像处理 Scala 流一样处理 SQL ResultSet [英] Treating an SQL ResultSet like a Scala Stream

问题描述

当我查询一个数据库并收到一个(只进，只读)ResultSet 时，ResultSet 就像一个数据库行列表.

When I query a database and receive a (forward-only, read-only) ResultSet back, the ResultSet acts like a list of database rows.

我试图找到某种方法来像对待 Scala Stream 一样处理这个 ResultSet.这将允许诸如 filter、map 等操作，同时不会消耗大量 RAM.

I am trying to find some way to treat this ResultSet like a Scala Stream. This will allow such operations as filter, map, etc., while not consuming large amounts of RAM.

我实现了一个尾递归方法来提取单个项目，但这要求所有项目同时在内存中，如果 ResultSet 非常大，就会出现问题:

I implemented a tail-recursive method to extract the individual items, but this requires that all items be in memory at the same time, a problem if the ResultSet is very large:

// Iterate through the result set and gather all of the String values into a list
// then return that list
@tailrec
def loop(resultSet: ResultSet,
         accumulator: List[String] = List()): List[String] = {
  if (!resultSet.next) accumulator.reverse
  else {
    val value = resultSet.getString(1)
    loop(resultSet, value +: accumulator)
  }
}

推荐答案

我没有测试过，但为什么它不起作用?

I didn't test it, but why wouldn't it work?

new Iterator[String] {
  def hasNext = resultSet.next()
  def next() = resultSet.getString(1)
}.toStream

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