Treating an SQL ResultSet like a Scala Stream

问题:

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

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.

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)
  }
}

回答1:

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

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

回答2:

Utility function for @elbowich's answer:

def results[T](resultSet: ResultSet)(f: ResultSet => T) = {
  new Iterator[T] {
    def hasNext = resultSet.next()
    def next() = f(resultSet)
  }
}

Allows you to use type inference. E.g.:

stmt.execute("SELECT mystr, myint FROM mytable")

// Example 1:
val it = results(stmt.resultSet) {
  case rs => rs.getString(1) -> 100 * rs.getInt(2)
}
val m = it.toMap // Map[String, Int]

// Example 2:
val it = results(stmt.resultSet)(_.getString(1))

回答3:

This sounds like a great opportunity for an implicit class. First define the implicit class somewhere:

import java.sql.ResultSet

object Implicits {

    implicit class ResultSetStream(resultSet: ResultSet) {

        def toStream: Stream[ResultSet] = {
            new Iterator[ResultSet] {
                def hasNext = resultSet.next()

                def next() = resultSet
            }.toStream
        }
    }
}

Next, simply import this implicit class wherever you have executed your query and defined the ResultSet object:

import com.company.Implicits._

Finally get the data out using the toStream method. For example, get all the ids as shown below:

val allIds = resultSet.toStream.map(result => result.getInt("id"))

回答4:

i needed something similar. Building on elbowich's very cool answer, I wrapped it a bit, and instead of the string, I return the result (so you can get any column)

def resultSetItr(resultSet: ResultSet): Stream[ResultSet] = {
    new Iterator[ResultSet] {
      def hasNext = resultSet.next()
      def next() = resultSet
    }.toStream
  }

I needed to access table metadata, but this will work for table rows (could do a stmt.executeQuery(sql) instead of md.getColumns):

 val md = connection.getMetaData()
 val columnItr = resultSetItr( md.getColumns(null, null, "MyTable", null))
      val columns = columnItr.map(col => {
        val columnType = col.getString("TYPE_NAME")
        val columnName = col.getString("COLUMN_NAME")
        val columnSize = col.getString("COLUMN_SIZE")
        new Column(columnName, columnType, columnSize.toInt, false)
      })

回答5:

Because ResultSet is just a mutable object being navigated by next, we need to define our own concept of a next row. We can do so with an input function as follows:

class ResultSetIterator[T](rs: ResultSet, nextRowFunc: ResultSet => T) 
extends Iterator[T] {

  private var nextVal: Option[T] = None

  override def hasNext: Boolean = {
    val ret = rs.next()
    if(ret) {
      nextVal = Some(nextRowFunc(rs))
    } else {
      nextVal = None
    }
    ret
  }

  override def next(): T = nextVal.getOrElse { 
    hasNext 
    nextVal.getOrElse( throw new ResultSetIteratorOutOfBoundsException 
  )}

  class ResultSetIteratorOutOfBoundsException extends Exception("ResultSetIterator reached end of list and next can no longer be called. hasNext should return false.")
}

EDIT: Translate to stream or something else as per above.

回答6:

This implementation, although longer and clumsier it is in better correspondence with the ResultSet contract. The side-effect has been removed from hasNext(...) and moved into next().

new Iterator[String] {
  private var available = resultSet.next()
  override def hasNext: Boolean = available
  override def next(): String = {
    val string = resultSet.getString(1)
    available = resultSet.next()
    string
  }
}

回答7:

I think most of above implementations has a nondeterministic hasNext method. Calling it two times will move cursor to the second row. I would advise to use something like that:

  new Iterator[ResultSet] {
    def hasNext = {
      !resultSet.isLast
    }
    def next() = {
      resultSet.next()
      resultSet
    }
  }