his blog. If these collectively do not exceed STATCHANGE percent of the row count, statistics or distributions are not updated. This applies even when you run an UPDATE STATISTICS command manually. Confusingly the command still returns ‘Statistics updated’ even when nothing is done; the only clue is that the prompt returns instantly. To get around this there is a new FORCE keyword for the UPDATE STATISTICS statement that reverts to the old behaviour.

I find turning AUTO_STAT_MODE on and setting STATCHANGE to zero works quite well: this just skips tables where no updates, inserts or deletes have occurred.

You can set the value of STATCHANGE manually on individual tables with a fast-alter operation:

alter table table statchange change_threshold;

As it’s an update to systables, be aware an exclusive table lock is briefly needed.

I’m not keen on setting STATCHANGE to non-zero value because we have a lot of tables with incrementing date/time fields, meaning query optimisation is often time-based. I would find the option to override the age-based AUS_AGE parameter on a per-table basis much more useful, something that can only be done by writing your own script. Fortunately, as auto update stats evaluates the tables with stale stats on a regular scheduled basis, any ad-hoc updates are taken into account in its scheduling.

Setting USTLOW_SAMPLE enables sampling for UPDATE STATISTICS LOW statements, which is generally a good thing and can dramatically the time these statements take. It can be overridden in your user environment. Sampling generally works well as long as the table is not heavily skewed in some way: if Informix thinks it is you’ll see messages like this in the online log:

Warning: update statistics low using sampling may generate inaccurate index statistics for index owner.index_name due to data skew

Whether this is an issue for you will depend very much on your queries.

The other major enhancement in 11.70 is fragment-level statistics but I shan’t cover in detail here. If your storage schema is compatible with it and your table access patterns mean that some table fragments are never updated, it’s extremely useful. Informix’s implementation is nice in that the table stats are still considered as a whole when the optimiser evaluates queries, so you don’t get into trouble with having no stats for new fragments.

Perhaps the most significant difference between dostats and auto update stats is that dostats gathers additional distributions on non-indexed columns using UPDATE STATISTICS MEDIUM. If you have such distributions already auto update stats will continue to maintain them but it won’t create any new ones. All automated tools are attempting to apply a set of general criteria and recommendations to all tables so there is no hard and fast rule about whether you need them. One case to consider is where you have some sort of status flag such as a boolean or a column with a limited set of allowed values, perhaps enforced by a check constraint. Here distributions could be useful where these columns are used as filter conditions in queries. Otherwise, I suspect that in a lot of cases they are not needed. You’ll need to decide what is appropriate for your system.

Dostats also gathers low statistics separately for different indices which takes extra time but in my tests using version 11.50.FC9W2 this didn’t make any difference to the end result.

So what about switching on auto update stats? For this you’ll need to turn on the task scheduler if it’s not running already, which can be done with:

database sysadmin;
execute function task("scheduler start");

I would strongly recommend reviewing the enabled (and/or disabled) tasks and switch off any you don’t want or are not sure about. Review the jobs with:

database sysadmin;
select tk_name, tk_description from ph_task where tk_enable='t';

The relevant jobs for auto update stats are mon_table_profile, Auto Update Statistics Evaluation and Auto Update Statistics Refresh. Most of the others are fairly benign but I disable auto_tune_cpu_vps, add_storage, Low Memory Reconfig and mon_low_storage:

database sysadmin;
update ph_task set tk_enable = 'f' where tk_name in ('auto_tune_cpu_vps', 'add_storage', 'Low Memory Reconfig', 'mon_low_storage');

You’ll also find in table ph_threshold several parameters related to auto update stats:

  • AUS_AGE
  • AUS_PDQ
  • AUS_CHANGE
  • AUS_AUTO_RULES
  • AUS_SMALL_TABLES

Most of these are well-documented in the manual but the explanation of parameter AUS_AUTO_RULES is unclear as it just talks about enforcing a base set of rules. My understanding of the parameter is that:

  • When set to zero, auto update stats just maintains whatever statistics and distributions you have already. This retains any custom resolutions and confidence values you may have.
  • When set to one, it does the above plus it also makes sure that low stats are gathered on all tables, distributions in high mode for all leading index columns and distributions in medium mode for columns that are part of an index but not a leading key.

You can just update the parameters with manual SQL updates on the ph_threshold table. Likewise you’ll need to review and possibly update the scheduled run times for the two auto update statistics tasks in the ph_task table.

By default you just get one process updating stats but it’s possible to have two or more running at the same time by inserting a new row into table ph_task. I’d make sure that the total effective PDQ priority of all these tasks does not exceed 100.

At this point we’re sort of ready to go but you’re now trusting your stats gathering to a new process and I would suggest setting up some kind of monitoring to make sure it’s working as you expect. I feel this is a slight weakness in the implementation because you are back to writing your own scripts. Maybe one answer is to use OAT but this is not a good solution in our environment.

I would suggest monitoring the following:

  • That all three tasks related to auto update stats are enabled and scheduled to run at least once a week.
  • That the db scheduler is running, perhaps by checking for its threads with onstat -g ath.
  • That the values for the various AUS* parameters are sane.
  • That UPDATE STATISTICS LOW was not run too long ago for all tables. If you set AUTO_STAT_MODE it gets a little more complicated because you’ll need to use the information in Keshava Murthy’s blog post, referenced above, to work out whether the table needs to be updated.
  • Something similar for your distributions.
  • For any issues encountered whilst running the UPDATE STATISTICS statements. For this query table aus_command and check for columns where aus_cmd_state is E for error. The SQL error code and ISAM error code will then be in columns aus_cmd_err_sql and aus_cmd_err_isam respectively.

One problem you might find is that your scheduled maintenance times are not long enough to keep pace with how frequently you require your stats to be updated. You can look at adding an extra process or extending the times in this case. Even better, consider reading John Miller’s excellent article on tuning update statistics. It’s now over ten years old but still completely relevant today.

There is a view in the sysadmin database called aus_cmd_comp which shows you all the commands run recently. It gets purged daily so if you want to keep a permanent or longer record you might want to consider writing a procedure to copy its contents elsewhere and creating a scheduled task to call it.

It’s worth noting that auto update stats doesn’t do anything with stored procedure plans, i.e. UPDATE STATISTICS FOR PROCEDURE. If there are open statement handles using procedures, doing so can risk a -710 error unless (and sometimes even if) AUTO_REPREPARE set in your onconfig. Whatever the situation on your system you’ll need to do this manually or by another means.

Finally you might be wondering how the Auto Update Statistics Evaluation task prioritises tables for updating. The answer to this is in the procedures in $INFORMIXDIR/etc/sysadmin/sch_aus.sql.

In summary I like auto update stats and recommend it as long as you have a good understanding of how it works and are aware of the points I’ve raised in this article. It integrates nicely with the new features in Informix 11.70. I like the fact that it is easy to set up, although I do believe you need to monitor it if up to date stats are critical to your system. By not gathering medium-mode distributions on non-indexed columns and not running update statistics low for leading index columns, it does significantly less work than dostats. I appreciate the nice touches it has, like retaining and maintaining the existing resolution of your statistics.

As I said at the start of the article, feedback is welcomed and encouraged.