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SQL Server s query optimization process is responsible for producing a query execution plan when a SELECT query is executed. The goal of the query optimizer is to generate an efficient (but not always the best) query execution plan. Under rare circumstances SQL Server may choose an inefficient plan over a more efficient one. If this happens you would be advised to investigate the query execution plan, table statistics, and other factors that are explored in more detail in 28. After researching the query performance, you may decide to override the decision making process of the SQL Server query optimizer by using hints. The next three recipes will demonstrate three types of hints: query, table, and join.

A join hint is a misnomer in this case, as a join hint will force the query optimizer to join the tables in the way you command. Join hints force the internal JOIN operation used to join two tables in a query. Available join hints are described in Table 1-4. Table 1-4. Join Hints

Strength reduction is an optimization that uses previous calculations or values to eliminate more expensive calls or calculations This option is activated by default when any GCC optimization level is used -freg-struct-return: This option when compiling C or C++ applications causes GCC to generate code that returns struct and union values in registers whenever possible By default, GCC uses whichever of the -fpcc-struct-return or -freg-struct-return options is appropriate for the target system -fregmove: This optimization option tells GCC to reassign register numbers in order to maximize the amount of register tying, and is synonymous with the -foptimize-register-move option This option is active by default when using GCC optimization level 2 or higher -frename-registers: This optimization option tells GCC to make use of any unallocated registers in order to attempt to avoid false dependencies in scheduled code This option is therefore most frequently used on systems with large numbers of registers.

LOOP joins operate best when one table is small and the other is large, with indexes on the joined columns. HASH joins are optimal for large unsorted tables. MERGE joins are optimal for medium or large tables that are sorted on the joined column. REMOTE forces the join operation to occur at the site of the table referenced on the right (the second table referenced in a JOIN clause). For performance benefits, the left table should be the local table, and should have fewer rows than the remote right table.

-freorder-blocks: This optimization option tells GCC to reorder basic blocks in compiled functions in order to improve code locality and minimize the number of branches taken This option is enabled at optimization levels 2 and 3 -freorder-blocks-and-partition: This optimization option tells GCC to perform the same reordering as specified by the -freorder-blocks option, but to also partition hot and cold basic blocks into separate sections of the assembly and object files in order to improve paging and cache locality -freorder-functions: This optimization option causes GCC to optimize function placement using profile feedback..

Before showing how the join hint works, the example starts off with the original, non-hinted query: -- (More on SHOWPLAN_TEXT in 28) SET SHOWPLAN_TEXT ON GO SELECT p.Name, r.ReviewerName, r.Rating FROM Production.Product p INNER JOIN Production.ProductReview r ON r.ProductID = p.ProductID GO SET SHOWPLAN_TEXT OFF GO This returns the following abridged results (SHOWPLAN_TEXT returns information about how the query may be processed, but it doesn t actually execute the query): StmtText -----------------------------------------------------------------------------------|--Nested Loops(Inner Join, OUTER REFERENCES:([r].[ProductID])) |--Clustered Index Scan(OBJECT:([AdventureWorks].[Production].[ProductReview].[PK_ProductReview_ ProductReviewID] AS [r])) |--Clustered Index Seek(OBJECT:([AdventureWorks].[Production].[Product].[PK_Product_ProductID] AS [p]), SEEK:([p].[ProductID]=[AdventureWorks].[Production].[ProductReview]. [ProductID] as [r].[ProductID]) ORDERED FORWARD) The next example submits the same query, only this time using a join hint: SET SHOWPLAN_TEXT ON GO SELECT p.Name, r.ReviewerName, r.Rating FROM Production.Product p INNER HASH JOIN Production.ProductReview r ON r.ProductID = p.ProductID GO SET SHOWPLAN_TEXT OFF GO This returns the following abridged results: StmtText -----------------------------------------------------------------------------------|--Hash Match(Inner Join, HASH:([p].[ProductID])=([r].[ProductID])) |--Index Scan(OBJECT:([AdventureWorks].[Production].[Product]. [AK_Product_Name] AS [p])) |--Clustered Index Scan(OBJECT:([AdventureWorks].[Production]. [ProductReview].