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From "Steven Wong (JIRA)" <>
Subject [jira] [Commented] (HIVE-746) constant folding
Date Thu, 09 Jun 2011 21:47:58 GMT


Steven Wong commented on HIVE-746:

For a query like:

select * from src where partcol = unix_timestamp();

Don't we want the expression unix_timestamp() to be evaluated once and to be used in pruning
the partitions, *whether the UDF unix_timestamp is marked non-deterministic or not*?

> constant folding
> ----------------
>                 Key: HIVE-746
>                 URL:
>             Project: Hive
>          Issue Type: New Feature
>          Components: Query Processor
>            Reporter: Namit Jain
>            Assignee: Zheng Shao
> The constants are not folded at compile time:
> for eg:
> select 1+2 from src
> will evaluate 1+2 for every row.
> This becomes more interesting for scenarios like:
> select unix_timestamp() from src;
> The UDF should be evaluated only once, and the same value should be returned. However,
currently, we mark it as non-deterministic and evaluate it for every row.
> This can have bad side-effects on partition pruning etc.
> In MySQL, the same value is generated independent of the time taken for the query.

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