[HIVE-746] constant folding

• Type: New Feature
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: Query Processor
• Assignee: Zheng Shao
• Reporter: Namit Jain
• Created: 11/Aug/09 02:10
• Updated: 21/Jun/11 23:36

Description

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.

Issue Links

• incorporates: HIVE-757 Don't send constants from mapper to reducer for Group By queries
• is related to: HIVE-1986 partition pruner do not take effect for non-deterministic UDF