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This article is to share with you about the principle of RowBounds paging in Mybatis. The editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

Mybatis can paginate database data by passing RowBounds objects, but unfortunately, this paging operation is paging the ResultSet result set, which is often referred to as logical paging rather than physical paging.

The source code of the RowBounds object is as follows:

Public class RowBounds {public static final int NO_ROW_OFFSET = 0; public static final int NO_ROW_LIMIT = Integer.MAX_VALUE; public static final RowBounds DEFAULT = new RowBounds (); private int offset; private int limit; public RowBounds () {this.offset = NO_ROW_OFFSET; this.limit = NO_ROW_LIMIT;} public RowBounds (int offset, int limit) {this.offset = offset; this.limit = limit;} public int getOffset () {return offset;} public int getLimit () {return limit;}}

Paging the database data, depending on the two parameters offset and limit, indicates how many items should be taken from the beginning. It is also known as start,limit.

Let's take a look at how Mybatis is paginated.

Org.apache.ibatis.executor.resultset.DefaultResultSetHandler.handleRowValuesForSimpleResultMap () method source code.

Private void handleRowValuesForSimpleResultMap (ResultSetWrapper rsw, ResultMap resultMap, ResultHandler resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {DefaultResultContext resultContext = new DefaultResultContext (); / / Jump to offset location, ready to read skipRows (rsw.getResultSet (), rowBounds); / / read limit data while (shouldProcessMoreRows (resultContext, rowBounds) & & rsw.getResultSet (). Next ()) {ResultMap discriminatedResultMap = resolveDiscriminatedResultMap (rsw.getResultSet (), resultMap, null); Object rowValue = getRowValue (rsw, discriminatedResultMap); storeObject (resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet ()) }} private void skipRows (ResultSet rs, RowBounds rowBounds) throws SQLException {if (rs.getType ()! = ResultSet.TYPE_FORWARD_ONLY) {if (rowBounds.getOffset ()! = RowBounds.NO_ROW_OFFSET) {/ / directly locate rs.absolute (rowBounds.getOffset ());}} else {/ / can only scroll to the specified position for (int I = 0; I < rowBounds.getOffset (); iTunes +) {rs.next ();}

Note that the paging of Mybatis is the paging of the result set.

Suppose that the query result is a total of 100 records, and we only need 10 records after paging, does that mean that 100 records are in memory and we get 10 pieces of data from memory paging?

No, the JDBC driver does not load all the results into memory, but only loads a small part of the data into memory. If you need to fetch more records from the database, it will get some of the data again, which is the use of fetch size. And we withdraw money from the bank card is the same reason, the money in the card is yours, but we withdraw 200 yuan at a time, not enough to withdraw, at this time our fetch size = 200 yuan.

Therefore, the logical paging performance of Mybatis is not as bad as many people think, and many people think it is paging on memory.

The optimal solution, of course, is physical paging, that is, the query results, that is, our paging results, and the performance is the best. If you have to do physical paging, how to solve it?

1. Sql contains the offset,limit parameter. You can control the parameter value and query the paging result directly.

two。 Use the Mybatis paging plug-in developed by a third party.

3. Modify the Mybatis source code to append your own physical paging Subsql to Sql.

The above is what is the principle of RowBounds pagination in Mybatis. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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