Carlos Sierra's Tools and Tips

Tools and Tips for Oracle Performance and SQL Tuning

Archive for the ‘SQL Tuning’ Category

Scripts to deal with SQL Plan Baselines, SQL Profiles and SQL Patches

with 3 comments

To mitigate SQL performance issues, I do make use of SQL Plan Baselines, SQL Profiles and SQL Patches, on a daily basis. Our environments are single-instance 12.1.0.2 CDBs, with over 2,000 PDBs. Our goal is Execution Plan Stability and consistent performance, over CBO plan flexibility. The CBO does a good job, considering the complexity imposed by current applications design. Nevertheless, some SQL require some help in order to enhance their plan stability.

I have written and shared a set of scripts that simply make the use of a bunch of APIs a lot easier, with better documented actions, and fully consistent within the organization. I have shared with the community these scripts in the past, and I keep them updated as per needs change. All these “CS” scripts are available under the download section on the right column.

Current version of the CS scripts is more like a toolset. You treat them as a whole. All of them call some other script that exists within the cs_internal subdirectory, then I usually navigate to the parent sql directory, and connect into SQL*Plus from there. All these scripts can be easily cloned and/or customized to your specific needs. They are available as “free to use” and “as is”. There is no requirement to keep their heading intact, so you can reverse-engineer them and make them your own if you want. Just keep in mind that I maintain, enhance, and extend this CS toolset every single day; so what you get today is a subset of what you will get tomorrow. If you think an enhancement you need (or a fix) is beneficial to the larger community (and to you), please let me know.

SQL Plan Baselines scripts

With the set of SQL Plan Baselines scripts, you can: 1) create a baseline based on a cursor or a plan stored into AWR; 2) enable and disable baselines; 3) drop baselines; 4) store them into a local staging table; 5) restore them from their local staging table; 6) promote as “fixed” or demote from “fixed”; 7) “zap” them if you have installed “El Zapper” (iod_spm).

Note: “El Zapper” is a PL/SQL package that extends the functionality of SQL Plan Management by automagically creating SQL Plan Baselines based on proven performance of a SQL statement over time, while considering a large number of executions, and a variety of historical plans. Please do not confuse “El Zapper” with auto-evolve of SPM. They are based on two very distinct premises. “El Zapper” also monitors the performance of active SQL Plan Baselines, and during an observation window it may disable a SQL Plan Baseline, if such plan no longer performs as “promised” (according to some thresholds). Most applications do not need “El Zapper”, since the use of SQL Plan Management should be more of an exception than a rule.

SQL Profiles scripts

With the set of SQL Profiles scripts, you can: 1) create a profile based on the outline of a cursor, or from a plan stored into AWR; 2) enable and disable profiles; 3) drop profiles; 4) store them into a local staging table; 5) restore them from their local staging table; 6) transfer them from one location to another (very similar to coe_xfr_sql_profile.sql, but on a more modular way).

Note: Regarding the transfer of a SQL Profile, the concept is simple: 1) on source location generate two plain text scripts, one that contains the SQL text, and a second that includes the Execution Plan (outline); 2) execute these two scripts on a target location, in order to create a SQL Profile there. The beauty of this method is not only that you can easily move Execution Plans between locations, but that you can actually create a SQL Profile getting the SQL Text from SQL_ID “A”, and the Execution Plan from SQL_ID “B”, allowing you to do things like: removing CBO Hints, or using a plan from a similar SQL but not quite the same (e.g. I can tweak a stand-alone cloned version of a SQL statement, and once I get the plan that I need, I associate the SQL Text from the original SQL, with the desired Execution Plan out of the stand-alone customized version of the SQL, after that I create a SQL Plan Baseline and drop the staging SQL Profile).

SQL Patches scripts

With the set of SQL Patches scripts, you can: 1) create a SQL patch based on one or more CBO Hints you provide (e.g.: GATHER_PLAN_STATISTICS MONITOR FIRST_ROWS(1) OPT_PARAM(‘_fix_control’ ‘5922070:OFF’) NO_BIND_AWARE); 2) enable and disable SQL patches; 3) drop SQL patches; 4) store them into a local staging table; 5) restore them from their local staging table.

Note: I use SQL Patches a lot, specially to embed CBO Hints that generate some desirable diagnostics details (and not so much to change plans), such as the ones provided by GATHER_PLAN_STATISTICS and MONITOR. In some cases, after I use the pathfinder tool written by Mauro Pagano, I have to disable a CBO patch (funny thing: I use a SQL Patch to disable a CBO Patch!). I also use a SQL Patch if I need to enable Adaptive Cursor Sharing (ACS) for one SQL (we disabled ACS for one major application). Bear in mind that SQL Plan Baselines, SQL Profiles and SQL Patches happily co-exist, so you can use them together, but I do prefer to use SQL Plan Baselines alone, whenever possible.

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Written by Carlos Sierra

December 15, 2018 at 5:57 am

Adapting and adopting SQL Plan Management (SPM)

with 3 comments

Introduction

This post is about: “Adapting and adopting SQL Plan Management (SPM) to achieve execution plan stability for sub-second queries on a high-rate OLTP mission-critical application”. In our case, such an application is implemented on top of several Oracle 12c multi tenant databases, where a consistent average execution time is more valuable than flexible execution plans. We successfully achieved plan stability implementing a simple algorithm using PL/SQL calling DBMS_SPM public APIs.

Chart below depicts a typical case where the average performance of a large set of business-critical SQL statements suddenly degraded from sub-millisecond to 15 or 20ms, then beccome more stable around 3ms. Wide spikes are a typical trademark of an Execution Plan for one or more SQL statements flipping for some time. In order to produce a more consistent latency we needed to improve plan stability, and of course the preferred tool to achieve that on an Oracle database is SQL Plan Management.

Algorithm

We tested and ruled out adaptive SQL Plan Management, which is an excellent 12c new feature. But, due to the dynamics of this application, where transactional data shifts so fast, allowing this “adaptive SPM” feature to evaluate auto-captured plans using bind variable values captured a few hours earlier, rendered unfortunately false positives. These false positives “evolved” as execution plans that were numerically optimal for values captured (at the time the candidate plan was captured), but performed poorly when executed on “current” values a few hours later. Nevertheless, this 12c “adaptive SPM” new feature is worth exploring for other applications.

We adapted SPM so it would only generate SQL Plan Baselines on SQL that executes often, and that is critical for the business. The algorithm has some complexity such as candidate evaluation and SQL categorization; and besides SPB creation it also includes plan demotion and plan promotion. We have successfully implemented it in some PDBs and we are currently doing a rollout to entire CDBs. The algorithm is depicted on diagram on the left, and more details are included in corresponding presentation slides listed on the right-hand bar. I plan to talk about this topic on an international Oracle Users Group in 2018.

This algorithm is scripted into a sample PL/SQL package, which you can find on a subdirectory on my shared scripts. If you consider using this sample script for an application of your own, be sure you make it yours before attempting to use it. In other words: fully understand it first, then proceed to customize it accordingly and test it thoroughly.

Results

Chart below shows how average performance of business-critical SQL became more stable after implementing algorithm to adapt and adopt SPM on a pilot PDB. Not all went fine although: we had some outliers that required some tuning to the algorithm. Among challenges we faced: volatile data (creating a SPB when table was almost empty, then using it when table was larger); skewed values (create a SPB for non-popular value, then using it on a popular value); proper use of multiple optimal plans due to Adaptive Cursor Sharing (ACS); rejected candidates due to conservative initial restrictions on algorithm (performance per execution, number of executions, age of cursor, etc.)

Conclusion

If your OLTP application contains business critical SQL that executes at a high-rate, and where a spike on latency risks affecting SLAs, you may want to consider implementing SQL Plan Management. Consider then both: “adaptive SPM” if it satisfies your requirements, else build a PL/SQL library that can implement more complex logic for candidates evaluation and for SPBs maintenance. I do believe SPM works great, specially when you enhance its out-of-the-box functionality to satisfy your specific needs.

 

 

Written by Carlos Sierra

December 20, 2017 at 6:32 pm

SQL Monitoring without MONITOR Hint

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I recently got this question:

<<<Is there a way that I can generate SQL MONITORING report for a particular SQL_ID ( This SQL is generated from application code so I can’t add “MONITOR”  hint) from command prompt ? If yes can you please help me through this ?>>>

Since this question is of general interest, I’d rather respond here:

As you know, SQL Monitoring starts automatically on a SQL that executes a PX plan, or when its Serial execution has consumed over 5 seconds on CPU or I/O.

If you want to force SQL Monitoring on a SQL statement, without modifying the SQL text itself, I suggest you create a SQL Patch for it. But before you do, please be aware that SQL Monitoring requires the Oracle Tuning Pack.

How to turn on SQL Monitoring for a SQL that executes Serial, takes less than 5 seconds, and without modifying the application that issues such SQL

Use SQL Patch with the MONITOR Hint. An easy way to do that is by using the free sqlpch.sql script provided as part of the cscripts (see right-hand side of this blog under Downloads).

To use sqlpch.sql script, pass as parameter #1 your SQL_ID and for parameter #2 pass “GATHER_PLAN_STATISTICS MONITOR” (without the double quotes).

This sqlpch.sql script will create a SQL Patch for your SQL, which will produce SQL Monitoring (and the collection of A-Rows) for every execution of your SQL.

Be aware there is some overhead involved, so after you are done with your analysis drop the SQL Patch.

Script sqlpch.sql shows the name of the SQL Patch it creates (look at its spool file), and it gives you the command to drop such SQL Patch.

For the actual analysis and diagnostics of your SQL (after you have executed it with SQL Patch in place) use free tool SQLd360.

And for more details about sqlpch.sql and other uses of this script please refer to this entry on my blog.

Written by Carlos Sierra

February 29, 2016 at 10:16 am

Forcing a “Nested Loop only” Execution Plan

with 5 comments

Sometimes you do what you have to do. So here I confess doing something I usually avoid: forcing an Execution Plan (which is not the same as using a more conventional method for Plan stability).

This is a case on 11.2.0.3.0 base release where the application vendor sets the optimizer to 9i, and tweaks other CBO parameters in questionable ways, then some queries produce suboptimal plans (as expected); and you are called to help without changing the obvious.

There is a family of queries from an ad-hoc query generator that permits users to issue queries without a set of selective predicates. These queries join several large tables and their performance is poor (as expected as well!). On top of the previous, all these queries include the /*+ FIRST_ROWS */ CBO Hint and the questionable DISTINCT keyword. Note: it is quite common for developers to throw a DISTINCT keyword “to avoid duplicates” where the mere existence of duplicates would be an indication of an application bug; so “why fix it if I can hide it, right?”.

There is one caveat although: these queries include a generic predicate “rownum <= :b1”, and value passed defaults to 5000, so users rationale is “if I only want the first X rows my query should return fast”. This highlights still another questionable practice since it is hard to imagine a user scrolling 5000 rows and making any sense of such large set, especially when the full “filtered” set would be several million rows long. So the original problem is questionable in several ways. Nevertheless, sometimes we are called to help besides providing advice. And no, we are not allowed to slap hands 😉

The good news is that we can use this extra predicate on rownum and make these queries to return the first X rows really fast; and I mean less than 5 seconds instead of over one hour or more! And if users want not 5000 but 500 or even 50 rows, then we can be in the sub-second range!

You may be thinking FIRST_ROWS optimization, and that was my first try. Unfortunately, on 11.2.0.3.0, even reversing all the suboptimal CBO parameters at the session level, I would consistently get an Execution Plan with a few Hash Joins and a large Cost; and if I were to force a Nested Loop Plan, the cost would be several orders of magnitude larger so the CBO would not pick it! Nevertheless, such a “Nest Loop only” Execution Plan would fulfill the user’s expectations, regardless the validity of the initial request. And yes, CBO statistics are OK, not perfect but simply OK. One more piece of info: this is not Exadata! (if it were Exadata most probably these same Execution Plans with full table scans and Hash Joins would simply fly!).

So, my issue became: How do I force an Execution Plan that only contains Nested Loops? If I could do that, then the COUNT STOP operation could help me to halt my SQL execution once I fetched the first X rows (Hash Join does not allow me do that). Remember: these tables have literally millions of rows. I could pepper these queries with a ton of CBO Hints and I would get my desired “Nested Loop only” Execution Plan… But that would be a lot of work and tricky at best.

SQL Patch to the rescue

I could had used a SQL Profile, but I think this dirty trick of suppressing Hash Joins and Sort Merge Joins, would be better served with a SQL Patch. I also thought Siebel: They do tweak CBO parameters as well, and they suppress Hash Joins, but they change System and Session level parameters… Since I wanted my change to be very localized, SQL Patch could provide me just what I needed.

Under the Downloads section on the margin of this page, there is a “cscripts” link that includes the sqlpch.sql script. I used this script and passed as the second parameter the following string (1st parameter is SQL_ID). With a SQL Patch generated this way, I could systematically produce a “Nested-Loops only” Execution Plan for these few queries. I did not have to change the original SQL, nor change the CBO environment at the System or Session level, neither restrict the query generator, and I did not had to “educate” the users to avoid such unbounded queries.

OPT_PARAM("_optimizer_sortmerge_join_enabled" "FALSE") OPT_PARAM("_hash_join_enabled" "FALSE")

Conclusion

I have to concede doing something questionable, in this case using a SQL Patch to force a desired Execution Plan instead of fixing the obvious, simply because that was the shortest path to alleviate the user’s pain.

I consider this technique above a temporary work-around and not a solution to the actual issue. In this case the right way to handle this issue would be:

  1. Have the application vendor certify their application to the latest release of the database and reset all CBO related parameters, plus
  2. Have the application vendor remove CBO Hints and DISTINCT keyword from queries, plus
  3. Configure the ad-hoc query generator to restrict users from executing queries without selective predicates, then
  4. Tune those outlier queries that may still need some work to perform as per business requirements, and possibly
  5. Educate the users to provide as many selective predicates as possible

Anyways, the potential of using a SQL Patch to tweak an Execution Plan in mysterious ways is quite powerful, and something we may want to keep in the back of our minds for a rainy day…

Written by Carlos Sierra

August 8, 2015 at 8:30 am

edb360 taking a long time

with 2 comments

In most cases edb360 takes less than 1hr to execute. But I often hear of cases where it takes a lot longer than that. In a corner case it was taking several days and it had to be killed.

So the question is WHY edb360 takes that long?

Well, edb360 executes thousands of SQL statements sequentially (intentionally). Many of these queries read data from AWR and in particular from ASH. So, lets say your ASH historical table has 2B rows, and on top of that you have not gathered statistics on AWR tables in years, thus CBO under-estimates cardinality and tends to use index access and nested loops. In such extreme cases you may end up with suboptimal execution plans that expect to return a few rows, but actually read a couple of billion rows using index access operations and nested loops. A query like this may take hours to complete!

As of version v1515, edb360 has a shortcut algorithm that ends an execution after 8 hours. So you may get an incomplete output, but it ends normally and the partial output can actually be used. This is not a solution but a workaround for those long executions.

How to troubleshoot edb360 taking long?

Steps:

1. Review files 00002_edb360_dbname_log.txt, 00003_edb360_dbname_log2.txt, 00004_edb360_dbname_log3.txt and 00005_edb360_dbname_tkprof_sort.txt. First log shows the state of the statistics for AWR Tables. If stats are old then gather them fresh with script edb360/sql/gather_stats_wr_sys.sql

2. If number of rows on WRH$_ACTIVE_SESSION_HISTORY as per 00002_edb360_dbname_log.txt is several millions, then you may not be purging data periodically. There are some known bugs and some blog posts on this regard. Review MOS 387914.1 and proceed accordingly. Execute query below to validate ASH age:

SELECT TRUNC(sample_time, 'MM'), COUNT(*)
  FROM dba_hist_active_sess_history
 GROUP BY TRUNC(sample_time, 'MM')
 ORDER BY TRUNC(sample_time, 'MM')
 /

3. If edb360 version (first line on its readme) is older than 1 month, download and use latest version: https://github.com/carlos-sierra/edb360/archive/master.zip (link is also provided on the right-hand side of this blog under downloads).

4. Consider suppressing text and or csv reports. Each for an estimated gain of about 20%. Keep in mind that when suppressing reports, you start loosing some functionality. To suppress lets say text and csv reports, place the following two commands at the end of script edb360/sql/edb360_00_config.sql

DEF edb360_conf_incl_text = ‘N’;
DEF edb360_conf_incl_csv = ‘N’;

5. If after going through steps 1-4 above, edb360 still takes longer than a few hours, feel free to email author carlos.sierra.usa@gmail.com and provide 4 files from step 1.

Written by Carlos Sierra

May 6, 2015 at 8:19 pm

Discovering if a System level Parameter has changed its value (and when it happened)

with 4 comments

Quite often I learn of a system where “nobody changed anything” and suddenly the system is experiencing some strange behavior. Then after diligent investigation it turns out someone changed a little parameter at the System level, but somehow disregarded mentioning it since he/she thought it had no connection to the unexpected behavior.  As we all know, System parameters are big knobs that we don’t change lightly, still we often see “unknown” changes like the one described.

Script below produces a list of changes to System parameter values, indicating when a parameter was changed and from which value into which value. It does not filter out cache re-sizing operations, or resource manager plan changes. Both would be easy to exclude, but I’d rather see those global changes listed as well.

Note: This script below should only be executed if your site has a license for the Oracle Diagnostics pack (or Tuning pack), since it reads from AWR.

WITH
all_parameters AS (
SELECT snap_id,
       dbid,
       instance_number,
       parameter_name,
       value,
       isdefault,
       ismodified,
       lag(value) OVER (PARTITION BY dbid, instance_number, parameter_hash ORDER BY snap_id) prior_value
  FROM dba_hist_parameter
)
SELECT TO_CHAR(s.begin_interval_time, 'YYYY-MM-DD HH24:MI') begin_time,
       TO_CHAR(s.end_interval_time, 'YYYY-MM-DD HH24:MI') end_time,
       p.snap_id,
       p.dbid,
       p.instance_number,
       p.parameter_name,
       p.value,
       p.isdefault,
       p.ismodified,
       p.prior_value
  FROM all_parameters p,
       dba_hist_snapshot s
 WHERE p.value != p.prior_value
   AND s.snap_id = p.snap_id
   AND s.dbid = p.dbid
   AND s.instance_number = p.instance_number
 ORDER BY
       s.begin_interval_time DESC,
       p.dbid,
       p.instance_number,
       p.parameter_name
/

Sample output follows, where we can see a parameter affecting Degree of Parallelism was changed. This is just to illustrate its use. Enjoy this new free script! It is now part of edb360.
Screen Shot 2015-03-25 at 19.15.26

Written by Carlos Sierra

March 25, 2015 at 7:25 pm

SQLTXPLAIN under new administration

with 3 comments

During my 17 years at Oracle, I developed several tools and scripts. The largest and more widely used is SQLTXPLAIN. It is available through My Oracle Support (MOS) under document_id 215187.1.

SQLTXPLAIN, also know as SQLT, is a tool for SQL diagnostics, including Performance and Wrong Results. I am the original developer and author, but since very early stages of its development, this tool encapsulates the expertise of many bright engineers, DBAs, developers and others, who constantly helped to improve this tool on every new release by providing valuable feedback. SQLT is then nothing but the collection of many good ideas from many people. I was just the lucky guy that decided to build something useful for the Oracle SQL tuning community.

When I decided to join Enkitec back on 2013, I asked Mauro Pagano to look after my baby (I mean SQLT), and sure enough he did an excellent job. Mauro fixed most of my bugs, as he jokes about, and also incorporated some of his own :-). Mauro kept SQLT in good shape and he was able to continue improving it on every new release. Now Mauro also works for Enkitec, so SQLT has a new owner and custodian at Oracle.

Abel Macias is the new owner of SQLT, and as such he gets busy maintaining and enhancing this tool among other duties at Oracle. So, if you have enhancement requests, or positive feedback, please reach out to Abel at his Oracle account: abel.macias@oracle.com. If you come across some of my other tools and scripts, and they show my former Oracle account (carlos.sierra@oracle.com), please reach out to Abel and he might be able to route your concern or question.

Since one of my hobbies is to build free software that I also consume, my current efforts are on eDB360, eAdam and eSP. The most popular and openly available is eDB360, which basically gives your a 360-degree view of a database without installing anything. Then, Mauro is also building something cool on his own free time. Mauro is building the new SQLd360 tool, which is already available on the web (search for SQLd360). This SQLd360 tool, similar to eDB360, provides a 360-degree view, but instead of a database its focus is one SQL. And similarly than eDB360 it installs nothing on the database. Both are available as “free software” for anyone to download and use. That is the nice part: everyone likes free! (specially if any good).

What is the difference between SQLd360 and SQLT?

Both are exceptional tools. And both can be used for SQL Tuning and for SQL diagnostics. The main differences in my opinion are these:

  1. SQLT has it all. It is huge and it covers pretty much all corners. So, for SQL Tuning this SQLTXPLAIN is “THE” tool.
  2. SQLd360 in the other hand is smaller, newer and faster to execute. It gives me what is more important and most commonly used.
  3. SQLT requires to install a couple of schemas and hundreds of objects. SQLd360 installs nothing!
  4. To download SQLT you need to login into MOS. In contrast, SQLd360 is wide open (free software license), and no login is needed.
  5. Oracle Support requires SQLT, and Oracle Engineers are not exposed yet to SQLd360.
  6. SQLd360 uses Google charts (as well as eDB360 does) which enhance readability of large data sets, like time series for example. Thus SQLd360 output is quite readable.
  7. eDB360 calls SQLd360 on SQL of interest (large database consumers), so in that sense SQLd360 enhances eDB360. But SQLd360 can also be used stand-alone.

If you ask me which one would I recommend, I would answer: both!. If you can use both, then that is better than using just one. Each of these two tools (SQLT and SQLd360) has pros and cons compared to the other. But at the end both are great tools. And thanks to Abel Macias, SQLT continues its lifecycle with frequent enhancements. And thanks to Mauro, we have now a new kid on the block! I would say we have a win-win for our large Oracle community!

Written by Carlos Sierra

March 18, 2015 at 12:37 pm