Or, it assigns the provided plan for run.
MYSQL OPTIMIZER FULL
They are often used to mitigate performance challenges with third-party software.Ī plan guide consists of the statement whose run plan needs to be adjusted and either an OPTION clause that lists the desired query hints or a full XML query plan that is enforced as long it is valid.Īt run time, SQL Server matches the text of the query specified by the guide and attaches the OPTION hints.
If an application uses one-time queries instead of stored procedures, views, and functions, the only way to affect query plans is to use plan guides.
MYSQL OPTIMIZER CODE
While query or table hints need to be explicitly stated in the query text, they aren’t an option if you have no control over the source code generating these queries. The difference between query hints and plan guides is the way they are associated with a query. Plan guides can use either query hints or a full fixed, pre-generated plan attached to a query. Plan guides provide similar functionality to query hints in the sense they allow explicit user intervention and control over query optimizer plan choices. Query hints are specified after the query itself following the WITH options clause. Some common table hints include OPTIMIZE FOR, RECOMPILE, FORCE ORDER, and FAST. Query hints may be JOIN hints, table hints, or from a set of hints that are only relevant for query hints. Query hints affect the entire set of query operators, not just the individual clause in which they appear. Some common table hints are INDEX =, FORCESEEK, NOLOCK, and TABLOCKX. These hints don’t modify the defaults and apply only for the duration of the DML or DQL statement.
Table hints are used to explicitly force a particular locking strategy or access method for a table operation clause. Table hints override the default behavior of the query optimizer. REMOTE enables processing a join with a remote table on the local server. These hints force the optimizer to use nested loops, hash match, or merge physical join algorithms. For example, … Table1 INNER LOOP JOIN Table2 ON …. You can explicitly add LOOP, HASH, MERGE, and REMOTE hints to a JOIN. The term hint is misleading because, in reality, it forces an override to any other choice of the run plan. SQL Server hints are instructions that override automatic choices made by the query processor for DML and DQL statements.
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