You need to go through the EXPLAIN PLAN part and see what is the cost of the statement with INDEX and without INDEX. For example, if you have a very complex query, which consists of many table joins, and if you specify only the Note that the hints could have been also been in this format:By default, hints do not propagate inside a complex view. Because Oracle's default behavior for a range scan is to scan index entries in ascending order of their indexed values, this hint does not specify anything more than the INDEX hint. Only a subset of distribution method combinations for the joined tables is valid, as explained in Maps the rows of each table to consumer query servers, using a hash function on the join keys. If so, then you can force the optimizer to choose a nested loops join by using the In many cases, a nested loops join returns the first row faster than a sort merge join. 예 : …
Based on this information, you might be able to choose a more efficient execution plan than the optimizer. common hint, the There are a number of rules that need to be
In almost all cases, if the syntax of the hint is correct then Oracle will follow it, however if the index is in an unusable state then Oracle is likely to ignore the hint.Can you give a sample execution plan where Oracle uses an index for a query without WHERE condition? The OPTIMIZER_FEATURES_ENABLE hint acts as an umbrella parameter for enabling a series of optimizer features based on an Oracle Database release number. This chapter explains how to use hints to force various approaches.Hints let you make decisions usually made by the optimizer. This comment can only follow the The following syntax shows hints contained in both styles of comments that Oracle supports within a statement block.If you specify hints incorrectly, then Oracle ignores them but does not return an error:The optimizer recognizes hints only when using the cost-based approach. If you do not specify a view list, then Oracle searches for an eligible materialized view and always uses it regardless of its cost.Complex merging is not cost-based; that is, the accessing query block must include the This example requires that complex view merging be enabled.This hint lets the user have more influence over the way in which the view is accessed.Even if the hint is given, there is no guarantee that the transformation will take place.
This hint is recommended when the inner table is very small compared to the outer table. Pay careful attention to the row access method:From this execution plan, we see that the optimizer
Answer: In a true distributed query, the optimization is done on the sending site. The cost-based approach assumes that each value has an equal probability of appearing in each row. If you include a hint (except the When using hints, in some cases, you might need to specify a full set of hints in order to ensure the optimal execution plan. In such a case, use hints to force the optimizer to use the optimal execution plan.The use of hints involves extra code that must be managed, checked, and controlled.Hints provide a mechanism to direct the optimizer to choose a certain query execution plan based on the following criteria:Hints apply only to the optimization of the statement block in which they appear.
If not specified,
This hint is recommended when the outer table is very small compared to the inner table. the optimizer will use the best index on the table, but this is
Oracle provides the index hint, the and_equal hint, the index_asc hint, the index_combine hint, the index_desc hint, and the index_ffs hint to redirect the optimizer's use of indexes to access table rows.
Maps the rows of the inner table using the partitioning of the outer table. You can also use hints to improve the plan.
If you specify this hint in any of these statements, then the optimizer uses the cost-based approach and optimizes for best throughput.These estimates might not be as accurate as those gathered by the Oracle Corporation strongly advises the use of cost-based optimization. There could be many reasons for Index not being used. Explicitly chooses an index scan for the specified table. Here we see a query
Rule-based optimization will be deprecated in a future release.Each hint described in this section suggests an access path for a table.Specifying one of these hints causes the optimizer to choose the specified access path only if the access path is available based on the existence of an index or cluster and on the syntactic constructs of the SQL statement. However, Oracle recommends using INDEX_COMBINE rather than INDEX for bitmap indexes because it is a more versatile hint /*+ INDEX([@queryblock]
However, such hints can result in unexpected execution plans. The second value specifies how the table is to be split among the Oracle Real Application Clusters instances.
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