Paging for Oracle and MySQL

If we access the database through JDBC, then it is necessary to take different SQL paging statements according to the database type, for MySQL database, we can use the limit statement for paging, for the Oracle database, we can use rownum way of paging.

(1) The limit m,n statement for MySQL

In the two parameters after limit, the parameter m is the starting subscript, which starts at 0 and the parameter n is the number of records returned. We need to split the page to specify these two values.

(2) RowNum of Oracle Database

in Oracle database, paging is not as simple as MySQL, it relies on rownum to implement .
RowNum represents the line number of a record, and it is worth noting that it is given after each row has been fetched. Therefore, to specify the rownum of the interval to get paging data in a layer of query statements is not possible, to be paged also to make a query.

SELECT * FROM
(
SELECT a.*, ROWNUM RN
From (SELECT * from table_name) A
WHERE ROWNUM <= 40
)
WHERE RN >= 21

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.*, ROWNUM RN
From (SELECT * from table_name) A
)
WHERE RN between and 40

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.

Paging (RPM) for Oracle and MySQL