SQL Server Hints

SQL Server has three kinds hints, each has its own usage. Below are the hints with its usage.

Join Hints this will directly specify in the join format. such as "left outer hash join" "inner merge join"
Table Hints with( tablehints)
Query Hints option(queryhins)

Join hints:

<join_hint> ::= { LOOP | HASH | MERGE | REMOTE } apply to DELETE SELECT UPDATE

LOOP | HASH | MERGE

Specifies that the join in the query should use looping, hashing, or merging. Using LOOP |HASH | MERGE JOIN enforces a particular join between two tables. LOOP cannot be specified together with RIGHT or FULL as a join type.

REMOTE

Specifies that the join operation is performed on the site of the right table. This is useful when the left table is a local table and the right table is a remote table. REMOTE should be used only when the left table has fewer rows than the right table.

If the right table is local, the join is performed locally. If both tables are remote but from different data sources, REMOTE causes the join to be performed on the site of the right table. If both tables are remote tables from the same data source, REMOTE is not required.

REMOTE cannot be used when one of the values being compared in the join predicate is cast to a different collation using the COLLATE clause.

REMOTE can be used only for INNER JOIN operations.

Join hints are specified in the FROM clause of a query. Join hints enforce a join strategy between two tables. If a join hint is specified for any two tables, the query optimizer automatically enforces the join order for all joined tables in the query, based on the position of the ON keywords. When a CROSS JOIN is used without the ON clause, parentheses can be used to indicate the join order.

USE AdventureWorks2012;
GO
SELECT p.Name, pr.ProductReviewID
FROM Production.Product AS p
LEFT OUTER HASH JOIN Production.ProductReview AS pr
ON p.ProductID = pr.ProductID
ORDER BY ProductReviewID DESC;

USE AdventureWorks2012;
GO
DELETE FROM Sales.SalesPersonQuotaHistory 
FROM Sales.SalesPersonQuotaHistory AS spqh
    INNER LOOP JOIN Sales.SalesPerson AS sp
    ON spqh.SalesPersonID = sp.SalesPersonID
WHERE sp.SalesYTD > 2500000.00;
GO

USE AdventureWorks2012;
GO
SELECT poh.PurchaseOrderID, poh.OrderDate, pod.ProductID, pod.DueDate, poh.VendorID 
FROM Purchasing.PurchaseOrderHeader AS poh
INNER MERGE JOIN Purchasing.PurchaseOrderDetail AS pod 
    ON poh.PurchaseOrderID = pod.PurchaseOrderID;
GO

Table Hints

Table hints override the default behavior of the query optimizer for the duration of the data manipulation language (DML) statement by specifying a locking method, one or more indexes, a query-processing operation such as a table scan or index seek, or other options. Table hints are specified in the FROM clause of the DML statement and affect only the table or view referenced in that clause.

WITH  ( <table_hint> [ [, ]...n ] )

<table_hint> ::= 
[ NOEXPAND ] { 
    INDEX  ( index_value [ ,...n ] ) | INDEX =  ( index_value )    | FORCESEEK [( index_value ( index_column_name  [ ,... ] ) ) ]
  | FORCESCAN
  | FORCESEEK
  | HOLDLOCK 
  | NOLOCK 
  | NOWAIT
  | PAGLOCK 
  | READCOMMITTED 
  | READCOMMITTEDLOCK 
  | READPAST 
  | READUNCOMMITTED 
  | REPEATABLEREAD 
  | ROWLOCK 
  | SERIALIZABLE 
  | SPATIAL_WINDOW_MAX_CELLS = integer
  | TABLOCK 
  | TABLOCKX 
  | UPDLOCK 
  | XLOCK 
} 

<table_hint_limited> ::=
{
    KEEPIDENTITY 
  | KEEPDEFAULTS 
  | HOLDLOCK 
  | IGNORE_CONSTRAINTS 
  | IGNORE_TRIGGERS 
  | NOLOCK 
  | NOWAIT
  | PAGLOCK 
  | READCOMMITTED 
  | READCOMMITTEDLOCK 
  | READPAST 
  | REPEATABLEREAD 
  | ROWLOCK 
  | SERIALIZABLE 
  | TABLOCK 
  | TABLOCKX 
  | UPDLOCK 
  | XLOCK 
}

http://msdn.microsoft.com/en-us/library/ms187373(v=sql.110).aspx

Query Hints

Query hints specify that the indicated hints should be used throughout the query. They affect all operators in the statement. If UNION is involved in the main query, only the last query involving a UNION operation can have the OPTION clause. Query hints are specified as part of the OPTION clause. If one or more query hints cause the query optimizer not to generate a valid plan, error 8622 is raised.

<query_hint > ::= 
{ { HASH | ORDER } GROUP 
  | { CONCAT | HASH | MERGE } UNION 
  | { LOOP | MERGE | HASH } JOIN 
  | EXPAND VIEWS 
  | FAST number_rows 
  | FORCE ORDER 
  | IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX
  | KEEP PLAN 
  | KEEPFIXED PLAN
  | MAXDOP number_of_processors 
  | MAXRECURSION number 
  | OPTIMIZE FOR ( @variable_name { UNKNOWN | = literal_constant } [ , ...n ] )
  | OPTIMIZE FOR UNKNOWN
  | PARAMETERIZATION { SIMPLE | FORCED }
  | RECOMPILE
  | ROBUST PLAN 
  | USE PLAN N'xml_plan'
  | TABLE HINT ( exposed_object_name [ , <table_hint> [ [, ]...n ] ] )
}

<table_hint> ::=
[ NOEXPAND ] { 
    INDEX ( index_value [ ,...n ] ) | INDEX = ( index_value )
  | FORCESEEK [( index_value ( index_column_name [,... ] ) ) ]
  | FORCESCAN
  | HOLDLOCK 
  | NOLOCK 
  | NOWAIT
  | PAGLOCK 
  | READCOMMITTED 
  | READCOMMITTEDLOCK 
  | READPAST 
  | READUNCOMMITTED 
  | REPEATABLEREAD 
  | ROWLOCK 
  | SERIALIZABLE 
  | SPATIAL_WINDOW_MAX_CELLS = integer
  | TABLOCK 
  | TABLOCKX 
  | UPDLOCK 
  | XLOCK
}

http://msdn.microsoft.com/en-us/library/ms181714(v=sql.110).aspx

NOLOCK（不加锁）

此选项被选中时，SQL Server 在读取或修改数据时不加任何锁。在这种情况下，用户有可能读取到未完成事务（Uncommited Transaction）或回滚(Roll Back)中的数据, 即所谓的“脏数据”。

HOLDLOCK（保持锁）
此选项被选中时，SQL Server 会将此共享锁保持至整个事务结束，而不会在途中释放。

UPDLOCK（修改锁）
此选项被选中时，SQL Server 在读取数据时使用修改锁来代替共享锁，并将此锁保持至整个事务或命令结束。使用此选项能够保证多个进程能同时读取数据但只有该进程能修改数据。

TABLOCK（表锁）
此选项被选中时，SQL Server 将在整个表上置共享锁直至该命令结束。这个选项保证其他进程只能读取而不能修改数据。

PAGLOCK（页锁）
此选项为默认选项，当被选中时，SQL Server 使用共享页锁。

TABLOCKX（排它表锁）
此选项被选中时，SQL Server 将在整个表上置排它锁直至该命令或事务结束。这将防止其他进程读取或修改表中的数据。

HOLDLOCK 持有共享锁，直到整个事务完成，应该在被锁对象不需要时立即释放，等于SERIALIZABLE事务隔离级别

NOLOCK 语句执行时不发出共享锁，允许脏读，等于 READ UNCOMMITTED事务隔离级别

PAGLOCK 在使用一个表锁的地方用多个页锁

READPAST 让sql server跳过任何锁定行，执行事务，适用于READ UNCOMMITTED事务隔离级别只跳过RID锁，不跳过页，区域和表锁

ROWLOCK 强制使用行锁

TABLOCKX 强制使用独占表级锁，这个锁在事务期间阻止任何其他事务使用这个表

UPLOCK 强制在读表时使用更新而不用共享锁

注意: 锁定数据库的一个表的区别
SELECT * FROM table WITH (HOLDLOCK) 其他事务可以读取表，但不能更新删除
SELECT * FROM table WITH (TABLOCKX) 其他事务不能读取表,更新和删

posted @ 2014-06-26 16:58 princessd8251 阅读(465) 评论(0) 收藏举报

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