Grokbase Groups Pig dev July 2010
FAQ
I see. There looks like some disconnect about "Scenario 1". To me, all
filtering logics that can be pushed above JOIN can be figured out
without use of CNF, which is scenario 1; while CNF helps to derive the
filtering logic after (or, in your example, below) JOIN, which is
Scenario 2.



In your example, C1 and C2, or their equivalent, above JOIN can be
easily figured out without resorting to CNF; C3 may have to be figured
out with CNF, but evaluation cost of the post-Join filtering logic thus
generated may not be cheaper than the original one before pushing up.



In summary, if we want to support scenario 2(and 1), we should use CNF;
if we JUST want to support scenario 1, which will push up all possible
filters closer to source and have all benefits on pruned I/O, we should
not use CNF.



Thanks,



Yan



-----Original Message-----
From: Swati Jain
Sent: Monday, July 12, 2010 4:04 PM
To: pig-dev@hadoop.apache.org
Subject: PIG Logical Optimization: Use CNF in SplitFilter



Yan,



What I meant in my last email was that scenario 2 optimizations would
lead

to more opportunities for scenario 1 kind of optimizations.



Consider the conjunct list [C1;C2;C3] as the source of a JOIN.



(a) Suppose none of these are computable on a join input, in this case
we

retain the original expression and discard the CNF.



(b) Suppose C1 is computable on join input J1 and C2 is computable on
join

input J2 but C3 requires a combination of both join inputs. In this
case, we

push C1 above J1, C2 above J2 and leave C3 as is below the JOIN. Note
that

C1 and C2 may be further pushed up (with additional iterations of the

optimizer). If they are now the source of single input operators, it is

similar to scenario 1.



Thanks,

Swati





On Mon, Jul 12, 2010 at 3:14 PM, Yan Zhou wrote:


Hopefully by this week. I'm still in the debugging phase of the work.
While you are welcome to reuse some of my algorithms, I doubt you can reuse
the code as much as you want. It's basically for my DNF use. You might need
to factor out some general codes which you can find >
reusable.
>

>

>
I fully understand the I/O benefits as I put in my first message. And it is
classified as "Scenario 1". There is no doubt that it should be
considered
as part of your work. However, for this, CNF is not necessary.
>

>

>
For scenario 2, the benefits will be from less in-core logical
expression
evaluation costs and no I/O benefits as I can see. And use of CNF may or may
not lead to cheaper evaluations as the example in my first message shows. In
other words, after use of CNF, you should >
compare the eval cost with that in the original expression eval before
deciding either the CNF or the original form should be evaluated.
>

>

>
Please let me know if I miss any of your points.
>

>

>
Thanks,
>

>

>
Yan
------------------------------ >
*From:* Swati Jain
*Sent:* Monday, July 12, 2010 11:52 AM >
*To:* Yan Zhou
*Cc:* pig-dev@hadoop.apache.org
*Subject:* Re: PIG Logical Optimization: Use CNF in SplitFilter
>

>

>
I was wondering if you are not going to check in your patch soon then it
would be great if you could share it with me. I believe I might be able to
reuse some of your (utility) functionality directly or get some ideas. >
About your cost-benefit question:
1) I will control the complexity of CNF conversion by providing a
configurable threshold value which will limit the OR-nesting.
2) One benefit of this conversion is that it will allow pushing parts of a
filter (conjuncts) across the joins which is not happening in the current
PushUpFilter optimization. Moreover, it may result in a cascading effect to
push the conjuncts below other operators by other rules that may be fired as
a result. The benefit from this is really data dependent, but in big-data
workloads, any kind of predicate pushdown may eventually lead to big savings
in amount of data read or amount of data transfered/shuffled across the
network (I need to understand the LogicalPlan to PhysicalPlan
conversion
better to give concrete examples). >
Thanks!
Swati >
On Mon, Jul 12, 2010 at 10:36 AM, Yan Zhou wrote: >
Yes, I already implemented the "NOT push down" upfront, so you do not need
to do that.
>

>

>
The support of CNF will probably be the most difficulty part. But as I
mentioned last time, you should compare the cost after the trimming CNF to
get the post-split filtering logic. Given the complexity of
manipulating CNF
and undetermined benefits, I am not sure it should be in scope at this
moment or not.
>

>

>
To handle CNF, I think it's a good idea to create a new plan and connect
the nodes in the new plan to the base plan as you envisioned. In my changes,
which uses DNF instead of CNF but processing is similar otherwise, I use a
LogicalExpressionProxy, which contains a "source" member that is just the
node in the original plan, to link the nodes in the new plan and old plan.
The original LogicalExpression is enhanced with a counter to trace the # of
proxies of the original nodes since normal form creation will "spread" the
nodes in the original tree across many normalized nodes. The benefit, aside
from not setting the plan, is that the original expression is trimmed
according to the processing results from DNF; while DNF is created
separately and as a kinda utility so that complex features can be used. In
my changes, I used multiple-child tree in DNF while not changing the
original binary expression tree structure. Another benefit is that the
original tree is kept as much as it is at the start, i.e., I do not attempt
to optimize its overall structure beyond trimming based upon the
simplification logics. (I also control the size of DNF to 100 nodes.) The
down side of this is added complexity.
>

>

>
But in your case, for scenario 2 which is the whole point to use CNF, you
would need to change the original expression tree structurally beyond
trimming for post-split filtering logic. The other benefit of using
multiple-child expression is depending upon if you plan to support such
expression to replace current binary tree >
in the final plan. Even though I think it's a good idea to support that,
but it is not in my scope now.
>

>

>
I'll add my algorithm details soon to my jira. Please take a look and
comment as you see appropriate.
>

>

>
Thanks,
>

>

>
Yan
>

>

>

>
------------------------------ >
*From:* Swati Jain
*Sent:* Friday, July 09, 2010 11:00 PM
*To:* Yan Zhou
*Cc:* pig-dev@hadoop.apache.org
*Subject:* Re: PIG Logical Optimization: Use CNF in SplitFilter
>

>

>
Hi Yan, >
I agree that the first scenario (filter logic applied to individual input
sources) doesn't need conversion to CNF and that it will be a good idea to
add CNF functionality for the second scenario. I was also planning to
provide a configurable threshold value to control the complexity of CNF
conversion. >
As part of the above, I wrote a utility to push the "NOT" operator in
predicates below the "AND" and "OR" operators (Scenario 2 in
PIG-1399). I am
considering making this utility to push NOT a separate rule in itself. Lmk
if you have already implemented this. >
While implementing this utility I am facing some trouble in
maintaining
OperatorPlan consistent as I rewrite the expression. This is because each
operator is referencing the main filter logical plan. Here is my current
approach of implementation: >
1. I am creating a new LogicalExpressionPlan for the converted boolean
expression.
2. I am creating new logical expressions while pushing the NOT
operation,
converting AND into OR, OR into AND eliminating NOT NOT pairs.
3. However, I am having trouble updating the LogicalExpressionPlan if it
reaches the base case ( i.e. root operator is not NOT,AND,OR). >
D = Filter J2 by ( (c2 == 5) OR ( NOT( (c1 < 10) AND (c3+b3 > 10 ) ) )
);

>
In the above, for example, I am not sure how to integrate base
expression
(c2 == 5) into the new LogicalExpressionPlan. There is no routine to set the
plan for a given operator and its children. Also, there is currently no way
to deepCopy an expression into a new OperatorPlan. It would be great if you
could give me some suggestions on what approach to take for this. >
One approach I thought of is to visit the base expression and create and
connect the base expression to the LogicalExpressionPlan as I visit
it.

>
Thoughts?
Swati >
ps: About your other point regarding binary vs multi way trees, the way I
am creating the normal form is a list of conjuncts, where each
conjunct is a
list of disjuncts. This is logically similar to a multi waytree. However,
the current modeling of boolean expressions (modeled as binary
expressions)
requires a conversion back to the binary tree model when adding back to the
main plan. >
On Tue, Jul 6, 2010 at 12:46 PM, Yan Zhou wrote: >
Swati, >
I happen to be working on the logical expression simplification effort
(https://issues.apache.org/jira/browse/PIG-1399), but not on the filter
split front. So I guess our interests will have some overlaps. >
I think the filter logic split problem can be divided into 2 parts:
1) the filtering logic that can be applied to individual input sources;
and 2) the filtering logic that has to be applied when merged(or joined)
inputs are processed. >
The benefits for 1) are any benefits if the underlying storage supports
predicate pushdown; plus the memory/CPU savings by PIG for not
processing the unqualified rows. >
For 2), the purpose is not paying higher evaluation costs than
necessary. >
For 1), no normal form is necessary. The original logical expression
tree
can be trimmed off any sub-expressions that are not constants nor just
from a particular input source. The complexity is linear with the tree
size; while the use of normal form could potentially lead to
exponential
complexity. The difficulty with this approach is how to generate the
filtering logic for 2); while CNF can be used to easily figure out the
logic for 2). However, the exact logic in 2) might not be cheaper to
evaluate than the original logical expression. An example is "Filter J2
by ((C1 < 10) AND (a3+b3>10)) OR ((C2 == 5) AND (a2+b2 >5))". In 2) the
filtering logic after CNF will be "((C1 < 10) OR (a2+b2 > 5)) AND
((a3+b3>10) OR (C2 == 5)) AND ((a3+b3 >10) OR (a2+b2 > 5))". The cost
will be 5 logical evaluations (3 OR plus 2 AND), which could be reduced
to 4, compared with 3 logical evaluations in the original form. >
In summary, if only 1) is desired, the tree trimming is enough. If 2) is
desired too, then CNF could be used but its complexity should be
controlled and the cost of the filtering logic evaluation in 2) should
be computed and compared with the original expression evaluation cost.
Further optimization is possible in this direction. >
Another potential optimization to consider is to support logical
expression tree of multiple children vs. the binary tree after taking
into consideration of the commutative property of OR and AND
operations.
The advantages are less tree traversal costs and easier to change the
evaluation ordering within the same sub-tree in order to maximize the
possibilities to short-cut the evaluations. Although this is general for
all logical expressions, this tends to be more suitable for normal form
handlings as normal forms group the sub-expressions by the operators
that act on the sub-expressions. >
Thanks, >
Yan
>

>
-----Original Message-----
From: Swati Jain
Sent: Monday, July 05, 2010 2:34 AM
To: pig-dev@hadoop.apache.org
Cc: Daniel Dai
Subject: PIG Logical Optimization: Use CNF in SplitFilter >
Hi, >
I am interested in implementing logical optimization rules and to target
this I have studied currently implemented logical rules and the rule
framework. In particular, I felt that rules dealing with LOfilter are
not
able to handle complicated boolean expressions. I would like to share
suggestions to improve handling of boolean expressions in LOFilter to
enable
better optimization. >
1. SplitFilter Rule : 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 current rule is not able to 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) ) >
*Suggestions:* 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. >
I have started thinking about the design for implementing this
conversion
(arbitrary boolean expression to CNF) and would appreciate any feedback
or
ideas. >
Thanks!
Swati
>

>

>

>

>

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