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 22.214.171.124.0 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 126.96.36.199.0, 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")
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:
- Have the application vendor certify their application to the latest release of the database and reset all CBO related parameters, plus
- Have the application vendor remove CBO Hints and DISTINCT keyword from queries, plus
- Configure the ad-hoc query generator to restrict users from executing queries without selective predicates, then
- Tune those outlier queries that may still need some work to perform as per business requirements, and possibly
- 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…
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?
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 firstname.lastname@example.org and provide 4 files from step 1.
Every release of edb360 includes some “new goodies”. Latest version v1510 reduces the number of execution parameters from 2 to just 1. It also incorporates a new configuration file which is mostly static, but it can be user-modified for special cases. And new sections below are all now part of edb360. Enjoy!
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 /
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: email@example.com. If you come across some of my other tools and scripts, and they show my former Oracle account (firstname.lastname@example.org), 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:
- SQLT has it all. It is huge and it covers pretty much all corners. So, for SQL Tuning this SQLTXPLAIN is “THE” tool.
- SQLd360 in the other hand is smaller, newer and faster to execute. It gives me what is more important and most commonly used.
- SQLT requires to install a couple of schemas and hundreds of objects. SQLd360 installs nothing!
- To download SQLT you need to login into MOS. In contrast, SQLd360 is wide open (free software license), and no login is needed.
- Oracle Support requires SQLT, and Oracle Engineers are not exposed yet to SQLd360.
- 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.
- 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!
New tool sqld360 is now available! Mauro Pagano released this cool new tool a few hours ago. And yes, it is free for all!
So, what is sqld360? Well, it is an install-nothing free software that tells you a lot about one SQL statement. And if your site has an Oracle Tuning Pack or Diagnostics Pack license, then you get a lot more from sqld360. Sample snapshot below shows you the dynamic menu for a simple SQL. This sqld360 new tool uses similar techniques than edb360, so it displays information as html, text, csv and in some cases it uses some cool Google charts.
The obvious question is why would I use this sqld360 instead of SQLT or SQLHC? The answer is: you can use them all. What makes sqld360 different to SQLT is that sqld360 installs nothing on the database. And what makes sqld360 different to SQLHC is that sqld360 is available as free software on a GitHub repository, so you don’t have to have a MOS account available. Any Oracle user can download and use sqld360 starting today!
In terms of content, I can say that SQLHC gives you some Observations and sqld360 does not (yet). Besides that, I think sqld360 is superior to SQLHC simply because it is more mature and developed from scratch using newer techniques.
SQLT is a different animal. It provides tons of functionality developed over a decade. This sqld360 is on its first release, but it will certainly grow over time but only in the right directions. From the beginning, sqld360 focuses on what is important for tuners and for those in need to diagnose a SQL misbehaving. So simply put, sqld360 is faster and better focused than SQLT and SQLHC. We can call it “the next generation”.
Why free? The answer is: Why not? When Mauro and I started discussing sqld360, we decided to continue developing cool scripts and tools on our own personal time, on our own hardware, and without taking anything from anyone. So this is kind of our contribution to our Oracle community. I have provided edb360 and Mauro is providing sqld360. Mauro and myself would love to blog more often, but if we have to decide where to put our personal time, we both are more inclined to invest on free tools (after of course our family duties).
It is hard to appreciate with a sample execution from one of our systems, but if you look at chart below you may get to see how a SQL of interest compares to the system load. This kind of chart is helpful when you are trying to understand how a particular SQL affects a given load for a certain time window. Or when you need to documents your findings to business leaders.
I could continue telling more about sqld360, but the best way to appreciate it is by actually trying it on one of your own SQL statements. You can go to Mauro’s blog post about sqld360 and download this tool following instructions there. Then give it a shot, after all you have nothing to loose.
Forgot to mention this: sqld360 as well as edb360 work through a SQL*Plus connection, either on the database server or on a PC client. This is important since quite often we road-warriors and even in-house developers may not have access to OEM or some other persistently-connected tools. Then, with sqld360 and edb360, using a SQL*Plus connection you can extract enough metadata to analyze and comprehend what is happening on the entire database or around one particular SQL, all without having to be persistently-connected to the database. Yes, you can do all your analysis remotely from home! yay!
And, by the way, as of today edb360 calls sqld360 on SQL of interest. So if you are already using edb360 you may want to download version v1504 dated February 15, 2015. This edb360 v1504 contains sqld360.
I hope you enjoy both tools. Life is Good!