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When you specify both the ALL_ROWS or perhaps the FIRST_ROWS trace inside a SQL statement, and if the info dictionary does not have studies about tables accessed by the statement, then the optimizer takes advantage of default statistical values (such as allocated storage for these types of tables) to estimate the lacking figures and, subsequently, to settle on an execution prepare.
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If a view's question contains a bunch BY clause or Unique operator in the Decide on listing, then the optimizer can merge the view's query to the accessing statement only if sophisticated view merging is enabled.
Even when the trace is offered, there is absolutely no warranty that the transformation will happen. The optimizer only generates the subqueries if it seems fair to do so.
With nonmergeable views, optimization method and intention hints inside the view are disregarded; the highest-amount query decides the optimization manner.
In case the UNNEST hint is applied, Oracle initially verifies In the event the statement is valid. If the assertion is just not legitimate, then subquery unnesting cannot continue. The assertion ought to then should go a heuristic examination.
If this hint specifies no indexes, then the optimizer considers the expense of a scan on Each and every available index on the desk and after that performs the index scan with the bottom Price tag.
When applying hints, sometimes, you would possibly should specify a complete list of hints in an effort to make sure the ideal execution plan. For instance, Should you have an exceptionally sophisticated question, which includes lots of desk joins, and when you specify only the INDEX trace for any offered table, then the optimizer wants to ascertain the remaining accessibility paths for use, plus the corresponding join techniques.
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For example, a compound query consisting of two ingredient queries blended with the UNION operator has two statement blocks, one for every component question. For this reason, hints in the very first ingredient question utilize only to its optimization, not for the optimization of the 2nd component query.