Improvement to detection if MV can be used to rewrite queries in non-trivial cases.
Pasting the email conversation below that happened over this which briefly discusses the approach taken:
---------- Forwarded message ----------
From: Amogh Margoor <firstname.lastname@example.org>
Date: Fri, Jun 26, 2015 at 10:11 AM
Subject: Re: Detect if materialized view can be used to rewrite a query in non-trivial cases
To: email@example.com, Rajat Venkatesh <firstname.lastname@example.org>
Thanks a lot Julian for your feedback. I have inlined my response below which also includes the commit done.
On Fri, Jun 26, 2015 at 1:05 AM, Julian Hyde <email@example.com> wrote:
This is great work. Certainly consistent with where I am heading.
I would not be inclined to use DNF (because of its tendency to inflate
certain predicates) but if you are able to get something effective I
will happily use it. I think you should package it behind a method –
"find out what is left to satisfy p when you have already satisfied q"
or something – and write lots of tests of that method, and it doesn't
really matter what algorithm is behind it.
Take a look at SubstitutionVisitor.simplfy(RexNode) and how it focuses
on finding whether
p1 AND p2 AND p3 AND NOT (q1 AND q2 AND q3)
>> I saw this method. I will try to use this in improvements to follow.
>> It didnot seem to solve this currently: (x>10 => x>30) i.e., find if
>> NOT (NOT(x>10 ) OR x >30) is satisfiable.
>> We have currently packaged it as "if X => Y" (see RexImplicationChecker
>> in the commit I shared below), but agree it should be
>> more generic like what you suggested above and something we can try to achieve.
Later we will want to know not just "can I satisfy query Q using
materialization M?" but "can I satisfy part of Q using M, and what is
left over?". I can convert most of Q to use an aggregate table over
years 2012 .. 2014 and 2015 Jan - May, and then scan the raw data for
June 1st onwards, that is a big win.
>> This certainly should be something we should aim at.
What branch are you working on? Your master branch
https://github.com/qubole/incubator-calcite/tree/master seems to be
the same as apache/master right now.
>> We work on https://github.com/qubole/incubator-calcite/tree/qds-1.3 .
>> This is the commit: https://github.com/qubole/incubator-calcite/pull/1/files?diff=unified
>> We are in the process of writing UTs for it. We did most of the testing through our client code till now.
>> We have created new Visitor extending SustitutionVisitor because did not want to mess with the existing code.
>> More rules need to be added to the new Visitor.
>> Will raise a PR once UTs are added and testing is complete.
If you can divide this work into pull requests with unit tests, I will
happy commit each change as you make progress.
By the way, I logged a few jira cases connected to materialized view
rewrite today. They were motivated by the phoenix team wanting to use
secondary indexes. But they could by applied to any scan-project-sort
>> Thanks for sharing this info Julian. Will definitely take a look.
On Thu, Jun 25, 2015 at 11:46 AM, Amogh Margoor <firstname.lastname@example.org> wrote:
> We were working on a problem to detect if materialized view can be used to
> rewrite a query in non-trivial cases. Will briefly describe the problem and
> approach below and would appreciate feedback on the same.
> For instance there exists a table "logs" and a partition (materialized
> view) named "log_after_01_Jan" created on it and described by SQL :
> "Select * from logs where log_date > '01-01-2015' ".
> Assume that the table "log_after_01_Jan" is much smaller than table "logs".
> For user query:
> "Select log_body from logs where log_date > '03-03-2015' and
> char_length(log_body) < 100",
> we should detect that the materialized view "log_after_01_Jan" can be used
> and transform the query into:
> "Select log_body from log_after_01_Jan where log_date > '03-03-2015' and
> char_length(log_body) < 100"
> One of the fundamental problems we would come across here is to check if a
> boolean condition X implies (=>) Y. This quickly reduces to the
> Satisfiability problem which is NP complete for propositional logic. But
> there are many instances like above which can be detected easily. We have
> implemented an approach to handle several useful cases for few operators
> and types of operands. Will be extending it further for more types of
> Top Level approach:
> 1. Currently, VolcanoPlanner:useMaterialization tries to rewrite original
> query using MV using SubstitutionVisitor. Have extended SubstitutionVisitor
> to detect above cases and do the substitution.
> 2. To check if a condition X => Y,
> a. Convert both of them into Disjunctive Normal Form.
> Say X is transformed into x_1 or x_2 or x_3 ... or x_m and
> Y is transformed into y_1 or y_2 ,... or y_i, where any x_i and y_i
> are conjunctions of atomic predicates.
> For instance condition "(a>10 or b>20) and c <90" will be converted
> to DNF: (a>10 and c<90) or (b>20 and c<90).
> b. For X=>Y to be a tautology i.e., hold always true, every conjunction
> x_i should imply atleast one of the conjunction y_j.
> We wrote some set of simple heuristics to check if a conjunction of
> atomic predicates implies another.
> This also involves executing RexNode using RexImplExecutor.
> We have checked in code for this in our fork of
> calcite(qubole/incubator-calcite). This is ongoing work and we will be
> making many more improvements to it. If this is useful or anybody is
> interested in giving feedback then I can share the commit so that we can
> discuss about it and take it forward.
> Member of Technical Staff