Explain the Explain Plan: Cardinality Estimates

In last week’s post, I began a series on how to read and interpret Oracle execution plans by explaining what an execution plan is and how to generate one. This week I’m going to tackle the most important piece of information the Optimizer shares with you via the execution plan, it’s cardinality estimates.

What is a Cardinality Estimate?

A cardinality estimate is the estimated number of rows, the optimizer believes will be returned by a specific operation in the execution plan. The Optimizer determines the cardinality for each operation based on a complex set of formulas that use table and column level statistics as input (or the statistics derived by dynamic sampling). It’s considered the most important aspect of an execution plan because it strongly influences all of the other decisions the optimizer makes.

In part 4 of our series, I share some of the formulas used by the optimizer to estimate cardinalities, as well as showing you how to identify cardinalities in a plan. I also demonstrate multiple ways to determine if the cardinality estimates are accurate.

What can cause a Cardinality Misestimate and how do I fix it?

Several factors can lead to incorrect cardinality estimates even when the basic table and column statistics are up to date. In part 5 of our series, I explain the leading causes of cardinality misestimates and how you can address them.

Next weeks, instalment will be all about the different access methods available to the Optimizer and what you can do to encourage the optimizer to select the access method you want! Don’t forget more information on the Oracle Optimizer can always be found on the Optimizer blog.

Explaining the Explain Plan – How to Read and Interpret Execution Plans


Examining the different aspects of an execution plan, from cardinality estimates to parallel execution, and understanding what information you should glean from it can be overwhelming even for the most experienced DBA.

That’s why I’ve put together a series of short videos that will walk you through each aspect of the plan and explain what information you can find there and what to do if the plan isn’t what you were expecting.

What is an Execution Plan?

The series starts at the very beginning with a comprehensive overview of what an execution plan is and what information is displayed in each section. After all, you can’t learn to interpret what is happening in a plan, until you know what a plan actually is.

How to Generate an Execution Plan?

Although multiple different tools will display an Oracle Execution Plan for you, there really are only two ways to generate the plan. You can use the Explain Plan command, or you can view the execution plan of a SQL statement currently in the Cursor Cache using the dictionary view V$SQL_Plan. This session covers both techniques for you and provides insights into what additional information you can get the Optimizer to share with you when you generate a plan. It also explains why you don’t always get the same plan with each approach, as I discussed in an earlier post.

How to use DBMS_XPLAN to FORMAT an Execution Plan

The FORMAT parameter within the DBMS_XPLAN.DISPLAY_CURSOR function is the best tool to show you detailed information about a what’s happened in an execution plan including the bind variable values used, the actual number of rows returned by each step, and how much time was spent on each step.  I’ve also covered a lot of the content in this video in a previous post.

Part 2 of the series will cover Cardinality Estimates and what you can do to improve them!

Remember you can always get more information on the Oracle Optimizer on the Optimizer team’s blog.

What are Query Block Names and how to find them

I got a lot of follow-up questions on what Query Block names are and how to find them, after my recent post about using SQL Patches to influence execution plans. Rather than burying my responses in the comment section under that post, I thought it would be more useful to do a quick post on it.

What are query blocks?

query block is a basic unit of SQL. For example, any inline view or subquery of a SQL statement are considered separate query blocks to the outer query.

The simple query below has just one sub-query, but it has two Query Blocks—one for the outer SELECT and one for the subquery SELECT.

Oracle automatically names each query block in a SQL statement based on the keyword using the following sort of name; sel$1, ins$2, upd$3, del$4, cri$5, mrg$6, set$7, misc$8, etc.

Given there are two SELECT statements in our query, the query block names will begin with SEL. The outer query will be SEL$1 and the inner query SEL$2.

How do I find the name of a query block?

To find the Query Block name, you can set the FORMAT parameter to ‘+alias’ in the DBMS_XPLAN.DISPLAY_CURSOR command. This will display the contents of the OBJECT_ALIAS column in the PLAN_TABLE, as a new section under the execution plan.

The new section will list the Query Block name for each of the lines in the plan.

SQL_ID 4c8bfsduxhyht, child NUMBER 0
SELECT e.ename, e.deptno FROM emp e WHERE e.deptno IN (SELECT d.deptno 
FROM dept d WHERE d.loc='DALLAS')
Plan hash VALUE: 2484013818
| Id  | Operation	   | Name | ROWS  | Bytes | Cost (%CPU)| TIME	  |
|   0 | SELECT STATEMENT   |	  |	  |	  |	5 (100)|	  |
|*  1 |  HASH JOIN SEMI    |	  |	5 |   205 |	5  (20)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP  |    14 |   280 |	2   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| DEPT |	1 |    21 |	2   (0)| 00:00:01 |
Query Block Name / Object Alias (IDENTIFIED BY operation id):
1 - SEL$5DA710D3
2 - SEL$5DA710D3 / E@SEL$1
3 - SEL$5DA710D3 / D@SEL$2
Predicate Information (IDENTIFIED BY operation id):
1 - access("E"."DEPTNO"="D"."DEPTNO")

As you can see, @SEL1 is the Query Block name for the outer query, where the EMP table is used, and @SEL2 is the Query Block name for the sub-query, where the DEPT tables is used.

Continue reading “What are Query Block Names and how to find them”

SQL Tuning Workshop

Last week I had the pleasure of delivering a five-part SQL Tuning Workshop for my local Oracle User Group –  Northern California Oracle User Group. The workshop explains the fundamentals of the cost-based optimizer, the statistics that feed it, the hints that influence it and key tools you need to exam executions plans.

The workshop also provides a methodology for diagnosing and resolving the most common SQL execution performance problems. Given the volume of interest in this content, I want to share all of the material from the workshop here and give you links to additional material on each of the 5 topics.

Part 1 Understanding the Oracle Optimizer

The first part of the workshop covers the history of the Oracle Optimizer and explains the first thing the Optimizer does when it begins to optimize a query – query transformation.

Query transformations or the rewriting of the SQL statement into a semantically equivalent statement allows the Optimizer to consider alternative methods of processing or executing that query, which are often more efficient than the original SQL statement would allow. the majority of Oracle’s query transactions are now cost-based, which means the Optimizer will cost the plan with and with the query transformation and pick the plan with the lowest cost. With the help of the Optimizer development team, I’ve already blogged about a number of these transformations including:

You can also download the slides here.

Part 2 Best Practices for Managing Optimizer Statistics

Part 2 of the workshop focuses on Optimizer Statistics and the best practices for managing them, including when and how to gather statistics, including fixed object statistics.
Continue reading “SQL Tuning Workshop”

Getting started with Oracle Autonomous Transaction Processing

Getting started with Oracle Autonomous Transaction Processing is actually much easier than you might think. In fact, with Oracle’s $300 in free cloud credits you can probably get your first 30 days on the service for free. Please note, you will require an active email address and credit card in order to sign up for a trial account. Of course, if you have existing cloud credits you can skip this step.

Once you sign up for trail account you’ll get an email with your tenancy, username and password. Armed with this information, head on over to https://cloud.oracle.com to sign in. The video below explains in detailed the simple steps needed to provision a new Autonomous Transaction Processing database. I’ve also listed these steps below the video, for  easy reference.

Continue reading “Getting started with Oracle Autonomous Transaction Processing”

JEFF Talks From Kscope18

The first day of the ODTUG Kscope conference is always symposium Sunday. This year’s Database symposium, organized by @ThatJeffSmith, consisted of multiple, short, rapid  sessions, covering a wide variety of database and database tool topics, similar to Ted Talks but we called then JEFF Talks!

I was lucky enough to present 3 of this year’s JEFF Talks that I thought I would share on my blog since there wasn’t a way to uploaded to the conference site.

In the first session I covered  5 useful tips for getting the most out of your Indexes, including topics like reverse key indexes, partial indexes, and invisible indexes.

Next up was my session on JSON and the Oracle Database. In this session, I covered topics like what data type you should use to store JSON documents (varchar2, clob or blob) the pros and cons of using an IS JSON check constraint, and how to load, index, and query JSON documents.

In my finally JEFF talk I covered some of the useful PL/SQL packages that are automatically supplied with the Oracle Database. Since the talk was only 15 minutes I only touched on 4 of the 300 supplied packages you get with Oracle Database 18c but hopefully it will give you enough of a taste to get you interested in investigating some of the others!



How to determine which view to use

Often times DBAs or application architects create views to conceal complex joins or aggregations in order to help simplify the SQL queries developers need to write.  However, as an application evolves, and the number of views grow, it can often be difficult for a developer to know which view to use.

It also become easier for a developer to write an apparently simple query, that results in an extremely complex SQL statement being sent to the database, which may execute unnecessary joins or aggregations.

The DBMS_UTILITY.EXPAND_SQL_TEXT procedure, introduced in Oracle Database 12.1, allows developers to expand references to views, by turning them into subqueries in the original statement, so you can see just exactly what tables or views are being accessed and what aggregations are being used.

Let’s imagine we have been asked to determine the how many “Flat Whites” we sold in our coffeeshops this month. As a developer, I know I need to access the SALES table to retrieve the necessary sales data and the PRODUCTS table to limit it to just our “Flat Whites” sales but I also know that the DBA has setup a ton of views to make developers lives easier. In order to determine what views I have access to, I’m going to query the dictionary table USER_VIEWS.

SELECT  view_name 
FROM    user_views
WHERE   view_name LIKE '%SALES%';

Based on the list of views available to me, I would likely pick the view called SALES_REPORTING_V or SALES_REPORTING2_V but which would be better?

Let’s use the DBMS_UTILITY.EXPAND_SQL_TEXT procedure to find out. In order to see the underlying query for each view, we can use is a simple “SELECT *” query from each view. First, we will try ‘SELECT * FROM sales_reporting_v‘.

Continue reading “How to determine which view to use”

Does the Explain Plan command really show the execution plan that will be used?

When it comes to SQL tuning we often need to look at the execution plan for a SQL statement to determine where the majority of the time is spent. But how we generate that execution plan can have a big impact on whether or not the plan we are looking at is really the plan that is used.

The two most common methods used to generate the execution plan for a SQL statement are:

EXPLAIN PLAN command – This displays an execution plan for a SQL statement without actually executing the statement.

V$SQL_PLAN A dynamic performance view introduced in Oracle 9i that shows the execution plan for a SQL statement that has been compiled into a cursor and stored in the cursor cache.

My preferred method is always to use V$SQL_PLAN (even though it requires the statement to at least begin executing) because under certain conditions the plan shown by the EXPLAIN PLAN command can be different from the plan that will actually be used when the query is executed.

So, what can cause the plans to differ?

Continue reading “Does the Explain Plan command really show the execution plan that will be used?”