Streaming Query

MyBatis-Plus has supported streaming queries since version 3.5.4, which is a native feature of MyBatis implemented through the ResultHandler interface. This query method is suitable for batch data processing or business scenarios involving large datasets.

In BaseMapper, several overloaded methods have been added, including selectList, selectByMap, selectBatchIds, selectMaps, and selectObjs, which can be used in combination with streaming queries.

Note that in older versions of MyBatis-Plus, errors may occur when custom ResultHandler is combined with pagination queries. In such cases, the count query needs to be manually disabled. Specific issues and solutions can be found in the related GitHub issue.

Common Methods

• getResultObject: Retrieves each record from the database.
• getResultCount: Gets the current count of processed result sets. The counter increments by 1 for each processed record, starting from 1.
• stop: Stops further processing of the result set, equivalent to using a break statement in a loop.

Usage Example

The following example demonstrates how to use streaming queries, showing how to fetch data in batches from the database with pagination and how to process all records in a table.

// Using pagination to fetch data in batches for processing, e.g., retrieving 100,000 records from the database for data processing
Page<H2User> page = new Page<>(1, 100000);
baseMapper.selectList(page, Wrappers.emptyWrapper(), new ResultHandler<H2User>() {
    int count = 0;
    @Override
    public void handleResult(ResultContext<? extends H2User> resultContext) {
        H2User h2User = resultContext.getResultObject();
        System.out.println("Currently processing record " + (++count) + ": " + h2User);
        // Perform your business logic here, such as task distribution
    }
});

// Fetch all records from the database for processing
baseMapper.selectList(Wrappers.emptyWrapper(), new ResultHandler<H2User>() {
    int count = 0;
    @Override
    public void handleResult(ResultContext<? extends H2User> resultContext) {
        H2User h2User = resultContext.getResultObject();
        System.out.println("Currently processing record " + (++count) + ": " + h2User);
        // Perform your business logic here, such as task distribution
    }
});

In the above example, we created a Page object to specify pagination parameters, then called the selectList method and passed a ResultHandler to process each record. In the handleResult method of ResultHandler, we can retrieve the currently processed record and perform corresponding business logic. The counter count allows us to track the number of records processed.