How to Efficiently Stream Large MySQL Result Sets in Spring Without OutOfMemory Errors?

Streaming Large MySQL Result Sets

Problem Description

When loading large MySQL tables in a Spring application, OutOfMemory exceptions occur due to the driver's attempts to load the entire table into memory. Despite setting the fetch size to Integer.MIN_VALUE, closing ResultSets leads to hanging issues. The hang persists even for small tables and prevents closing the connection, resulting in the following stack trace:

[Stack trace of the hang inserted here]

Solution

Setting only the fetch size is insufficient. The MySQL JDBC driver requires a specific Statement creation method:

stmt = conn.createStatement(java.sql.ResultSet.TYPE_FORWARD_ONLY, java.sql.ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);

Cautions and Implications

This approach has several caveats:

• All rows in the result set must be read before other queries can be executed.
• Locks are not released until the transaction or statement is complete, or the active result set is closed.

Additional Considerations

If the OutOfMemory exception persists, consider implementing immediate processing of data retrieved from the database to avoid excessive memory allocation. This may require significant code restructuring and a different flow control approach.

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