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1411ca6a00
Added an expression rewrite rule to convert a disjunctive predicate to
conjunctive normal form (CNF). Converting to CNF enables multi-table
predicates that were only evaluated by a Join operator to be converted
into either single-table conjuncts that are eligible for predicate pushdown
to the scan operator or other multi-table conjuncts that are eligible to
be pushed to a Join below. This helps improve performance for such queries.
Since converting to CNF expands the number of expressions, we place a
limit on the maximum number of CNF exprs (each AND is counted as 1 CNF expr)
that are considered. Once the MAX_CNF_EXPRS limit (default is unlimited) is
exceeded, whatever expression was supplied to the rule is returned without
further transformation. A setting of -1 or 0 allows unlimited number of
CNF exprs to be created upto int32 max. Another option ENABLE_CNF_REWRITES
enables or disables the entire rewrite. This is False by default until we
have done more thorough functional testing (tracking JIRA IMPALA-9539).
Examples of rewrites:
original: (a AND b) OR c
rewritten: (a OR c) AND (b OR c)
original: (a AND b) OR (c AND d)
rewritten: (a OR c) AND (a OR d) AND (b OR c) AND (b OR d)
original: NOT(a OR b)
rewritten: NOT(a) AND NOT(b)
Testing:
- Added new unit tests with variations of disjunctive predicates
and verified their Explain plans
- Manually tested the result correctness on impala shell by running
these queries with ENABLE_CNF_REWRITES enabled and disabled
- Added TPC-H q7, q19 and TPC-DS q13 with the CNF rewrite enabled
- Preliminary performance testing of TPC-DS q13 on a 10TB scale factor
shows almost 5x improvement:
Original baseline: 47.5 sec
With this patch and CNF rewrite enabled: 9.4 sec
Change-Id: I5a03cd7239333aaf375416ef5f2b7608fcd4a072
Reviewed-on: http://gerrit.cloudera.org:8080/15462
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>