The Halloween Problem – Part 1

Much has been written over the years about understanding and optimizing SELECT queries, but rather less about data modification. This series looks at an issue that is specific to INSERT, UPDATE, DELETE and MERGE queries – the Halloween Problem.

The phrase “Halloween Problem” was originally coined with reference to a SQL UPDATE query that was supposed to give a 10% raise to every employee who earned less than $25,000. The problem was that the query kept giving 10% raises until everyone earned at least $25,000.

We will see later on in this series that the underlying issue also applies to INSERT, DELETE and MERGE queries, but for this first entry, it will be helpful to examine the UPDATE problem in a bit of detail.

Incorrect Results with Indexed Views

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:

• SQL Server 2012
• SQL Server 2008 R2
• SQL Server 2008

The Knowledge Base article does not go into detail, or provide a reproduction script, but this blog post does.

A creative use of IGNORE_DUP_KEY

Let’s say you have a big table with a clustered primary key, and an application that inserts batches of rows into it. The nature of the business is that the batch will inevitably sometimes contain rows that already exist in the table.

The default SQL Server INSERT behaviour for such a batch is to throw error 2627 (primary key violation), terminate the statement, roll back all the inserts (not just the rows that conflicted) and keep any active transaction open:

Optimizing T-SQL queries that change data

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.

MERGE Bug with Filtered Indexes

A MERGE statement can fail, and incorrectly report a unique key violation when:

• The target table uses a unique filtered index; and
• No key column of the filtered index is updated; and
• A column from the filtering condition is updated; and
• Transient key violations are possible

Cardinality Estimation Bug with Lookups

Estimated row counts on Key or RID Lookups where a filtering predicate is applied can be wrong in SSMS execution plans.

This error does not affect the optimizer’s ultimate plan selection, but it does look odd.

There are other cases where estimated row counts are inconsistent (for defensible reasons) but the behaviour shown in this post in certainly a bug.

Why Doesn’t Partition Elimination Work?

Given a partitioned table and a simple SELECT query that compares the partitioning column to a single literal value, why does SQL Server read all the partitions when it seems obvious that only one partition needs to be examined?

Compute Scalars, Expressions and Execution Plan Performance

The humble Compute Scalar is one of the least well-understood of the execution plan operators, and usually the last place people look for query performance problems. It often appears in execution plans with a very low (or even zero) cost, which goes some way to explaining why people ignore it.

Some readers will already know that a Compute Scalar can contain a call to a user-defined function, and that any T-SQL function with a BEGIN…END block in its definition can have truly disastrous consequences for performance (see When is a SQL function not a function? by Rob Farley for details).

This post is not about those sorts of concerns.

Deletes that Split Pages and Forwarded Ghosts

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.

Temporary Table Caching Explained

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):

• Named constraints (bad idea anyway, since concurrent executions can cause a name collision)
• DDL after creation (though what is considered DDL is interesting)
• Creation using dynamic SQL
• Table created in a different scope
• Procedure executed using WITH RECOMPILE

Temporary objects are often created and destroyed at a high rate in production systems, so caching can be an important optimization.

Temporary Table Caching in Stored Procedures

Introduction

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.

Parallel Execution Plans Suck

Summary: A deep dive into SQL Server parallelism, and a potential performance problem with parallel plans that use TOP.

Query Optimizer Deep Dive - Part 4

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

Beating the Optimizer

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.

Query Optimizer Deep Dive – Part 3

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

Storage of Alternative Plans

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.

Query Optimizer Deep Dive – Part 2

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

Cost-Based Optimization Overview

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.

Query Optimizer Deep Dive - Part 1

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

Introduction

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.

Fun with Scalar and Vector Aggregates

There are interesting things to be learned from even the simplest queries.

For example, imagine you are asked to write a query that lists AdventureWorks product names, where the product has at least one entry in the transaction history table, but fewer than ten.

Dynamic Seeks and Hidden Implicit Conversions

A LIKE predicate with only a trailing wildcard can usually use an index seek, as the following AdventureWorks sample database query shows:

SELECT 
    P.[Name]
FROM Production.Product AS P
WHERE 
    P.[Name] LIKE N'D%';

Forcing a Parallel Query Execution Plan

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.

SQL Server Optimizer Bug with JOIN and GROUP BY

I came across a SQL Server optimizer bug recently that made me wonder how on earth I never noticed it before.

As the title of this post suggests, the bug occurs in common JOIN and GROUP BY queries. While it does not cause incorrect results to be returned, it will often cause a poor query plan to be selected by the optimizer.

If you are just interested in the bug itself, you will find a description in the section headed “the bug revealed”. It relates to cardinality estimation for serial partial aggregates.

As the regular reader will be expecting though, I am going to work up to it with a bit of background. The lasting value of this post (once the bug is fixed) is in the background details anyway.

Is Distinct Aggregation Still Considered Harmful?

Back in 2008, Marc Friedman of the SQL Server Query Processor Team wrote a blog entry entitled “Distinct Aggregation Considered Harmful”.

Marc shows a way to work around the poor performance that often results simply from adding the keyword DISTINCT to an otherwise perfectly reasonable aggregate function in a query.

This post is an update to that work, presenting a query optimizer enhancement in SQL Server 2012 that reduces the need to perform the suggested rewrite manually.

Finding the Statistics Used to Compile an Execution Plan

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.

Can a SELECT query cause page splits?

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.

SQL Server, Seeks, and Binary Search

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).

Avoiding Uniqueness for Performance

In my last post, Enforcing Uniqueness for Performance, I showed how using a unique index could speed up equality seeks by around 40%.