"But It Worked In Development!" - 3 Hard SQL Server Problems

Slides & scripts:

BrentOzar.com/go/3hardproblems

Agenda

About my server & workload

Problem #1: RESOURCE_SEMAPHORE

Problem #2: THREADPOOL

Problem #3: LOCK ESCALATION

SQL2016A

VM with 4 cores, 8GB RAM

Max server memory: 6GB RAM

Stack Overflow database: 100GB

dbo.Users table: <1GB

Problem 1 Demo:

RESOURCE_

SEMAPHORE

Symptoms

RESOURCE_SEMAPHORE waits present

Page life expectancy drops

(sometimes without long-running-queries happening)

SQLServer: Memory Manager – Memory Grants Pending

counter goes > 0

Queries take an unpredictably long time to start

Today’s fix

The query:

SELECT TOP 250 *FROM dbo.UsersORDER BY Reputation DESC;

The fix:

• Get just the fields we need

• Get less rows

Where to learn more

Find queries with high memory grants:

sp_BlitzCache @SortOrder = 'memory grant'

Bonus points: train developers to watch this in dev, spotting

high grant queries before going to production

Paul White: Sorting, Row Goals, and TOP 100

http://sqlblog.com/blogs/paul_white/archive/2010/08/27/sort

ing-row-goals-and-the-top-100-problem.aspx

Problem 2 Demo:

THREADPOOL

Symptoms

Apps, users can’t connect to the SQL Server

Monitoring tools show gaps in history with no metrics

SQL Server error log doesn’t show anything unusual

Today’s fixCreate an index to let the readers go past:

CREATE INDEX IX_Reputation ON dbo.Users(Reputation) INCLUDE (DisplayName, Location);

Other great options:

• Commit/shorten your transactions, or

• Consider different isolation levels, or

• Ease up on app server quantity & workloads

Where to learn more

Enable & use the Dedicated Admin Connection:

https://www.mssqltips.com/sqlservertip/1801/enable-sql-

server-dedicated-administrator-connection/

When it hits, train someone to log in and save sp_BlitzWho

or sp_WhoIsActive results to a table

Use sp_BlitzIndex to find “aggressive indexes” – indexes

with long lock waits, and tune those tables

Problem 3 Demo:

LOCK ESCALATION(which isn’t usually typed in all capitals, but we’re on a roll here)

Symptoms

Production & development have different data sizes

Queries that worked fine suddenly start blocking

Queries that take wildly variable workloads:

• Data warehouse nightly loads

• User-driven batch processing

Varying execution plans for writes

(narrow vs wide plans for different parameters)

Today’s fixIf writers are blocking readers (or vice versa),

let’s try Read Committed Snapshot Isolation (RCSI):

ALTER DATABASE [StackOverflow] SET READ_COMMITTED_SNAPSHOT ON WITH NO_WAIT;

Other great options:

• Work in bigger batches (update everything at once), or

• Purposely work in small batches, committing as you go to

avoid triggering escalation, or

• Consider different isolation levels

Where to learn moreMichael J. Swart on batch coding:http://michaeljswart.com/2014/09/take-care-when-scripting-batches/

SQLCAT team’s Fast Ordered Deletes technique, archived by John Sansom:https://www.johnsansom.com/fast-sql-server-delete/

Support KB #323630 on lock escalation: https://support.microsoft.com/en-us/help/323630/

As data gets bigger, so

do your problems.