Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/15664#discussion_r94000876
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JdbcUtils.scala
---
@@ -211,6 +211,52 @@ object JdbcUtils extends Logging {
}
/**
+ * Returns the schema if the table already exists in the JDBC database.
+ */
+ def getSchema(conn: Connection, url: String, table: String):
Option[StructType] = {
+ val dialect = JdbcDialects.get(url)
+
+ Try {
+ val statement = conn.prepareStatement(dialect.getSchemaQuery(table))
+ try {
+ getSchema(statement.executeQuery(), dialect)
+ } finally {
+ statement.close()
+ }
+ } match {
+ case Success(v) =>
+ Some(v)
+ case Failure(e) =>
+ None
+ }
+ }
+
+ /**
+ * Returns the saving schema using rddSchema's sequence and
tableSchema's name.
--- End diff --
Here, we need to explain why we need to use the column sequences in
`rddSchema` and why we need to use the column names in `tableSchema`
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