Java 8 JPA存储库流在Postgresql中逐行 [英] Java 8 JPA Repository Stream row-by-row in Postgresql

问题描述

阅读此定义了必需所必需的某些查询提示（ @QueryHints 批注），以使MySQL处理结果集逐行显示。有谁知道PostgreSQL需要什么（如果有的话）？

Reading this about MySQL defines certain query hints (@QueryHints annotation) that are required in order to make MySQL process the result set row-by-row. Does anyone know for sure what (if anything) is required for PostgreSQL?

这是MySQL存储库中的引用查询定义：

Here is the reference query definition in the repository for MySQL:

@QueryHints(value = @QueryHint(name = HINT_FETCH_SIZE, value = "" + Integer.MIN_VALUE))
@Query(value = "select t from Todo t")
Stream<Todo> streamAll();

推荐答案

相当于PostgreSQL的是：

Its PostgreSQL equivalent is:

@QueryHints(value = @QueryHint(name = HINT_FETCH_SIZE, value = "1")

对比 setFetchSize（rows） 应该在时抛出 SQLException 条件行> = 0 不满足。MySQL的JDBC驱动程序不支持此条件。

For contrast setFetchSize(rows) should throw SQLException when the condition rows >= 0 is not satisfied. This is not honored by MySQL's JDBC driver.

尽管在PostgreSQL中，您实际上可以设置大于 1 的值，以允许PostgreSQL的JDBC驱动程序进行某些缓存。文档示例的 50 似乎是合理的（除非您有您也可以选择此值作为预期行数的一部分。在部署y之前测试一些变体我们的应用程序。

Though in PostgreSQL you can actually set up values greater than 1 to allow some caching by PostgreSQL's JDBC driver. The docs' example of 50 seems reasonable (unless you have irrationally wide rows). You could also choose this value to be a portion of the expected row count. Test a few variants before deploying your application.

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