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Join hints (Transact-SQL)

  • Článok

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Join hints specify that the query optimizer enforce a join strategy between two tables in SQL Server. For general information about joins and join syntax, see FROM clause plus JOIN, APPLY, PIVOT.

Caution

Because the SQL Server query optimizer typically selects the best execution plan for a query, we recommend that hints be used only as a last resort by experienced developers and database administrators.

Applies to

Transact-SQL syntax conventions

Syntax

<join_hint> ::=
     { LOOP | HASH | MERGE | REMOTE }

Arguments

{ LOOP | HASH | MERGE }

Specifies that the join in the query should use looping, hashing, or merging. Using LOOP, HASH, or MERGE JOIN enforces a particular join between two tables. LOOP can't be specified together with RIGHT or FULL as a join type. For more information, see Joins.

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 isn't required.

REMOTE can't 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.

Remarks

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.

Examples

The Transact-SQL code samples in this article use the AdventureWorks2022 or AdventureWorksDW2022 sample database, which you can download from the Microsoft SQL Server Samples and Community Projects home page.

A. Use HASH

The following example specifies that the JOIN operation in the query is performed by a HASH join.

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;

B. Use LOOP

The following example specifies that the JOIN operation in the query is performed by a LOOP join.

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

C. Use MERGE

The following example specifies that the JOIN operation in the query is performed by a MERGE join.

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

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