Github user scwf commented on a diff in the pull request:
https://github.com/apache/spark/pull/4015#discussion_r22856728
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala ---
@@ -40,6 +41,104 @@ import
org.apache.spark.sql.sources.{DataSourceStrategy, BaseRelation, DDLParser
/**
* :: AlphaComponent ::
+ * The entry point for parsing the SQL text to logical plan, we call this
SQL Dialect.
+ * Currently we support the default dialect named "sql", associated with
the class
+ * [[DefaultSQLDialect]]
+ *
+ * And we can also provide custom SQL Dialect, register it before using.
+ * For example, assume we need to provide a Hive Dialect for HiveContext,
we can
+ * register the Dialect during the HiveContext creation
+ * {{{
+ * class HiveContext extends SQLContext {
+ * ...
+ * SQLDialectFactory.registerSQLDialect("hiveql", classOf[HiveQLDialect])
+ * ...
+ * }
+ * }}}
+ *
+ * And in Spark SQL CLI:
+ * {{{
+ * -- switch to "hiveql" dialect
+ * spark-sql>SET spark.sql.dialect=hiveql;
+ * spark-sql>SELECT * FROM src LIMIT 1;
+ * -- switch to "sql" dialect
+ * spark-sql>SET spark.sql.dialect=sql;
+ * spark-sql>SELECT * FROM src LIMIT 1;
+ * }}}
+ */
+@AlphaComponent
+abstract class SQLDialect {
+ /**
+ * We assume the DDLParser has higher priority than any of the other SQL
Parsers,
--- End diff --
This assumption may lead to some problem, an example (from
https://github.com/apache/spark/pull/3935#issuecomment-69220158).
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