[jira] [Created] (SPARK-25597) SQL query with limit iterates the whole iterator when WholeStage code generation is enabled

["Hannu Kröger (JIRA)" <ji...@apache.org>]

Hannu Kröger created SPARK-25597:
------------------------------------

             Summary: SQL query with limit iterates the whole iterator when WholeStage code generation is enabled
                 Key: SPARK-25597
                 URL: https://issues.apache.org/jira/browse/SPARK-25597
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.3.0
            Reporter: Hannu Kröger


When _SELECT * FROM table LIMIT 1_ is executed, the WholeStageCodegenExec generates code that iterates the source iterator completely instead of just first element (per partition). This is a major performance penalty over the expected functionality. This does not happen if _spark.sql.codegen.wholeStage_ is set to false in spark context conf.

In codegen mode {{LocalLimitExec}} overrides {{stopEarly}} method of {{BufferedRowIterator}} to stop consuming rows when enough rows are fetched. However, {{stopEarly}} is only called by code generated by {{InputAdapter}} and {{InputAdapter}} is not added to the plan for some reason.

One potential way to fix this would be to check {{stopEarly}} in {{hasNext}} method of {{BufferedRowIterator}}. Interesting fact is that the {{next}} does not call {{hasNext}}. Therefore {{next}} might return empty result even if there is data to be fetched by the {{processNext}} method. Or add a check to stopEarly in shouldStop method.

This commit might have broken the original functionality: [https://github.com/apache/spark/commit/b41ec997786e2be42a8a2a182212a610d08b221b]



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)

---------------------------------------------------------------------
To unsubscribe, e-mail: issues-unsubscribe@spark.apache.org
For additional commands, e-mail: issues-help@spark.apache.org