ORACLE Paging Query

Oracle's paging query

Oracle's paging query statements can be applied basically in the format given in this article.

Paged Query format:

SELECT *  from SELECT A.* from (SELECT*fromWHERE<=  + WHERE >=  +

The most internal query, select * FROM table_name, represents the original query statement without paging. ROWNUM <= 40 and RN >= 21 control the range of pages per page for paged queries.

The paging query statement given above has high efficiency in most cases. The purpose of paging is to control the output result set size and return the results as soon as possible. In the above paged query statement, this consideration is mainly reflected in the where ROWNUM <= 40 sentence.

There are two ways to select the 21st to 40th record, one of which is shown in the above example in the second layer of the query through the rownum <= 40来 control The maximum value, at the outermost level of the query control the minimum value. The other way is to remove the ROWNUM <= 40 statement that queries the second layer, controlling the minimum and maximum paging values at the outermost level of the query. This is the query statement as follows:

SELECT *  from SELECT A.* from (SELECT*fromWHEREbetween   + +

In contrast to these two formulations, the first query is much more efficient than the second in most cases.

This is because in the CBO optimization mode, Oracle can push the outer query condition into the inner query to improve the execution efficiency of the inner query. For the first query statement, the second level of the query condition where ROWNUM <= 40 can be pushed into the inner query by Oracle, so that if the results of an Oracle query exceed the ROWNUM limit, the result is returned to the terminating query.

and the second query statement, because the query condition between and 40 is present in the third layer of the query, and Oracle cannot push the third layer of the query conditions to the most inner layer (even pushing to the inner layer is meaningless, because the most inner query does not know what RN represents). Therefore, for the second query statement, the Oracle's inner layer is returned to the middle tier as all data that satisfies the criteria, and the middle tier returns to the outermost of all the data. Data filtering is done at the outermost layer, and obviously this is much less efficient than the first query.

The query analyzed above is not just a simple query for a single table, but is as effective for the most inner query as a complex multi-table union query or a case in which the inner query contains a sort.

The query containing the sort is not explained here, and the next article is explained in detail by examples. The following is a brief discussion of the multi-table syndication scenario. For the most common equivalent table connection queries, the CBO may generally use two connection modes nested loop and hash join (the MERGE join efficiency is less efficient than the hash join, and the general CBO will not consider it). Here, because paging is used, the maximum number of records returned is specified, and the NESTED loop can stop immediately when the number of returned records exceeds the maximum value and return the result to the middle tier, and the hash join must process all the result sets (also the MERGE join). In most cases, it is more efficient to select nested loop as the query's connection method for paging queries (most of the time when paging queries are querying the first few pages of data, the lower the number of pages accessed).

Therefore, if you do not mind using hint in your system, you can rewrite the paging query statement to:

Select/*+ first_rows */* FROM (select A.*, ROWNUM rn from (SELECT * FROM table_name) A where ROWNUM <=) where RN >= 21