有没有办法在通过 Glue 插入 Redshift 时简单地截断列？

Question

我有一列大于 varchar(max) 数据类型，据我了解，这是 AWS Glue 使用的最大数据类型，当我尝试加载我的因为它的桌子。我不想截断该列，因为它并不是那么重要，并且无法弄清楚如何在 Glue 中做到这一点。我知道如果我在 EC2 实例中使用 psql 连接到我的数据库并且实际上可以让我的表以这种方式成功加载，我可以使用 TRUNCATECOLUMNS 作为复制命令上的标签。但是，我的老板坚持要我使用 Glue 来完成这项工作，所以我正在寻找一种使用 Glue 脚本截断列的方法。我浏览了很多文档，但找不到类似的东西。谢谢。

这里有一些工作代码，供其他可能遇到此问题并需要完整参考的人使用。请注意，varchar(65535) 是 Redshift 中一列可以包含的最大字符数：

val truncColUdf = udf((str: String) => if (str.length > 29999) str.substring(0, 29999) else str)

val datasource30 = glueContext.getCatalogSource(database = "database", tableName = "entry", redshiftTmpDir = "", transformationContext = "datasource30").getDynamicFrame()
val revDF30 = datasource30.toDF()
  .withColumn("message", truncColUdf(col("message")))
val truncDynamicFrame30 = DynamicFrame(revDF30, glueContext)
val applymapping30 = truncDynamicFrame30.applyMapping(mappings = Seq(("id", "bigint", "id", "bigint"), ("message", "string", "message", "varchar(65535)"), ("state", "string", "state", "varchar(256)"), ("created_at", "timestamp", "created_at", "timestamp"), ("depth", "int", "depth", "int")), caseSensitive = false, transformationContext = "applymapping30")
val resolvechoice30 = applymapping30.resolveChoice(choiceOption = Some(ChoiceOption("make_cols")), transformationContext = "resolvechoice30")
val dropnullfields30 = resolvechoice30.dropNulls(transformationContext = "dropnullfields30")
val datasink30 = glueContext.getJDBCSink(catalogConnection = "databaseConnection", options = JsonOptions("""{"dbtable": "entry", "database": "database"}"""), redshiftTmpDir = args("TempDir"), transformationContext = "datasink30").writeDynamicFrame(dropnullfields30)

这是正在读取的示例数据行：

01,"<p>Here is the message where the quotations are in case of commas within the message, like so.</p>",active,2017-08-27 23:38:40,1

Answer 1

将 DynamicFrame 转换为 spark 的 DataFrame，然后使用用户定义的函数截断列值（Scala）：

import com.amazonaws.services.glue.DynamicFrame
import org.apache.spark.sql.functions._

val truncColUdf = udf((str: String) => if (str.length > 20) str.substring(0, 20) else str)
val truncDataFrame = dynamicFrame.toDF()
  .select("text_long")
  .withColumn("text_short", truncColUdf(col("text_long")))
  .withColumn("text_short_length", length(col("text_short")))

truncDataFrame.show(5, false)

val truncDynamicFrame = DynamicFrame(truncDataFrame, glueContext)

...

//write to sink

输出：

+-----------------------+--------------------+-----------------+
|text_long              |text_short          |text_short_length|
+-----------------------+--------------------+-----------------+
|I'd rather not answer  |I'd rather not answe|20               |
|Agree                  |Agree               |5                |
|Custom Answer Favorable|Custom Answer Favora|20               |
|Agree                  |Agree               |5                |
|Sometimes              |Sometimes           |9                |
+-----------------------+--------------------+-----------------+

Answer 2

您可以在 DynamicFrameWriter 的“extracopyoptions”参数中传递“TRUNCATECOLUMNS”：https://aws.amazon.com/premiumsupport/knowledge-center/sql-commands-redshift-glue-job/