Nested loops join query plans can be a lot more interesting (and complicated) than is commonly realized.
One query plan area I get asked about a lot is prefetching. It is not documented in full detail anywhere, so this seems like a good topic to address in a blog post.
The examples used in this article are based on questions asked by Adam Machanic.
I love SQL Server execution plans. It is often easy to spot the cause of a performance problem just by looking at one closely. That task is considerably easier if the plan includes run-time information (a so-called ‘actual’ execution plan), but even a compiled plan can be very useful.
Nevertheless, there are still times when the execution plan does not tell the whole story, and we need to think more deeply about query execution to really understand a problem. This post looks at one such example, based on a question I answered.
If you use MERGE, indexed views and foreign keys, your queries might return incorrect results. Microsoft have released a fix for incorrect results returned when querying an indexed view. The problem applies to:
The Knowledge Base article does not go into detail, or provide a reproduction script, but this blog post does.
Most tuning efforts for data-changing operations concentrate on the SELECT side of the query plan. Sometimes people will also look at storage engine considerations (like locking or transaction log throughput) that can have dramatic effects. A number of common practices have emerged, such as avoiding large numbers of row locks and lock escalation, splitting large changes into smaller batches of a few thousand rows, and combining a number of small changes into a single transaction in order to optimize log flushes.
This is all good, but what about the data-changing side of the query plan — the INSERT, UPDATE, DELETE, or MERGE operation itself — are there any query processor considerations we should take into account? The short answer is yes.
The query optimizer considers different plan options for the write-side of an execution plan, though there isn’t a huge amount of T-SQL language support that allows us to affect these choices directly. Nevertheless, there are things to be aware of, and things we can look to change.
Can DELETE operations cause pages to split?
Yes. It sounds counter-intuitive on the face of it. Deleting rows frees up space on a page, and page splitting occurs when a page needs additional space. Nevertheless, there are circumstances when deleting rows causes them to expand before they can be deleted.
SQL Server (since 2005) caches temporary tables and table variables referenced in stored procedures for reuse, reducing contention on tempdb allocation structures and catalogue tables.
A number of things can prevent this caching (none of which are allowed when working with table variables):
WITH RECOMPILETemporary objects are often created and destroyed at a high rate in production systems, so caching can be an important optimization.
Ask anyone what the primary advantage of temporary tables over table variables is, and the chances are they will say that temporary tables support statistics and table variables do not.
This is true, of course. The indexes that enforce PRIMARY KEY and UNIQUE constraints on table variables do not have populated statistics associated with them. Neither do any non-constraint table variable indexes (using inline index definitions, available starting with SQL Server 2014). Finally, it is not possible to manually create statistics on table variables.
Intuitively, then, any query that has alternative execution plans to choose from ought to benefit from using a temporary table rather than a table variable. This is also true, up to a point.
This is the final part in a series of posts based on the content of the Query Optimizer Deep Dive presentations I have given over the last month or so at the Auckland SQL Users’ Group, and SQL Saturday events in Wellington, New Zealand and Adelaide, Australia.
Links to other parts of this series: Part 1 Part 2 Part 3
Our AdventureWorks test query produces an optimized physical execution plan that is quite different from the logical form of the query.
The estimated cost of the execution plan shown below is 0.0295 units.
Since we know the database schema very well, we might wonder why the optimizer did not choose to use the unique nonclustered index on Name in the Product table to filter rows based on the LIKE predicate.
This is the third in a series of posts based on the content of the Query Optimizer Deep Dive presentations I have given over the last month or so at the Auckland SQL Users’ Group, and SQL Saturday events in Wellington, New Zealand and Adelaide, Australia.
Links to other parts of this series: Part 1 Part 2 Part 4
We saw in part 2 how optimizer rules are used to explore logical alternatives for parts of the query tree, and how implementation rules are used to find physical operations to perform each logical steps.
To keep track of all these options, the cost-based part of the SQL Server query optimizer uses a structure called the Memo. This structure is part of the Cascades general optimization framework developed by Goetz Graefe.
This is the second in a series of posts based on the content of the Query Optimizer Deep Dive presentations I have given over the last month or so at the Auckland SQL Users’ Group, and SQL Saturday events in Wellington, New Zealand and Adelaide, Australia.
Links to other parts of this series: Part 1 Part 3 Part 4
The input to cost-based optimization is a tree of logical operations produced by the previous optimization stages discussed in part one.
Cost-based optimization takes this logical tree, explores logical alternatives (different logical tree shapes that will always produce the same results), generates physical implementations, assigns an estimated cost to each, and finally chooses the cheapest physical option overall.
The goal of cost-based optimization is not to find the best possible physical execution plan by exploring every possible alternative. Rather, the goal is to find a good plan quickly.
This is the first in a series of posts based on the content of the Query Optimizer Deep Dive presentations I have given over the last month or so at the Auckland SQL Users’ Group, and SQL Saturday events in Wellington, New Zealand and Adelaide, Australia.
Links to other parts of this series: Part 2 Part 3 Part 4
The motivation behind writing these sessions is finding that relatively few people have a good intuition for the way the optimizer works. This is partly because the official documentation is rather sparse, and partly because what information is available is dispersed across many books and blog posts.
The content presented here is very much geared to my preferred way of learning. It shows the concepts in what seems to me to be a reasonably logical sequence, and then provides tools to enable the interested reader to explore further, if desired.
This article is for SQL Server developers who have experienced the special kind of frustration that only comes from spending hours trying to convince the query optimizer to generate a parallel execution plan.
This situation often occurs when making an apparently innocuous change to the text of a moderately complex query — a change which somehow manages to turn a parallel plan that executes in ten seconds, into a five-minute serially-executing monster.
In this post, I show you how to determine the statistics objects used by the query optimizer in producing an execution plan.
Note: This technique only applies to queries compiled using the original (70) cardinality estimation model.
The SQL Server documentation has this to say about page splits:
When a new row is added to a full index page, the Database Engine moves approximately half the rows to a new page to make room for the new row. This reorganization is known as a page split. A page split makes room for new records, but can take time to perform and is a resource intensive operation. Also, it can cause fragmentation that causes increased I/O operations.
Given that, how can a SELECT statement be responsible for page splits?
Well, I suppose we could SELECT from a function that adds rows to a table variable as part of its internal implementation, but that would clearly be cheating, and no fun at all from a blogging point of view.
The following table summarizes the results from my last two articles, Enforcing Uniqueness for Performance and Avoiding Uniqueness for Performance. It shows the CPU time used when performing 5 million clustered index seeks into a unique or non-unique index:
In test 1, making the clustered index unique improved performance by around 40%.
In test 2, making the same change reduced performance by around 70% (on 64-bit systems – more on that later).
You probably already know that it’s important to be aware of data types when writing queries, and that implicit conversions between types can lead to poor query performance.
Some people have gone so far as to write scripts to search the plan cache for CONVERT_IMPLICIT elements, and others routinely inspect plans for that type of thing when tuning.
Now, that’s all good, as far as it goes. It may surprise you to learn that not all implicit conversions are visible in query plans, and there are other important factors to consider too.
As usual, here’s a sample table:
CREATE TABLE #Example
(
pk numeric IDENTITY PRIMARY KEY NONCLUSTERED,
col1 sql_variant NULL,
col2 sql_variant NULL,
thing sql_variant NOT NULL,
);
Some sample data:
And an index that will be useful shortly:
CREATE INDEX nc1
ON #Example
(col1, col2, thing);
There’s a complete script to create the table and add the data at the end of this post. There’s nothing special about the table or the data (except that I wanted to have some fun with values and data types).
The diagram below shows two data sets, with differences highlighted:
To find changed rows using T-SQL, we might write a query like this:
The logic is clear: Join rows from the two sets together on the primary key column, and return rows where a change has occurred in one or more data columns.
Unfortunately, this query only finds one of the expected four rows:
The problem is that our query does not correctly handle NULLs.
If you look up Table Hints in the official documentation, you’ll find the following statements:
If a clustered index exists, INDEX(0) forces a clustered index scan and INDEX(1) forces a clustered index scan or seek.
If no clustered index exists, INDEX(0) forces a table scan and INDEX(1) is interpreted as an error.
The interesting thing there is that both hints can result in a scan. If that is the case, you might wonder if there is any effective difference between the two.
This blog entry explores that question, and highlights an optimizer quirk that can result in a much less efficient query plan when using INDEX(0). I’ll also cover some stuff about ordering guarantees.
A detailed look at costing, and more undocumented optimizer fun.
The SQL Server query optimizer generates a number of physical plan alternatives from a logical requirement expressed in T-SQL. If full cost-based optimization is required, a cost is assigned to each iterator in each alternative plan, and the plan with the lowest overall cost is ultimately selected for execution.