Hi Gavin, in a nutshell, a logical plan consists of logical operators (say, a join), which can be implemented in several ways at the physical level (say, merge join, hash join, etc.), and are therefore associated with some corresponding physical operators.
How the logical vs physical planning is performed depends on the optimization framework you use. SQL is not a plan, it's a declarative language, it only dictates "what" you will be getting, the logical plan sketches how you will get it, the physical plan fills the missing details. Then there is also query compilation which translates to "real instructions" (be it machine code, a DAG for an execution engine like for Spark or Hive, etc.). In federated queries, you will plan and split the work across different databases, you will pass them either SQL or some native query language (CQL for Cassandra, for example), but then the DB will do all the steps again, parse, validate, etc., build a logical plan, optimize it and run it as it thinks it's best, which can be radically different from what you envisioned. I have never heard of systems where you can directly inject a physical/execution plan, but I haven't really looked for that. HTH, Alessandro On Tue, 22 Mar 2022 at 22:10, Gavin Ray <ray.gavi...@gmail.com> wrote: > I'm on my second pass of the book "How Query Engines Work" by Arrow's own > Andy Grove > (Really great read, huge recommendation: > https://leanpub.com/how-query-engines-work) > > Something I'm not sure I'm fully understanding is what qualifies something > as a Physical Plan > A logical plan is straightforward, say I have an expression, "Select name > from users where ID is less than 5" > > Then I can represent this as an abstract, logical operation like: > > LogicalPlan( > project = ["name"], > filter = Filter(LessThan(Column("id"), Literal(5))) > ) > > Now say I want to give this plan to a database and have it run it. > I need to write an implementation for translating this to an executable > expression > (Probably SQL) > > Is the class that implements the translation to SQL that gets executed the > Physical Plan > Or is there no Physical Plan, and that's the database's job to figure out? >
