Github user marmbrus commented on a diff in the pull request:

    https://github.com/apache/spark/pull/4015#discussion_r28013127
  
    --- Diff: sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala ---
    @@ -129,19 +191,33 @@ class SQLContext(@transient val sparkContext: 
SparkContext)
       @transient
       protected[sql] lazy val optimizer: Optimizer = DefaultOptimizer
     
    -  @transient
    -  protected[sql] val ddlParser = new DDLParser(sqlParser.apply(_))
    -
    -  @transient
    -  protected[sql] val sqlParser = {
    -    val fallback = new catalyst.SqlParser
    -    new SparkSQLParser(fallback(_))
    +  protected[sql] def dialectCache = tlSession.get().dialectCache
    +  protected[sql] def setDialectCache(d: SQLDialect) = {
    +    tlSession.get().dialectCache = d
       }
     
    -  protected[sql] def parseSql(sql: String): LogicalPlan = {
    -    ddlParser(sql, false).getOrElse(sqlParser(sql))
    +  protected[sql] def getSQLDialect(): SQLDialect = {
    +    try {
    +      val clazz = Utils.classForName(dialectClassName)
    +      if (dialectCache == null || dialectCache.getClass != clazz) {
    +        
setDialectCache(clazz.newInstance().asInstanceOf[SQLDialect].initialize(this))
    +      }
    +
    +      dialectCache
    --- End diff --
    
    This caching logic is pretty complicated and hard to follow.  Its not even 
caching the reflective part which is likely the most expensive part.  I would 
remote it and just construct dialects on demand unless you have a benchmark to 
show that is expensive.


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