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  Oracle SQL statement to retrieve data paging table
  Add Date : 2018-11-21      
  About the Oracle SQL syntax to retrieve data paging, network everywhere, many tricks herein, this is also the author himself on the web search more representative of grammar, not my original, posted here, is simply to allow the "Data tab series "looks a little fuller and complete, so the first statement about here especially, to avoid attracting condemning!

First introduced two more representative data paging to retrieve SQL instance.
• No ORDER BY sort of wording. (Most efficient)

(After testing, this method is the lowest cost, only one nesting fastest! Amount of data retrieved even bigger, almost unaffected, the speed is still!)

      FROM emp t
      WHERE hire_date BETWEEN TO_DATE ( '20060501', 'yyyymmdd')
AND TO_DATE ( '20060731', 'yyyymmdd')
          AND ROWNUM <= 20) table_alias
WHERE table_alias.rowno> = 10;

• There ORDER BY sort of wording. (Most efficient)

(Tested this method with the expansion of the search range, the speed will be more slow Oh!)

      FROM (SELECT t. *
            FROM emp t
            WHERE hire_date BETWEEN TO_DATE ( '20060501', 'yyyymmdd')
                AND TO_DATE ( '20060731', 'yyyymmdd')
                ORDER BY create_time DESC, emp_no) tt
      WHERE ROWNUM <= 20) table_alias
WHERE table_alias.rowno> = 10;

Refer to the above two examples, basically, the following SQL statement on behalf of the conventional paging retrieval format, according to the actual needs and personal proficiency in SQL can be freely converted, to give the pagination SQL statement you need.

      FROM (SELECT * FROM table_name) a
WHERE rn> = 21

Wherein the innermost retrieve SELECT * FROM TABLE_NAME means no flip the original search statement. ROWNUM <= 40 and RN> = 21 controls tab page range retrieval.

Given above this tab search statement, in most cases, it has a high efficiency. The purpose is to control the output of pagination result set size, the results are returned as soon as possible. Tab above the search statement, this consideration is mainly reflected in the WHERE ROWNUM <= 40 on the phrase.

There are two methods to select the first 21-40 record, one is shown in the above example to retrieve a second layer by ROWNUM <= 40 to control the maximum, minimum in the outermost layer of control to retrieve. The other way is to remove the second layer to retrieve WHERE ROWNUM <= 40 statement, minimum and maximum values ​​in the retrieval of the outermost control tab. In this case, the search statement is as follows:

      FROM (SELECT * FROM table_name) a)

Compare these two way, the vast majority of cases, the first retrieval efficiency is much higher than the second.

This is because under the CBO optimization mode, Oracle can be pushed to the search condition outer inner search, in order to improve the efficiency of the inner search. For the first search statement, the second layer of the search condition WHERE ROWNUM <= 40 Oracle can be pushed into the inner search, so Oracle search results once more than ROWNUM restrictions on the termination of retrieval results returned.

And the second search statement, since the search condition BETWEEN 21 AND 40 are present in the retrieval of the third layer, and the third search condition Oracle can not be pushed to the innermost layer (even when pushed to the innermost layer has no meaning because innermost retrieval representatives do not know what RN). Thus, for the second retrieval statement, Oracle innermost layer is returned to the middle of all the data to meet the conditions, and the intermediate layer is returned to the outermost layer of all the data. Filtering data in the outermost layer is completed, it is clear that efficiency is much lower than the first retrieval.

The above analysis is not just for the simple search to retrieve a single table to retrieve the innermost layer is a complex multi-table retrieval or the joint innermost layer contains the sort of retrieval as effective.
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