[jira] Commented: (HIVE-178) SELECT without FROM; dropping unnecessary table references

["David Phillips (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-178?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12660813#action_12660813 ] 

David Phillips commented on HIVE-178:
-------------------------------------

bq. (select from empty table) should also just return '2' ... so it produces no output.

Selecting from an empty table should not produce results (unless it is an aggregate without grouping).  An empty table is an empty set, thus it would produce an empty set (no output rows):

{noformat}
mysql> create table empty (x int);
Query OK, 0 rows affected (0.00 sec)

mysql> select 1+1 from empty;
Empty set (0.00 sec)
{noformat}



> SELECT without FROM; dropping unnecessary table references
> ----------------------------------------------------------
>
>                 Key: HIVE-178
>                 URL: https://issues.apache.org/jira/browse/HIVE-178
>             Project: Hadoop Hive
>          Issue Type: Bug
>          Components: Query Processor
>            Reporter: Adam Kramer
>
> SELECT 1+1;
> should just return '2', but instead hive fails because no table is listed.
> SELECT 1+1 FROM (empty table);
> should also just return '2', but instead hive "succeeds" because there is "no possible output," so it produces no output. This is the reason I filed under "bug" instead of "improvement:" If it does not return '2' hive should fail.
> So, currently we have to run 
> SELECT 1+1 FROM (actual table with data);
> ...which runs a whole mapreduce step to ignore all of the data in the table before spitting out '2'. This will be exceptionally frustrating if hadoop makes us wait for mapper(s).
> It would be nice to be able to SELECT without specifying a table, and perhaps relatedly, to be able to edit the above query (which worked) to not need to actually process (actual table with data) given that no data is being pulled from it (as we see from the SELECT statement, that table's name or alias does not appear).

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