Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/16422#discussion_r126344451
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/command/tables.scala ---
@@ -626,6 +624,120 @@ case class DescribeTableCommand(
}
}
+/**
+ * A command to list the info for a column, including name, data type,
column stats and comment.
+ * This function creates a [[DescribeColumnCommand]] logical plan.
+ *
+ * The syntax of using this command in SQL is:
+ * {{{
+ * DESCRIBE [EXTENDED|FORMATTED] table_name column_name;
+ * }}}
+ */
+case class DescribeColumnCommand(
+ table: TableIdentifier,
+ colNameParts: Seq[String],
+ isExtended: Boolean)
+ extends RunnableCommand {
+
+ override val output: Seq[Attribute] = {
+ // The displayed names are based on Hive.
+ // (Link for the corresponding Hive Jira:
https://issues.apache.org/jira/browse/HIVE-7050)
+ if (isExtended) {
+ Seq(
+ AttributeReference("col_name", StringType, nullable = false,
+ new MetadataBuilder().putString("comment", "name of the
column").build())(),
+ AttributeReference("data_type", StringType, nullable = false,
+ new MetadataBuilder().putString("comment", "data type of the
column").build())(),
+ AttributeReference("min", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "min value of the
column").build())(),
+ AttributeReference("max", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "max value of the
column").build())(),
+ AttributeReference("num_nulls", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "number of nulls of
the column").build())(),
+ AttributeReference("distinct_count", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "distinct count of
the column").build())(),
+ AttributeReference("avg_col_len", StringType, nullable = true,
+ new MetadataBuilder().putString("comment",
+ "average length of the values of the column").build())(),
+ AttributeReference("max_col_len", StringType, nullable = true,
+ new MetadataBuilder().putString("comment",
+ "maximum length of the values of the column").build())(),
+ AttributeReference("comment", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "comment of the
column").build())())
+ } else {
+ Seq(
+ AttributeReference("col_name", StringType, nullable = false,
+ new MetadataBuilder().putString("comment", "name of the
column").build())(),
+ AttributeReference("data_type", StringType, nullable = false,
+ new MetadataBuilder().putString("comment", "data type of the
column").build())(),
+ AttributeReference("comment", StringType, nullable = true,
+ new MetadataBuilder().putString("comment", "comment of the
column").build())())
+ }
+ }
+
+ override def run(sparkSession: SparkSession): Seq[Row] = {
+ val catalog = sparkSession.sessionState.catalog
+ val resolver = sparkSession.sessionState.conf.resolver
+ val relation = sparkSession.table(table).queryExecution.analyzed
+ val field = {
+ relation.resolve(colNameParts, resolver).getOrElse {
+ throw new AnalysisException(s"Column
${UnresolvedAttribute(colNameParts).name} does not " +
+ s"exist")
+ }
+ }
+ if (!field.isInstanceOf[Attribute]) {
+ // If the field is not an attribute after `resolve`, then it's a
nested field.
+ throw new AnalysisException(s"DESC TABLE COLUMN command is not
supported for nested column:" +
+ s" ${UnresolvedAttribute(colNameParts).name}")
+ }
+
+ val catalogTable = catalog.getTempViewOrPermanentTableMetadata(table)
+ val colStats = catalogTable.stats.map(_.colStats).getOrElse(Map.empty)
+ val cs = colStats.get(field.name)
--- End diff --
nit: `val colStats = catalogTable.stats.flatMap(_.colStats.get(field.name))`
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