Spark数据框:选择不同的行 [英] Spark Dataframe: Select distinct rows

问题描述

我尝试了两种方法从镶木地板中查找不同的行，但似乎不起作用.
尝试1: Dataset< Row>df = sqlContext.read().parquet("location.parquet").distinct();
但是抛出

I tried two ways to find distinct rows from parquet but it doesn't seem to work.
Attemp 1: Dataset<Row> df = sqlContext.read().parquet("location.parquet").distinct();
But throws

Cannot have map type columns in DataFrame which calls set operations
(intersect, except, etc.), 
but the type of column canvasHashes is map<string,string>;;

尝试2:尝试运行SQL查询:

Attemp 2: Tried running sql queries:

Dataset<Row> df = sqlContext.read().parquet("location.parquet");
    rawLandingDS.createOrReplaceTempView("df");
    Dataset<Row> landingDF = sqlContext.sql("SELECT distinct on timestamp * from df");

我得到的错误:

= SQL ==
SELECT distinct on timestamp * from df
-----------------------------^^^

在读取镶木地板文件时是否有获取不同记录的方法?我可以使用的任何读取选项.

Is there a way to get distinct records while reading parquet files? Any read option I can use.

推荐答案

您遇到的问题已在异常消息中明确说明-因为 MapType 列既不可散列也不可排序，不能用作一部分对表达式进行分组或分区.

The problem you face is explicitly stated in the exception message - because MapType columns are neither hashable nor orderable cannot be used as a part of grouping or partitioning expression.

您对SQL解决方案的看法在逻辑上不等同于 Dataset 上的 distinct .如果要基于一组兼容的列对数据进行重复数据删除，则应使用 dropDuplicates :

Your take on SQL solution is not logically equivalent to distinct on Dataset. If you want to deduplicate data based on a set of compatible columns you should use dropDuplicates:

df.dropDuplicates("timestamp")

等同于

SELECT timestamp, first(c1) AS c1, first(c2) AS c2,  ..., first(cn) AS cn,
       first(canvasHashes) AS canvasHashes
FROM df GROUP BY timestamp

不幸的是，如果您的目标是实际的 DISTINCT ，那么就不会那么容易了.一种可能的解决方案是利用Scala * Map 哈希.您可以这样定义 Scala udf :

Unfortunately if your goal is actual DISTINCT it won't be so easy. On possible solution is to leverage Scala* Map hashing. You could define Scala udf like this:

spark.udf.register("scalaHash", (x: Map[String, String]) => x.##)

，然后在Java代码中使用它来派生可用于 dropDuplicates 的列:

and then use it in your Java code to derive column that can be used to dropDuplicates:

 df
  .selectExpr("*", "scalaHash(canvasHashes) AS hash_of_canvas_hashes")
  .dropDuplicates(
    // All columns excluding canvasHashes / hash_of_canvas_hashes
    "timestamp",  "c1", "c2", ..., "cn" 
    // Hash used as surrogate of canvasHashes
    "hash_of_canvas_hashes"         
  )

与SQL等效

SELECT 
  timestamp, c1, c2, ..., cn,   -- All columns excluding canvasHashes
  first(canvasHashes) AS canvasHashes
FROM df GROUP BY
  timestamp, c1, c2, ..., cn    -- All columns excluding canvasHashes

---

*请注意，带有 hashCode 的 java.util.Map 无效，因为 hashCode 不一致.

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