关键词：hdp , hive , StorageHandler
了解Hive StorageHandler的同学都知道，StorageHandler作为Hive适配不同存储的拓展类，同时肩负着HiveStoragePredicateHandler的角色对相关存储做下推优化，核心方法如下：
/** * HiveStoragePredicateHandler is an optional companion to {@link * HiveStorageHandler}; it should only be implemented by handlers which * support decomposition of predicates being pushed down into table scans. */ public interface HiveStoragePredicateHandler { /** * Gives the storage handler a chance to decompose a predicate. The storage * handler should analyze the predicate and return the portion of it which * cannot be evaluated during table access. For example, if the original * predicate is <code>x = 2 AND upper(y)='YUM'</code>, the storage handler * might be able to handle <code>x = 2</code> but leave the "residual" * <code>upper(y)='YUM'</code> for Hive to deal with. The breakdown * need not be non-overlapping; for example, given the * predicate <code>x LIKE 'a%b'</code>, the storage handler might * be able to evaluate the prefix search <code>x LIKE 'a%'</code>, leaving * <code>x LIKE '%b'</code> as the residual. * * @param jobConf contains a job configuration matching the one that * will later be passed to getRecordReader and getSplits * * @param deserializer deserializer which will be used when * fetching rows * * @param predicate predicate to be decomposed * * @return decomposed form of predicate, or null if no pushdown is * possible at all */ public DecomposedPredicate decomposePredicate( JobConf jobConf, Deserializer deserializer, ExprNodeDesc predicate); /** * Struct class for returning multiple values from decomposePredicate. */ public static class DecomposedPredicate { /** * Portion of predicate to be evaluated by storage handler. Hive * will pass this into the storage handler's input format. */ public ExprNodeGenericFuncDesc pushedPredicate; /** * Serialized format for filter */ public Serializable pushedPredicateObject; /** * Portion of predicate to be post-evaluated by Hive for any rows * which are returned by storage handler. */ public ExprNodeGenericFuncDesc residualPredicate; } }
核心方法便是decomposePredicate方法，返回一个 DecomposePredicate 对象，其中，对象中的属性成员 Serializable pushedPredicateObject 是一个自由度非常高的属性，你可以把你任何下推的结果、配置、甚至在下推中解析表达树得到的一些函数声明等都可以传递出去，给到InputFormat侧去决定如何读取数据。但是在HDP 2.2.6-2800（对应Hive 0.14.0.2.2.6-2800）和 HDP 2.4.2.0-258 （对应 Hive 1.2.1000.2.4.2.0-258） 中，经测试，DecomposePredicate的另外两个属性都能起效，唯独pushedPredicateObject怎么都拿不到，在InputFormat侧一直为null。
单步跟了Hive 0.14.0.2.2.6.0的源码，pushedPredicateObject测试能用，本地打包上传测试服务器替换原来的hive-exec jar包重启HiveServer2，居然也测试成功能用。由于HDP的代码小版本号太多，而且也不确定后面横线后的版本号对应的数字是代表什么意思（revision?），所以暂时找不到确定的源码了，认为最近似的源码2.2.6.0手动编译打包的是没问题的。
只能姑且认为是HDP的一个莫名的坑，有基于HDP的Hive做下推优化的同学需要留意一下这个问题。