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Paul White: Page Free Space

A technical SQL Server blog from New Zealand.

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  • Nested Loops Prefetching

    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 Read More...
  • Execution Plan Analysis: The Mystery Work Table

    I love SQL Server execution plans. It is often easy to spot the cause of a performance problem just by looking at one. The task is considerably easier if the plan includes run-time information (a so-called ‘actual’ execution plan), but even a compiled Read More...
  • 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 important storage engine considerations like locking and transaction log throughput that can have dramatic effects. As Read More...
  • Parallel Execution Plans Suck

    Summary: A deep dive into SQL Server parallelism, and a potential performance problem with parallel plans that use TOP. There was an interesting question asked by Mark Storey-Smith on dba.stackexchange.com back in October 2011. He was looking at the execution Read More...
  • Fun with Aggregates

    There are interesting things to be learned from even the simplest queries. For example, imagine you are given the task of writing a query to list AdventureWorks product names where the product has at least one entry in the transaction history table, but Read More...
  • Forcing a Parallel Query Execution Plan

    This post is for SQL Server developers who have experienced the special kind of frustration, which only comes from spending hours trying to convince the query optimizer to generate a parallel execution plan.  This situation often occurs when making Read More...
  • Join Performance, Implicit Conversions, and Residuals

    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 Read More...
  • Beware Sneaky Reads with Unique Indexes

    A few days ago, Sandra Mueller (twitter | blog) asked a question using twitter’s #sqlhelp hash tag: “Might SQL Server retrieve (out-of-row) LOB data from a table, even if the column isn’t referenced in the query?” Leaving aside trivial cases (like selecting a computed column that does reference the LOB data), one might be tempted to say that no, SQL Server does not read data you haven’t asked for. In general, that’s quite correct; however there are cases where SQL Server might sneakily retrieve a LOB column… Read More...
  • Myth: SQL Server Caches a Serial Plan with every Parallel Plan

    Many people believe that whenever SQL Server creates an execution plan that uses parallelism, an alternative serial plan is also cached. The idea seems to be that the execution engine then decides between the parallel and serial alternatives at runtime. I’ve seen this on forums, in blogs, and even in books. In fairness, a lot of the official documentation is not as clear as it might be on the subject. In this post I will show that only a single (parallel) plan is cached. I will also show that SQL Server can execute a parallel plan on a single thread… Before diving into the demonstration, I want to quickly run through some background information about the SQL Server plan cache. Compiled Plans Queries are expensive to compile and optimize, so SQL Server uses caching to improve efficiency through plan reuse. The server optimizes an entire batch all at once, and the result is known as a compiled plan (or sometimes ‘query plan’). The dynamic management view sys.dm_exec_cached_plans contains one row for each compiled plan, with a plan_handle that uniquely identifies the compiled plan among those currently in cache (plan handles can be reused over time). This plan handle can be passed to the dynamic management function sys.dm_exec_query_plan to show the compiled plan in XML format. When displayed in Management Studio, we can click on this XML representation to view the familiar graphical plan. A compiled plan is a compile-time object – no user or runtime context is stored. You might find it helpful to think of the compiled plan as a template – or perhaps as being similar to the estimated execution plans seen in Management Studio. Execution Contexts An execution context (or ‘executable plan’) - internally known as an MXC – is generated when a compiled plan is prepared for execution. Execution contexts contain specific runtime information, for a single execution, for a single user. Read More...
  • A Tale of Two Index Hints

    If you look up Table Hints in Books Online, you’ll find the following statement: 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. Read More...
  • Row Goals and Grouping

    You might recall (from my last post) that query plans containing a row goal tend to favour nested loops or merge join over hashing. This is because a hash join has to fully process its build input (to populate its hash table) before it can start probing for matches from its second input. Hash join therefore has a high start-up cost, which is balanced by a lower per-row cost once probing begins. In this post, I’ll take a look at how row goals affect grouping operations. Grouping Strategies While the start-up cost of hash join often makes it unsuitable for plans with a row goal, there are times when hashing operations may feature in such plans, since the Hash Match iterator also supports a streaming mode. As an example, say we are asked to list one hundred unique first names from the AdventureWorks Contacts table: Read More...
  • Inside the Optimizer: Row Goals In Depth

    Background One of the core assumptions made by the SQL Server query optimiser’s model is that clients will consume all of the rows produced by a query. This results in plans that favour the overall execution cost, though it may take longer to begin producing rows. Let’s look at an example: The optimiser chooses to perform the logical join using a Hash Match physical iterator, resulting in a plan with a total estimated cost of around 1.4 units. By forcing alternative physical joins using a query hint, we see that a plan based on Sort-Merge would have an estimated cost of just under 10, and using Nested Loops would cost over 18 units. All these cost estimates are based on the assumption that all rows are required. Hash Match As detailed in a previous post, the Hash Match iterator starts by consuming all rows produced by its build input (the Product table) in order to build a hash table. This makes Hash Match a semi-blocking iterator: it can only start producing output rows once the build phase is complete. If we need the first few rows from the query quickly, this join type may not be optimal. Read More...
  • The Impact of Non-Updating Updates

    From time to time, I encounter a system design that always issues an UPDATE against the database after a user has finished working with a record – without checking to see if any of the data was in fact altered. The prevailing wisdom seems to be that “the database will sort it out”. This raises an interesting question: how smart is SQL Server in these circumstances? In this post, I’ll look at a generalisation of this problem: what is the impact of updating a column to the value it already contains? The specific questions I want to answer are: Does this kind of UPDATE generate any log activity? Do any data pages get marked as dirty (and so eventually get written out to disk)? Does SQL Server bother doing the update at all? Read More...
  • Iterators, Query Plans, and Why They Run Backwards

    SQL Server uses an extensible architecture for query optimisation and execution, using ‘iterators’ as basic building-blocks. Iterators are probably most familiar in their graphical showplan representation, where each icon represents a single iterator. They also show up in XML query plan output as RelOp nodes. Each iterator performs a single simple function, such as applying a filtering condition, or performing an aggregation. It can represent a logical operation, a physical operation, or (most often) both. For example, ‘Aggregate’ is a logical operation, and Stream Aggregate and Hash Aggregate are physical operations. Similarly, ‘Inner Join’ is a logical operation; Nested Loops a physical one. Both graphical plans and XML showplan output show these properties: Read More...
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