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Query Transformations

5.1 OR Expansion

In OR expansion, the optimizer transforms a query with a WHERE clause containing OR operators into a query that uses the UNION ALL operator. The database can perform OR expansion for various reasons. For example, it may enable more efficient access paths or alternative join methods that avoid Cartesian products. As always, the optimizer performs the expansion only if the cost of the transformed statement is lower than the cost of the original statement.

5.2 View Merging

In view merging, the optimizer merges the query block representing a view into the query block that contains it. View merging can improve plans by enabling the optimizer to consider additional join orders, access methods, and other transformations.

For example, after a view has been merged and several tables reside in one query block, a table inside a view may permit the optimizer to use join elimination to remove a table outside the view. For certain simple views in which merging always leads to a better plan, the optimizer automatically merges the view without considering cost. Otherwise, the optimizer uses cost to make the determination. The optimizer may choose not to merge a view for many reasons, including cost or validity restrictions.

If OPTIMIZER_SECURE_VIEW_MERGING is true (default), then Oracle Database performs checks to ensure that view merging and predicate pushing do not violate the security intentions of the view "Anadrol 50" creator. To disable these additional security checks for a specific view, you can grant the MERGE VIEW privilege to a user for this view. To disable additional security checks for all views for a specific user, you can grant the MERGE ANY VIEW privilege to that user.

5.2.1 Masteron E 200 Query Blocks in View Merging

The "Achat Anabolisant Belgique" optimizer represents each nested subquery or unmerged view by a separate query block. The database optimizes query blocks separately from the bottom up. Thus, the database optimizes the innermost query block first, generates the part of the plan for it, and then generates the plan for the outer query block, representing the entire query.

The parser expands each view referenced "Anaboliset Aineet" in a query into a separate query block. The block essentially represents the view definition, and thus the result of a view. One option for the optimizer is to analyze the view query block separately, generate a view subplan, and then process the rest of the query by using the view subplan to generate an overall execution plan. However, this technique may lead to a suboptimal execution plan because the view is optimized separately.

View merging can sometimes improve performance. As shown in Example 5 4, view merging merges the tables from the view into the "Anaboliset Aineet" outer query block, removing the inner query block. Thus, separate optimization of the view is not necessary.

5.2.3 Complex View Masteron Effects Merging

In complex view merging, the optimizer merges views containing GROUP BY and DISTINCT views. Like simple view merging, complex merging enables the optimizer to consider additional join orders and access paths.

The optimizer can delay Oral Steroids Side Effects In Babies evaluation of GROUP BY or DISTINCT operations until after it "buy cheap jintropin online" has evaluated the joins. Delaying these operations can improve or worsen performance depending on the data characteristics. If the joins use filters, then delaying the operation until after joins can reduce the data set on which the operation is to be performed. Evaluating the operation early can reduce the amount of data to be processed by subsequent joins, or the joins could increase the amount of data to be processed by the operation. The optimizer uses cost to evaluate view merging and merges the view only when it is the lower cost option.

Aside from cost, the optimizer may be unable to perform complex view merging for the following reasons:

The outer query tables do not have a rowid or unique column.

The view appears in a CONNECT BY query block.

The view contains GROUPING SETS, ROLLUP, or PIVOT clauses.

The view or outer query block contains the MODEL clause.

5.4 Subquery Unnesting

In subquery unnesting, the optimizer transforms a nested query into an equivalent join statement, and then optimizes the join. This transformation enables the optimizer to consider the subquery tables during access path, join method, and join order selection. The optimizer can perform this transformation only if the resulting join statement is guaranteed to return the same rows as the original statement, and if subqueries do not contain aggregate functions such as AVG.

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