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Drill, mail # dev - updates to logical plan spec


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Ted Dunning 2012-10-13, 02:37
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Re: updates to logical plan spec
Julian Hyde 2012-10-13, 09:03
I guess I've been thinking about something a little different. I've been thinking about what would be an appropriate algebra to internally represent, manipulate, and optimize Drill queries.

My conclusion is that the best candidate is relational algebra, augmented with data types that allow collections of nested records and with "explode" and "implode" operators. ("explode" takes a record with nested records and converts it to a sequence of flat records, plus a "location" value that indicates how the nested record fits into the parent; "implode" is the inverse: it takes a sequence of flat records with a "location" field and converts into a nested record.) Here is how I came to that conclusion.

The algebra is lower level than DrQL: viz, it would have fewer operators, and the operators would be easier to reason about and to write transformation rules for. (Anyone who thinks that DrQL's 'where' operator is straightforward should ponder why BigQuery will sometimes give the error "Cannot query the cross product of repeated fields".)

The algebra is (probably) higher level than what Ted calls a "logical plan". His operators produce two outputs, and while that makes perfect sense for physical operators, it is difficult to reason about.

Here are a few reasons why I consider DrQL to be a less clean model than the relational model. As I've said before, the "where" operator has a much more complex behavior than SQL's where. It is best understood as decomposing records, applying a filtering predicate, then re-composing the fragments of the row that made it through the filter. The "within" clause is a nice shorthand, but is too limited to be considered a full operator. Trees (collections within rows) are similar to relations (collections of rows) but are handled using different operators. If the "tree" model was as powerful as advertised then we wouldn't need the concept of "relation" at all.

That doesn't mean that DrQL is not a good query language. It seems to be concise, and users learn it quickly and like it. Syntactic sugar operators like "within" is totally appropriate (just like syntactic sugar "select distinct" and "having" in SQL).

To fix up DrQL's "where" operator, we convert it to "explode" followed by "filter" followed by "implode". To fix up aggregate "within", we apply "explode" then aggregate. We find that we never need to operate on trees. If we need to operate on a tree, we explode into several records, apply relational operators on those records, and if necessary implode back again. We're operating in relational algebra.

This is good news, because relational algebra is well behaved and well understood.

And by the way, even if the algebra is about exploded sets of flat records, there's no reason that the physical operators can't operate on tree-structured records. We could recognize explode-followed-by-filter-followed-by-implode and implement an operator that does precisely the same as a DrQL "where" clause.

Am I over-engineering here? It's possible. Maybe Drill doesn't need query optimization. Maybe queries can go straight from a DrQL parse tree to a DAG of operators using a straightforward mapping. But I'll argue that many people will come to Drill with SQL queries, or queries very similar to SQL, data sets with minimal nesting, and will be saddened when Drill can't execute their queries. This particular user kicked the tires, was impressed with the speed of the car, but was disappointed that he couldn't drive it where he wanted to go: http://cwebbbi.wordpress.com/2012/05/20/a-look-at-google-bigquery/.

Julian

On Oct 12, 2012, at 7:37 PM, Ted Dunning <[EMAIL PROTECTED]> wrote:

> I talked to Jason some more.  He had some very good suggestions.
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> a) some operators need to have multiple outputs.  For instance, the group
> operator needs to output the main data stream and a reference to the
> grouped field
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> b) what Julian was calling nest/unnest is more naturally called explode and
> flatten.  The idea is that some field has a list-like value and the output
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Ted Dunning 2012-10-13, 13:29