Using of "get the N first lines only" does / * + FIRST_ROWS ([N]) * / redundant index?

I know FIRST_ROWS indicator shows the optimizer to minimize the time of the first row.  I know that the new feature of 12 c for "fetch [FIRST |]» [NEXT] [N] LINES [ONLY |] WITH LINKS] "get first/next N lines only / with ties" will implement the query using ROW_NUMBER().  Should I leave hint in case it improves performance, or the clause FETCH FIRST made this redundant suspicion?

Hi Wes and Hoek,

Oracle said on the indicators in the 12 c setting guide. Each version of this statement becomes stronger.

The disadvantage of the advice is additional code that you must manage, audit and control. Tips have been introduced in Oracle7, when users have little recourse if the optimizer generated suboptimal plans. Because changes in the database and host environment can make obsolete tips or negative consequences, it is a good practice to test the use of indicators, but use other techniques to manage the execution plans.

Oracle provides several tools, including how to set up SQL, SQL plan management and SQL Performance Analyzer to solve performance problems unresolved by the optimizer. Oracle strongly recommends that you use these tools instead of advice because they provide new solutions like the change of environment data and database.

Oracle presents advice in V7, basically as an admission that its optimizer based on CSSTidy based cost did not get things right all the time and tried to get rid of them since. In addition, the preferred method of setting when you are browsing the major updates was to review advice to remove them. It will be interesting to what extent can it be pushed in V12.

In what concerns the first lines index and ROWNUM limiting, unless you just try to get the garbage data, it's meaningless without the presence of an ORDER BY. Once you have an ORDER BY, the query must retrieve all the data before it can return anything. The exception to this rule is if there is an index that the database can use to retrieve already ordered data, that is to say on the order of columns. Therefore, the essence of the indication of FIRST LINES. It will be aggressive looking in the index in favor if the index is in line with the order of. (Try the setting of a SIEBEL instance if you need proof)

I don't have a 12 c to test at the moment, but looking at the examples of Martin, it appears the optimizer is aware of the new windowing function in the new FETCH FIRST/NEXT structure and selects a plan that gives the best answer. If you go through the effort to review suggested rownum limited requests to remove the tips if possible, maybe you should just rewrite with new windowing function.

Concerning

André

Tags: Database

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