对以下说法的质疑： $in 操作一次是快，但是在 list 量大的时候会造成全局的阻塞，造成大量的慢查询（查询查日志中都有记录）。因为目前一个库要对应的查询各种各样，每秒的读操作要上千或者几千，并且 mongodb 是单进程在跑，所以我们要遵循的原则就是单次请求要尽量快的完成。$in 操作，尽量少用，即使要用，list 中的元素也尽量的少。

Benchmark.java

package com.woodywang;

import com.mongodb.*;

public class Benchmark {

	/**
	 * 使用线程模拟多个客户端并发请求
	 */
	static class Finder implements Runnable {

		private int size;
		private int loop;
		private String id;

		private DBCollection collection;

		/**
		 * 构造执行 find 操作的客户端
		 *
		 * @param size $in 条件长度
		 * @param loop 重复执行次数（使统计结果更明显）
		 * @param id   线程标识
		 * @throws Exception
		 */
		public Finder(int size, int loop, String id) throws Exception {
			this.size = size;
			this.loop = loop;
			this.id = id;

			MongoClient client = new MongoClient("127.0.0.1", 27017);
			DB db = client.getDB("test");

			this.collection = db.getCollection("test");
		}

		private String[] generateIds() {
			String[] ids = new String[this.size];
			for (int i = 0; i < this.size; i++) {
				ids[i] = String.format("%08d", (int) (Math.random() * 1000000));
			}
			return ids;
		}

		public void run() {
			long totalTime = 0;
			for (int i = 0; i < this.loop; i++) {
				String[] ids = generateIds();
				BasicDBList idList = new BasicDBList();
				for (String id : ids) {
					idList.add(id);
				}
				DBObject condition = new BasicDBObject();
				condition.put("uid", new BasicDBObject("$in", idList));
				long startTime = System.currentTimeMillis();
				DBCursor cursor = collection.find(condition);
				cursor.count();
				long endTime = System.currentTimeMillis();
				totalTime += endTime - startTime;

			}

			System.out.println(id + ": DataSize: " + (this.loop * this.size) + "; Time: " + totalTime);
		}
	}

	public static void main(String[] args) throws Exception {

		// 创建一个带有大量 ID 的 $in 条件查询测试，如执行 $in 条件有 2000 个ID，执行一次
		Thread tLong = new Thread(new Finder(2000, 1, "LongThread"));

		// 创建若干个小数据量的查询测试
		int countShortThreads = 100;
		Thread[] threads = new Thread[countShortThreads + 1];

		for (int i = 0; i < countShortThreads; i++) {
			// 每个小数据量的测试定义为：每次查询只使用 1 个 ID，反复执行 2000 次
			threads[i] = new Thread(new Finder(1, 2000, "ShortThread-" + i));
		}
		threads[countShortThreads] = tLong;
		for (Thread t : threads) {
			t.start();
		}
		for (Thread t : threads) {
			t.join();
		}
	}
}

Generator.java

package com.woodywang;

import com.mongodb.BasicDBObject;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.MongoClient;

/**
 * Created with IntelliJ IDEA.
 * User: woody
 * Date: 4/24/13
 * Time: 8:16 PM
 * To change this template use File | Settings | File Templates.
 */
public class Generator {

    private static void fillData(DBCollection collection, int size) throws Exception {
		for (int i = 0; i < size; i++) {
			String uid = String.format("%08d", i);
			BasicDBObject doc = new BasicDBObject("uid", uid);
			doc.append("text", generateString(15));
			collection.insert(doc);
		}
	}

	private static String generateString(int size) {
		String str = "abcdefghijklmnopqrstuvwxyz";
		StringBuilder builder = new StringBuilder();
		for (int i = 0; i < size; i++) {
			int index = (int) (Math.random() * str.length());
			char c = str.charAt(index);
			builder.append(c);
		}
		return builder.toString();
	}

	public static void main(String[] args) throws Exception {
		MongoClient client = new MongoClient("localhost");
		DB db = client.getDB("test");

		DBCollection collection = db.getCollection("test");
		fillData(collection, 1000000);
	}
}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.woodywang</groupId>
    <artifactId>mongodb-benchmark</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>mongodb-benchmark</name>
    <url>http://maven.apache.org</url>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongo-java-driver</artifactId>
            <version>2.11.1</version>
        </dependency>
    </dependencies>
</project>