`
bupt04406
  • 浏览: 347680 次
  • 性别: Icon_minigender_1
  • 来自: 杭州
社区版块
存档分类
最新评论

hive auto map join

    博客分类:
  • Hive
阅读更多
旧的mapjoin实现,0.6及其以前的版本
新的mapjoin实现,0.7版本,HIVE-1641、HIVE-1754 将小表加载到分布式缓存
automapjoin依赖与新的mapjoin实现

https://issues.apache.org/jira/browse/HIVE-1642

Hiveconf:
hive.auto.convert.join


SemanticAnalyzer.genMapRedTasks(QB qb){

    PhysicalContext physicalContext = new PhysicalContext(conf,
        getParseContext(), ctx, rootTasks, fetchTask);
    PhysicalOptimizer physicalOptimizer = new PhysicalOptimizer(
        physicalContext, conf);
    physicalOptimizer.optimize();

}

PhysicalOptimizer:
  private void initialize(HiveConf hiveConf) {
    resolvers = new ArrayList<PhysicalPlanResolver>();
    if (hiveConf.getBoolVar(HiveConf.ConfVars.HIVESKEWJOIN)) {
      resolvers.add(new SkewJoinResolver());
    }
    if (hiveConf.getBoolVar(HiveConf.ConfVars.HIVECONVERTJOIN)) {
      resolvers.add(new CommonJoinResolver());
    }
    resolvers.add(new MapJoinResolver());
  }


MapJoinResolver:
    遍历所有的task,如果这个task有一个MapredLocalWork,那么创建一个MapredLocalTask运行这
个MapredLocalWork,前后task的依赖关系也相应的建立起来。
public class MapJoinResolver implements PhysicalPlanResolver {

         class LocalMapJoinTaskDispatcher implements Dispatcher {
                    if(localwork != null) {
           
                  private void processCurrentTask(Task<? extends Serializable> currTask,
        ConditionalTask conditionalTask) throws SemanticException {
                  // get the context info and set up the shared tmp URI
                  Context ctx = physicalContext.getContext();
                  String tmpFileURI = Utilities.generateTmpURI(ctx.getLocalTmpFileURI(), currTask.getId());
                  localwork.setTmpFileURI(tmpFileURI);
                  String hdfsTmpURI = Utilities.generateTmpURI(ctx.getMRTmpFileURI(), currTask.getId());
                  mapredWork.setTmpHDFSFileURI(hdfsTmpURI);
            }

         }

}

HashTableSinkOperator:
  public void closeOp(boolean abort) throws HiveException {
        // get tmp file URI
        String tmpURI = this.getExecContext().getLocalWork().getTmpFileURI();
      
        // get the tmp URI path; it will be a hdfs path if not local mode
        String tmpURIPath = Utilities.generatePath(tmpURI, tag, bigBucketFileName);
  }

MapJoinOperator:
  private void loadHashTable() throws HiveException {
       
  }

  

CommonJoinResolver 是auto map join的入口。
CommonJoinResolver.resolve(PhysicalContext pctx){

CommonJoinTaskDispatcher

}

CommonJoinTaskDispatcher.dispatch(Node nd, Stack<Node> stack, Object... nodeOutputs){



}

HiveConf
    //small table file size
    HIVESMALLTABLESFILESIZE("hive.smalltable.filesize",25000000L), //25M

ConditionalTask
MapJoinOperator

将每个包含 Map Join 的 Stage 替换并拆分成两个 Stage:
第一个 Stage 是一个包含 HashTableSinkOperator 的 MapredLocalTask,将小表生成 Hash Table 文件,并分发到 Distributed Cache。
第二个 Stage 是一个包含 MapJoinOperator 的 MapredTask,将小表(Hash Table)从Distributed Cache中加载进来,并执行实际的 Map Join 操作。

原先的一个MapredTask -> MapredLocalTask+MapredTask

MapredLocalTask :
MapredLocalTask.execute(){
   起子进程运行ExecDriver.main() {}
}
子进程运行
ExecDriver.main(){
   MapredLocalTask.executeFromChildJVM(){
       运行TableScanOperator+HashTableSinkOperator
       HashTableSinkOperator读取小表创建HashTable,写入本地的一个文件。
       //HashTableSinkOperator的processOp把数据put到HashMapWrapper。
       //HashTableSinkOperator的closeOp把HashMapWrappert Dump到本地的一个文件
   }
}
子进程MapredLocalTask里:
java.lang.Exception
        at org.apache.hadoop.hive.ql.exec.HashTableSinkOperator.processOp(HashTableSinkOperator.java:318)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
        at org.apache.hadoop.hive.ql.exec.TableScanOperator.processOp(TableScanOperator.java:55)
        at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.startForward(MapredLocalTask.java:330)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.executeFromChildJVM(MapredLocalTask.java:277)
        at org.apache.hadoop.hive.ql.exec.ExecDriver.main(ExecDriver.java:1054)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
        at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
        at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)


子进程MapredLocalTask里:
java.lang.Exception
        at org.apache.hadoop.hive.ql.exec.HashTableSinkOperator.closeOp(HashTableSinkOperator.java:438)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:558)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.Operator.close(Operator.java:567)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.startForward(MapredLocalTask.java:326)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.executeFromChildJVM(MapredLocalTask.java:277)
        at org.apache.hadoop.hive.ql.exec.ExecDriver.main(ExecDriver.java:1054)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
        at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
        at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)




MapredTask:
   调用return super.execute(driverContext);
       try {  //把小表存放在本地的文件中,写入hdfs
      // propagate the file to distributed cache
      MapredLocalWork localwork = work.getMapLocalWork();
      if (localwork != null) {
        boolean localMode = HiveConf.getVar(job, HiveConf.ConfVars.HADOOPJT).equals("local");
        if (!localMode) {
          Path localPath = new Path(localwork.getTmpFileURI());
          Path hdfsPath = new Path(work.getTmpHDFSFileURI());

          FileSystem hdfs = hdfsPath.getFileSystem(job);
          FileSystem localFS = localPath.getFileSystem(job);
          FileStatus[] hashtableFiles = localFS.listStatus(localPath);
          for(int i =0; i<hashtableFiles.length;i++){
            FileStatus file = hashtableFiles[i];
            Path path = file.getPath();
            String fileName = path.getName();
            String hdfsFile = hdfsPath + Path.SEPARATOR + fileName;
            LOG.info("Upload 1 HashTable from" +path+" to: "+hdfsFile);
            Path hdfsFilePath = new Path(hdfsFile);

            hdfs.copyFromLocalFile(path, hdfsFilePath);
            short replication = (short) job.getInt("mapred.submit.replication", 10);
            hdfs.setReplication(hdfsFilePath, replication);
          }
          FileStatus[] hashtableRemoteFiles = hdfs.listStatus(hdfsPath);
          for(int i =0; i<hashtableRemoteFiles.length;i++){
            FileStatus file = hashtableRemoteFiles[i];
            Path path = file.getPath();
            DistributedCache.addCacheFile(path.toUri(), job);

            LOG.info("add 1 hashtable file to distributed cache: " + path.toUri());
          }
        }
      }

   提交任务给JobTracker
   Map阶段:
   MapTask 运行 MapJoinOperator,MapJoinOperator在processOp的时候会loadHashTable
  
    MapTask执行MapJoinOperator时读取小表的栈:
    java.io.IOException: java.lang.IndexOutOfBoundsException: Index: 1, Size: 1
at org.apache.hadoop.hive.ql.exec.persistence.MapJoinDoubleKeys.readExternal(MapJoinDoubleKeys.java:117)
at java.io.ObjectInputStream.readExternalData(ObjectInputStream.java:1791)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1750)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
at java.util.HashMap.readObject(HashMap.java:1029)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:974)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1848)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1752)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
at org.apache.hadoop.hive.ql.exec.persistence.HashMapWrapper.initilizePersistentHash(HashMapWrapper.java:127)
at org.apache.hadoop.hive.ql.exec.MapJoinOperator.loadHashTable(MapJoinOperator.java:193)
at org.apache.hadoop.hive.ql.exec.MapJoinOperator.processOp(MapJoinOperator.java:225)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.SelectOperator.processOp(SelectOperator.java:84)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.SelectOperator.processOp(SelectOperator.java:84)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.CommonJoinOperator.genAllOneUniqueJoinObject(CommonJoinOperator.java:732)
at org.apache.hadoop.hive.ql.exec.CommonJoinOperator.checkAndGenObject(CommonJoinOperator.java:819)
at org.apache.hadoop.hive.ql.exec.MapJoinOperator.processOp(MapJoinOperator.java:265)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.FilterOperator.processOp(FilterOperator.java:87)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.TableScanOperator.processOp(TableScanOperator.java:55)
at org.apache.hadoop.hive.ql.exec.Operator.process(Operator.java:472)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:715)
at org.apache.hadoop.hive.ql.exec.MapOperator.process(MapOperator.java:402)
at org.apache.hadoop.hive.ql.exec.ExecMapper.map(ExecMapper.java:141)
at org.apache.hadoop.mapred.MapRunner.run(MapRunner.java:50)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:347)
at org.apache.hadoop.mapred.Child.main(Child.java:156)
Caused by: java.lang.IndexOutOfBoundsException: Index: 1, Size: 1
at java.util.ArrayList.RangeCheck(ArrayList.java:547)
at java.util.ArrayList.get(ArrayList.java:322)
at org.apache.hadoop.hive.ql.exec.persistence.MapJoinDoubleKeys.readExternal(MapJoinDoubleKeys.java:113)
... 50 more

 


初始化:

2011-09-01 05:59:00     Starting to launch local task to process map join;      maximum memory = 372834304
java.lang.NullPointerException
        at org.apache.hadoop.hive.ql.exec.persistence.HashMapWrapper.<init>(HashMapWrapper.java:83)
        at org.apache.hadoop.hive.ql.exec.HashTableSinkOperator.initializeOp(HashTableSinkOperator.java:264)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.initializeOp(FilterOperator.java:73)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.initializeOp(FilterOperator.java:73)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.initializeOp(FilterOperator.java:73)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.initializeOp(FilterOperator.java:73)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.FilterOperator.initializeOp(FilterOperator.java:73)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:434)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeChildren(Operator.java:390)
        at org.apache.hadoop.hive.ql.exec.Operator.initializeOp(Operator.java:375)
        at org.apache.hadoop.hive.ql.exec.Operator.initialize(Operator.java:358)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.initializeOperators(MapredLocalTask.java:429)
        at org.apache.hadoop.hive.ql.exec.MapredLocalTask.executeFromChildJVM(MapredLocalTask.java:267)
        at org.apache.hadoop.hive.ql.exec.ExecDriver.main(ExecDriver.java:1054)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
        at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
        at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
        at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)
FAILED: Execution Error, return code 2 from org.apache.hadoop.hive.ql.exec.MapredLocalTask



分享到:
评论

相关推荐

    hive参数配置说明大全

    如果设置为true,则Hive将自动将Reduce端的Common Join转化为Map Join,默认值为false。 5. hive.mapred.local.mem 该参数决定了Mapper/Reducer在本地模式的最大内存量,以字节为单位,0为不限制。如果设置为0,则...

    hive优化.docx

    * 使用MapJoin:set hive.auto.convert.join = true; * 设置小表刷入内存中的大小:set hive.mapjoin.smalltable.filesize = 2500000; * 设置最大Reduce个数:set hive.exec.reducers.max=200; * 设置Reduce个数:...

    hive on tez 常见报错问题收集

    当遇到此运行时异常时,可以尝试禁用向量化执行或者混合Grace Hash Join,设置`hive.vectorized.execution.enabled`为false,以及`hive.mapjoin.hybridgrace.hashtable`和`hive.vectorized.execution.reduce....

    hive on spark mr 数据开发常见问题解决

    对于Map阶段的内存溢出,尤其是由于MapJoin引起的,应设置`hive.auto.convert.join = false`,将MapJoin转换为Reduce端的Common Join。此外,可以调整以下参数以减少内存压力: - `hive.exec.reducers.bytes.per....

    Hive脚本任务参数优化配置.doc

    - `set hive.mapjoin.smalltable.size=2500000;` 设置小表大小阈值,小于该阈值的表会被加载到内存中用于Map Join。 - `set hive.auto.convert.join.noconditionaltask.size=512000000;` 设置无需条件任务的Map ...

    hive数仓、hive SQL 、 hive自定义函数 、hive参数深入浅出

    - 其他参数:如hive.exec.compress.output控制是否压缩输出,hive.auto.convert.join决定自动转换内连接的条件。 通过以上内容的学习,你将能熟练掌握Hive在大数据处理中的运用,理解其数仓设计原则,编写高效的...

    hive调优策略

    - **实现**:通过`SET hive.auto.convert.join.noconditionaltask=true`等配置来启用自动转换功能。 ##### 2. 小文件优化 - **问题**:大量小文件的存在不仅会导致Map任务增多,还可能导致数据倾斜等问题。 - **...

    hive配置说明

    4. **hive.auto.convert.join** - **含义**:控制是否根据输入小表的大小,自动将Reduce端的Common Join转换为Map Join,从而提高大表关联小表的速度。 - **默认值**:`false` - **建议设置**:可根据具体场景...

    Hive常见异常处理.docx

    set hive.auto.convert.join=false;”。 五、内存溢出报错异常 在Hive中,内存溢出报错是另一个常见的异常。当container执行需要的内存超出了物理内存限制时,Hive将抛出一个Container异常,错误信息为“Container...

    Hive+经纬度+数据导入ES

    left join (select pid, booth_type, booth_area, latest_journal_sd, lates -- 其他字段省略 from some_other_table) b on a.pid = b.pid; ``` #### 总结 通过以上步骤,可以有效地解决Hive与Elasticsearch之间的...

    大数据企业级调优的完整过程:9.1 Fetch抓取;9.2 本地模式;9.3 表的优化;9.4 数据倾斜;9.5 并行执行

    开启本地模式的设置是`hive.exec.mode.local.auto=true`,并可通过`hive.exec.mode.local.auto.inputbytes.max`和`hive.exec.mode.local.auto.input.files.max`设定阈值,以决定何时使用本地模式。通过合理设置这些...

    Hive性能优化

    本文来自于cnblogs,赘述了在工作中总结Hive的常用优化手段和在工作中使用Hive出现的问题。下面开始本篇文章的优化介绍:继续《那些年使用Hive踩过的坑》一文中的剩余部分.首先,我们来看看Hadoop的计算框架特性,在此...

    数据工程师培训题库(四).doc.docx

    在Hive中,通过在查询语句中添加`/*+MAPJOIN(table)*/`来指定较小的表作为map阶段的输入。对于10G的tableA和100G的tableB,将tableA设置为MapJoin的参数是合理的优化,即A选项正确。 10. HMaster在HBase中负责...

    集群配置参数生成.zip

    例如,`hive.exec.local.scratchdir` 设定本地临时目录,`hive.auto.convert.join` 控制是否自动转换内连接为MapJoin操作。正确配置这些参数可以显著提高查询效率。 YARN(Yet Another Resource Negotiator)是...

    数据工程师培训题库 优质文档.docx

    10. **优化查询效率**:在大型关联查询中,通过使用MapJoin(选项A和B)或者分区(选项C)可以优化效率。根据id字段进行分区比根据name字段更有效,因为id通常具有更好的分布特性。选项D不是最佳实践。 11. **...

Global site tag (gtag.js) - Google Analytics