The strategy selected by the query optimiser to process a query.

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20
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3answers
5k views

Parameter Sniffing vs VARIABLES vs Recompile vs OPTIMIZE FOR UNKNOWN

So we had a long running proc causing problems this morning (30 sec + run time). We decided to check to see if parameter sniffing was to blame. So, we rewrote the proc and set the incoming ...
18
votes
0answers
2k views

Guide to reading, comparing, and interpretting Execution Plans in SQL Server [closed]

I'm looking for a good, well-written and explained guide on reading and comparing execution plans in SQL Server. This is for more my own edification as well as for sharing with others. If you want ...
17
votes
2answers
2k views

Optimising plans with XML readers

Executing the query from here to pull the deadlock events out of the default extended events session SELECT CAST ( REPLACE ( REPLACE ( XEventData.XEvent.value ...
15
votes
4answers
623 views

Execution plan vs STATISTICS IO order

SQL Server graphical execution plans read right to left and top to bottom. Is there a meaningful order to the output generated by SET STATISTICS IO ON? The following query: SET STATISTICS IO ON; ...
15
votes
2answers
1k views

T-SQL query using completely different plan depending on number of rows I'm updating

I have a SQL UPDATE statement with a "TOP (X)" clause, and the row I'm updating values in has about 4 billion rows. When I use "TOP (10)", I get one execution plan that executes almost instantly, but ...
14
votes
3answers
13k views

Execution Plan Basics — Hash Match Confusion

I am starting to learn execution plans and am confused about how exactly a hash match works and why it would be used in a simple join: select Posts.Title, Users.DisplayName From Posts JOIN Users on ...
13
votes
3answers
3k views

Can you explain this execution plan?

I was researching something else when I came across this thing. I was generating test tables with some data in it and running different queries to find out how different ways to write queries affects ...
13
votes
1answer
1k views

Why does SQL Server “Compute Scalar” when I SELECT a persisted computed column?

The three SELECT statements in this code USE [tempdb]; GO SET NOCOUNT ON; CREATE TABLE dbo.persist_test ( id INT NOT NULL , id5 AS (id * 5) , id5p AS (id ...
12
votes
2answers
2k views

How (and why) does TOP impact an execution plan?

For a moderately complex query I am trying to optimize, I noticed that removing the TOP n clause changes the execution plan. I would have guessed that when a query includes TOP n the database engine ...
11
votes
1answer
252 views

How deterministic is execution plan creation in SQL Server?

Given the following constants: The same database with the same structure (tables, indexes, etc) The same data The same SQL Server and hardware configuration The same statistics The same SET options ...
10
votes
4answers
5k views

Index Seek vs Index Scan

Looking at an execution plan of a slow running query and I noticed that some of the nodes are index seek and some of them are index scan. What is the difference between and index seek and an index ...
10
votes
1answer
914 views

Does SQL Server 2008 store the creation date of execution plans?

We recently upgraded an application we use, which involved modifying the schema for the database. These changes could have forced cached execution plans to be discarded. If SQL Server was forced to ...
9
votes
1answer
562 views

Forcing an index spool

I know its something that should be avoided for performance reasons, but am trying to show a condition where it appears as a demo on how to make sure it does not appear. However, I end up with a ...
9
votes
3answers
636 views

Why are there execution plan differences between OFFSET … FETCH and the old-style ROW_NUMBER scheme?

The new OFFSET ... FETCH model introduces with SQL Server 2012 offers simple and faster paging. Why are there any differences at all considering that the two forms are semantically identical and very ...
9
votes
1answer
3k views

Are there any risks to granting users SQL Server SHOWPLAN permission?

I'm doing some performance tuning on a large SQL server 2008 database, and the IT group is unwilling to give SHOWPLAN permission. In the past, "Show Execution Plan" has been the most effective way to ...
9
votes
1answer
1k views

Interpreting SQL Server's Showplan XML

I just rolled out a feature on my site http://sqlfiddle.com that allows users to view the raw execution plans for their queries. In the case of PostgreSQL, MySQL, and (to some extent) Oracle, looking ...
8
votes
1answer
376 views

Should I be alarmed by this NO JOIN PREDICATE warning?

I'm troubleshooting the bits and pieces of a poorly-performing stored procedure. This section of the procedure is throwing a NO JOIN PREDICATE warning select method = case ...
8
votes
1answer
263 views

Re-run a specific actual query plan

I have captured an actual query plan for a specific query. After this I've changed a few things around (including updating the statistics) and re-ran that specific query. Now the actual query plan ...
8
votes
1answer
228 views

Does SQL Sentry Plan Explorer count the reads in UDFs?

I have a query like this: select dbo.fn_complexFunction(t.id) from mytable t In SQL Sentry Plan Explorer, I noticed I have to run Get Estimated Plan in SQL Sentry to make the Query Plan include the ...
8
votes
1answer
193 views

Are the cost percentages in this SQL Server plan over 100% for a valid reason?

I'm looking through the plan cache, looking for low-hanging optimization fruit and came across this snippet: Why are many of the costs listed above 100% ? Shouldn't that be impossible?
7
votes
2answers
471 views

Code creates different plan when ran ad-hoc vs. in a stored procedure

I have a delete statement that is using a bad plan when run inside a stored procedure, but is choosing a much better plan when run ad-hoc. I have rebuilt all the indexes for the tables used by the ...
7
votes
1answer
140 views

Query plan cache bloated by ad-hoc queries, even with “Optimize for Ad-hoc Workloads”

I've been noticing what I thought to be unusual issues with our query plan cache, where the plans in the cache were never more than a day old. Through running the following query (courtesy of ...
6
votes
3answers
423 views

Are SQL Server statistics stored in database or buffer pool?

Just wondering are statistics kept in the database but not in the memory? If I backup/restore the database from a prod server to a development server, would it keep the same statistics so that ...
6
votes
2answers
374 views

Query Plans not retained insufficient memory errors

We have been experiencing memory issues with SQL Server. We first realised we had a problem when we started getting timeouts and login errors: A connection was successfully established with the ...
6
votes
1answer
298 views

Why does the plan cache contain estimated and not actual execution plans?

I can think of many reasons behind this decision of storing estimated plans in the plan cache and not the actual plan. But I can't find the "correct" answer.
6
votes
2answers
2k views

How to optimise T-SQL query using Execution Plan

I have a SQL query that I have spent the past two days trying to optimise using trial-and-error and the execution plan, but to no avail. Please forgive me for doing this but I will post the entire ...
6
votes
0answers
141 views

Need help understanding query plan: estimates vs. actuals [closed]

I'd like to understand a section of the query plan that seems to mess up my query. The picture shows that there's a clustered index seek, followed by compute scalar, followed by sort. The part ...
5
votes
1answer
439 views

The use of NOT logic in relation to indexes

According to Microsoft's book on database development Exam 70-433: Microsoft SQL Server 2008 Database Development: Neither leading wildcard characters not NOT logic allow the query optimizer to ...
5
votes
2answers
287 views

Correlated subquery and join: still the same exectution plan?

I have a correlated subquery like this (from BOL): SELECT DISTINCT c.LastName, c.FirstName, e.BusinessEntityID FROM Person.Person AS c JOIN HumanResources.Employee AS e ON e.BusinessEntityID = ...
5
votes
1answer
375 views

how to improve performance by using or not using table variables

I have a table variable: DECLARE @to_process TABLE ( [Id] [bigint] NOT NULL, [SequenceId] [bigint] NOT NULL, ... ) INSERT INTO @to_process ( Id , SequenceId ... ) SELECT ...
5
votes
2answers
333 views

Index seek much slower with OR condition compared with separate SELECTs

Based on these questions and the answers given: SQL 2008 Server - performance loss possibly connected with a very large table Large table with historical data allocates too much of SQL Server 2008 ...
5
votes
1answer
858 views

Storage order vs Result order

This is a spin-off question from Sort order specified in primary key, yet sorting is executed on SELECT. @Catcall says this on the subject of storage order (clustered index) and the output order ...
5
votes
1answer
2k views

Why does it take so long to calculate an Execution Plan?

One of our customers has just upgraded to a new server. For a particular stored procedure the first time you execute it, is taking over three minutes to run. Subsequent runs are less than 1 second. ...
5
votes
1answer
512 views

Execution plan using B+ tree index, but also sorts

I'm using Oracle sqlplus. And I have the following query: SELECT fooID from foo MINUS SELECT fooID from bar; I have created two unclustered B+ tree indexes. One in the field fooIDof the table foo ...
5
votes
1answer
130 views

Capture Complex Actual Execution Plans

I have a stored procedure that among other things builds and executes a large number of Dynamic SQL statements. (Don't hate me, I didn't write it.) I would like to retrieve the actual execution plan ...
4
votes
4answers
1k views

When is a full table scan better than index scan?

What are some scenarios (in SQL Server) in which a full table scan is better than an index scan?
4
votes
3answers
1k views

Large set of execution plans hangs up SSMS

I have a stored procedure that takes about 3-5 seconds that I'm trying to understand so I want an execution plan for it. When I run it in SQL Server Management Studio with Execution Plan enabled, it ...
4
votes
1answer
161 views

Why clustered index scan?

I just started to learn to optimize my queries and to analyze their query plans. I thought this query would generate nonclustered index seek + key lookup. SELECT ct.* FROM Person.ContactType AS ct ...
4
votes
2answers
113 views

Performance of different precedence pseudocolumn solutions with LIMIT

I was wondering about the relative performance of the solutions provided in the answers to a question on stackoverflow, I decided to run some tests. The OP wanted to get the first matching row given ...
4
votes
3answers
1k views

SQL Server 2005 get execution plan of a overnight stored procedure

There is a overnight job runs long time. I have used profiler to get the trace and identified a statement (in a stored procedure) that takes most of execution time. Now I need to see the execution ...
4
votes
1answer
150 views

Optimize for ad-hoc workload

I know what this option does and how to enable it. My question is what things will happen if I were to enable this. Without giving too much info, our accounting system is a Microsoft Dynamics ...
4
votes
2answers
523 views

Why is my plan guide not being used?

We recently ran into the tipping point issue and some of our report queries that used to complete execution within a couple of seconds are now taking more than 2 min because the query optimizer simply ...
4
votes
2answers
1k views

How do I get the execution plan for a view?

I have a schema with a number of views. I need to check the execution plans to make sure the appropriate indexes are in place and being used. How do I do this? I'd rather not have to copy and paste ...
4
votes
2answers
361 views

Oracle build_plan_xml visualizer

Background: Please review my related question that involves SQL Server : Interpreting SQL Server's Showplan XML I now have a very nice looking Execution Plan display for SQL Server (as a result of my ...
4
votes
2answers
208 views

Performance on SQL Join

I have a query which takes ages to run mainly due to a join between 2 tables on a PK and FK. The Key is prefixed with 2 letters (which is always the same, RN) followed by 7 digits So RN1234567 for ...
4
votes
1answer
125 views

Filtered indices: Why include the filtered-on field?

This just drives me mad. Consider a simple table with irrelevant columns removed: create table boxes ( row_id int not null identity(1,1) primary key, location varchar(15) null, dismantled bit ...
4
votes
1answer
108 views

Are database query optimizers aware of storage performance differences?

As I understand it, the query optimizer in SQL Server (or any other RDBMS, really) isn't aware of the performance of the storage underneath the database, and will make decisions as if all storage has ...
4
votes
1answer
914 views

SQL Server 2008 query planner failing after index drop & recreate

Recently we ran a script to our production DB that would dynamically drop and recreate hundreds of indexes as filtered indexes. While this script had run perfectly in all other previous tests, now ...
3
votes
3answers
821 views

How does NHibernate handle execution plan?

I heard from a podcast that there are no ORMs that have a good solution for execution plan reuse. It will lead to increased execution plan cache which affects the performance. How does NHibernate ...
3
votes
1answer
279 views

How to determine cause of runtime increase given two query plans with SpillToTempDb warning

Two actual query plans were captured for two executions of one query: Plan "Fast" took ~1s and occurs about 90% of the time. Plan "Slow" took ~16s. Since the graphical plans look ...