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https://issues.apache.org/jira/browse/HIVE-4956?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13724133#comment-13724133
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Xuefu Zhang commented on HIVE-4956:
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The syntax, "select ... from T1, T2 ..." without join, might cause semantic
confusion as in some databases it really means crossing (Cartesian production),
which has a different meaning from yours. From a database point of view, A
table is a table, and two table are two tables. Treating two tables as one
seems going beyond what SQL defines. It might be conceptually clearer if we
allow tables have heterogeneous partitions. Of course, this may be more
involved.
> Allow multiple tables in from clause if all them have the same schema, but
> can be partitioned differently
> ---------------------------------------------------------------------------------------------------------
>
> Key: HIVE-4956
> URL: https://issues.apache.org/jira/browse/HIVE-4956
> Project: Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Amareshwari Sriramadasu
> Assignee: Amareshwari Sriramadasu
>
> We have a usecase where the table storage partitioning changes over time.
> For ex:
> we can have a table T1 which is partitioned by p1. But overtime, we want to
> partition the table on p1 and p2 as well. The new table can be T2. So, if we
> have to query table on partition p1, it will be a union query across two
> table T1 and T2. Especially with aggregations like avg, it becomes costly
> union query because we cannot make use of mapside aggregations and other
> optimizations.
> The proposal is to support queries of the following format :
> select t.x, t.y, .... from T1,T2 t where t.p1='x' OR t.p1='y' ...
> [groupby-clause] [having-clause] [orderby-clause] and so on.
> Here we allow from clause as a comma separated list of tables with an alias
> and alias will be used in the full query, and partition pruning will happen
> on the actual tables to pick up the right paths. This will work because the
> difference is only on picking up the input paths and whole operator tree does
> not change. If this sounds a good usecase, I can put up the changes required
> to support the same.
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