PIG Logical Optimization: Use CNF in PushUpFilter
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Key: PIG-1494
URL: https://issues.apache.org/jira/browse/PIG-1494
Project: Pig
Issue Type: Improvement
Components: impl
Affects Versions: 0.7.0
Reporter: Swati Jain
Priority: Minor
Fix For: 0.8.0
The PushUpFilter rule is not able to handle complicated boolean expressions.
For example, SplitFilter rule is splitting one LOFilter into two by "AND".
However it will not be able to split LOFilter if the top level operator is
"OR". For example:
*ex script:*
A = load 'file_a' USING PigStorage(',') as (a1:int,a2:int,a3:int);
B = load 'file_b' USING PigStorage(',') as (b1:int,b2:int,b3:int);
C = load 'file_c' USING PigStorage(',') as (c1:int,c2:int,c3:int);
J1 = JOIN B by b1, C by c1;
J2 = JOIN J1 by $0, A by a1;
D = *Filter J2 by ( (c1 < 10) AND (a3+b3 > 10) ) OR (c2 == 5);*
explain D;
In the above example, the PushUpFilter is not able to push any filter condition
across any join as it contains columns from all branches (inputs). But if we
convert this expression into "Conjunctive Normal Form" (CNF) then we would be
able to push filter condition c1< 10 and c2 == 5 below both join conditions.
Here is the CNF expression for highlighted line:
( (c1 < 10) OR (c2 == 5) ) AND ( (a3+b3 > 10) OR (c2 ==5) )
*Suggestion:* It would be a good idea to convert LOFilter's boolean expression
into CNF, it would then be easy to push parts (conjuncts) of the LOFilter
boolean expression selectively. We would also not require rule SplitFilter
anymore if we were to add this utility to rule PushUpFilter itself.
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