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Hash Warning Event Class

The Hash Warning event class can be used to monitor when a hash recursion or cessation of hashing (hash bailout) has occurred during a hashing operation.

Hash recursion occurs when the build input does not fit into available memory, resulting in the split of input into multiple partitions that are processed separately. If any of these partitions still do not fit into available memory, it is split into subpartitions, which are also processed separately. This splitting process continues until each partition fits into available memory or until the maximum recursion level is reached (displayed in the IntegerData data column).

Hash bailout occurs when a hashing operation reaches its maximum recursion level and shifts to an alternate plan to process the remaining partitioned data. Hash bailout usually occurs because of skewed data.

Hash recursion and hash bailout cause reduced performance in your server. To eliminate or reduce the frequency of hash recursion and bailouts, do one of the following:

  • Make sure that statistics exist on the columns that are being joined or grouped.

  • If statistics exist on the columns, update them.

  • Use a different type of join. For example, use a MERGE or LOOP join instead, if appropriate.

  • Increase available memory on the computer. Hash recursion or bailout occurs when there is not enough memory to process queries in place and they need to spill to disk.

Creating or updating the statistics on the column involved in the join is the most effective way to reduce the number of hash recursion or bailouts that occur.

NoteNote

The terms grace hash join and recursive hash join are also used to describe hash bailout. For more information, see Understanding Hash Joins.

Important noteImportant

To determine where the Hash Warning event is occurring when the query optimizer generates an execution plan, you should also collect a Showplan event class in the trace. You can choose any of the Showplan event classes except the Showplan Text and Showplan Text (Unencoded) event classes, which do not return a Node ID. Node IDs in Showplans identify each operation the query optimizer performs when it generates a query execution plan. These operations are called operators, and each operator in a Showplan has a Node ID. The ObjectID column for Hash Warning events corresponds to the Node ID in Showplans so you can determine which operator, or operation, is causing the error. For more information about the Showplan event classes, see Displaying Execution Plans by Using SQL Server Profiler Event Classes

Data column name

Data type

Description

Column ID

Filterable

ApplicationName

nvarchar

Name of the client application that created the connection to an instance of SQL Server. This column is populated with the values passed by the application rather than with the displayed name of the program.

10

Yes

ClientProcessID

int

ID assigned by the host computer to the process where the client application is running. This data column is populated if the client provides a client process ID.

9

Yes

DatabaseID

int

ID of the database specified by the USE database statement or the default database if no USE database statement has been issued for a given instance. SQL Server Profiler displays the name of the database if the ServerName data column is captured in the trace and the server is available. Determine the value for a database by using the DB_ID function.

3

Yes

DatabaseName

nvarchar

Name of the database in which the user statement is running.

35

Yes

EventClass

int

Type of event = 55.

27

No

EventSequence

int

Sequence of a given event within the request.

51

No

EventSubClass

int

Type of event subclass.

0=Recursion

1=Bailout

21

Yes

GroupID

int

ID of the workload group where the SQL Trace event fires.

66

Yes

HostName

nvarchar

Name of the computer on which the client is running. This data column is populated if the client provides the host name. To determine the host name, use the HOST_NAME function.

8

Yes

IntegerData

int

Recursion level (hash recursion only).

25

Yes

IsSystem

int

Indicates whether the event occurred on a system process or a user process. 1 = system, 0 = user.

60

Yes

LoginName

nvarchar

Name of the login of the user (either SQL Server security login or the Windows login credentials in the form of <DOMAIN>\<username>).

11

Yes

LoginSid

image

Security identification number (SID) of the logged-in user. You can find this information in the sys.server_principals catalog view. Each SID is unique for each login in the server.

41

Yes

NTDomainName

nvarchar

Windows domain to which the user belongs.

7

Yes

NTUserName

nvarchar

Windows user name.

6

Yes

ObjectID

int

Node ID of the root of the hash team involved in the repartition. Corresponds with the Node ID in Showplans.

22

Yes

RequestID

int

ID of the request that contains the statement.

49

Yes

ServerName

nvarchar

Name of the instance of SQL Server that is being traced.

26

SessionLoginName

nvarchar

Login name of the user who originated the session. For example, if you connect to SQL Server using Login1 and execute a statement as Login2, SessionLoginName shows Login1 and LoginName shows Login2. This column displays both SQL Server and Windows logins.

64

Yes

SPID

int

ID of the session on which the event occurred.

12

Yes

StartTime

datetime

Time at which the event started, if available.

14

Yes

TransactionID

bigint

System-assigned ID of the transaction.

4

Yes

XactSequence

bigint

Token that describes the current transaction.

50

Yes

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