Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/4015#discussion_r23197165
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala ---
@@ -42,6 +43,112 @@ import org.apache.spark.sql.types._
/**
* :: 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,
+ * We need to parse the DDL string first, if we can not get any result,
then will
+ * resort to the dialect parser.
+ */
+ @transient
+ protected[sql] val ddlParser = new DDLParser
+
+ protected[sql] final def apply(sqlText: String): LogicalPlan = {
+ ddlParser(sqlText).getOrElse(parse(sqlText))
+ }
+
+ // Will be called once after the instance created.
+ protected[sql] def initialize(sqlContext: SQLContext): SQLDialect = {
+ this
+ }
+
+ protected[sql] def parse(sqlText: String): LogicalPlan
+}
+
+class DefaultSQLDialect extends SQLDialect {
+ @transient
+ protected[sql] val sqlParser = {
+ val fallback = new catalyst.SqlParser
+ new SparkSQLParser(fallback(_))
+ }
+
+ protected[sql] override def parse(sqlText: String): LogicalPlan = {
+ sqlParser(sqlText)
+ }
+}
+
+@AlphaComponent
+object SQLDialectFactory {
+ import java.util.concurrent.ConcurrentHashMap
+
+ private val table = new ConcurrentHashMap[String, Class[_<:
SQLDialect]]()
+ private val parsers = new ThreadLocal[SQLDialect]()
+
+ def registerSQLDialect(dialect: String, className: String): Unit = {
+ val clazz = Utils.classForName(className)
+ registerSQLDialect(dialect, clazz)
+ }
+
+ def registerSQLDialect(dialect: String, clazz: Class[_]): Unit = {
+ if (classOf[SQLDialect].isAssignableFrom(clazz)) {
+ table.put(dialect, clazz.asInstanceOf[Class[_<:SQLDialect]])
--- End diff --
Nit: spaces around `<:`.
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