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From Julian Hyde <jul...@hydromatic.net>
Subject Re: Filter push
Date Thu, 02 Oct 2014 07:31:55 GMT
Dan,

First, can I clarify query semantics. SQL is a strongly-typed language, but there are a couple
of ways you can make it work on a “schema-less” or “schema-on-read” database. The
Splunk adapter does it one way (the columns you ask for effectively become the schema for
the duration of that query) and Drill working on JSON documents does it another way (you get
back a record with a single column whose value is a map, and then you can probe into that
map for values). I guess the former is what people call a key-value database, the latter a
document database.

The next question is what should be your basic table-scan operator. Let’s assume that you
want to pass in a list of columns to project, plus a boolean expression like ‘c1 > 10
and c1 < 20 and c2 = 4’ for the conditions you want to be executed in the table scan.
(Not sure exactly what expressions rocksdb can handle, but you should start simple.)

I think I complicated things by trying to pack too much functionality into SplunkTableAccessRel.
Here’s how I would do it better and simpler for your RocksDB adapter. (And by the way, the
MongoDB adapter works more like this.)

I’d write a RocksTableScan extends TableAccessRelBase. Also write a RocksProjectRel, whose
expressions are only allowed to be RexInputRefs (i.e. single columns), and a RocksFilterRel,
which is only allowed to do simple operations on PK columns. In other words, you write RocksDB
equivalents of the relational operators scan, project, filter, that do no more than — often
a lot less than — their logical counterparts. The mistake in the Splunk adapter was giving
a “table scan” operator too many responsibilities.

Create a RocksConvention, a RockRel interface, and some rules:

 RocksProjectRule: ProjectRel on a RocksRel ==> RocksProjectRel
 RocksFilterRule: FilterRel on RocksRel ==> RocksFilterRel

RocksProjectRule would only push down column references; it might need to create a ProjectRel
above to handle expressions that cannot be pushed down. Similarly RocksFilterRule would only
push down simple conditions.

Fire those rules, together with the usual rules to push down filters and projects, and push
filters through projects, and you will end up with a plan with

RocksToEnumerableConverter
  RocksProject
    RocksFilter
      RocksScan

at the bottom (RocksProject and RocksFilter may or may not be present). When you call the
RocksToEnumerableConverter.implement method, it will gather together the project, filter and
scan and make a single call to RocksDB, and generate code for an enumerable. The rest of the
query will be left behind, above the RocksToEnumerableConverter, and also get implemented
using code-generation.

ArrayTable would be useful if you want to cache data sets in memory. As always with caching,
I’d suggest you skip it in version 1.

Sounds like an interesting project. You ask really smart questions, so I’d be very happy
to help further. And when you have something, please push it to github so we can all see it.

Julian


On Oct 1, 2014, at 12:57 AM, Dan Di Spaltro <dan.dispaltro@gmail.com> wrote:

> First off, this project is awesome.  Great in code documentation.
> 
> I am trying to build a sql frontend for rocksdb.  The general idea is
> to iterate over a single key/value pairs and build them up to a map, 1
> layer deep.
> 
> foo\0bar = v1
> foo\0baz = v2
> f2\0bar = v3
> f2\0baz = v4
> 
> 
>        bar     baz
> foo  v1    v2
> f2   v3    v4
> 
> So I started looking at the Splunk code since it seems like middle of
> the road complexity with projection (unknown columns at metadata time)
> and filter push-down (via the search query).  The spark example seemed
> overly complex and the csv example doesn't have anything but
> projection (which is easy to grasp).  Here are some of my specific
> trouble areas:
> 
> #1 "primary key". with specific columns, I'd like pass them down to
> the db engine to filter.  So I've set up the structure very similar to
> the Splunk example, both with projections, filters and filter on
> projections and vice versa.  Is there a good pattern to do this
> basically to pass all the stuff I need to push down to the query
> layer?  If it's not a pk how do I let the in-memory system do the
> filtering?
> 
> #2 "alchemy".  There is a lot of alchemy in [1], is it complex because
> you're overloading a single class with multiple functions? Any good
> ideas where I'd learn the top vs bottom projections. That's probably a
> tough question, since I am pretty much a newb at query planning/sql
> optimizers.
> 
> #3 "array table". Would that be useful in this situation?
> 
> [1] https://github.com/apache/incubator-optiq/blob/master/splunk/src/main/java/net/hydromatic/optiq/impl/splunk/SplunkPushDownRule.java#L99
> 
> This is really neat stuff,
> 
> -Dan
> 
> -- 
> Dan Di Spaltro


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