Impalad summary

impalad-summary

## Git repo

Find modified impala [here](https://github.com/rampage644/impala-cut). First, have a look at [this](https://github.com/rampage644/impala-cut/blob/executor/README.md) *README* file.

## Task description

Original task was to prune impalad to some sort of *executor* binary which only executes part of query. Two approaches were suggested: top-down and bottom-up. I used bottom-up approach.

My intention was to write unittest that whill actually test the behavior we need. So, look at `be/src/runtime/plan-fragment-executior-test.cc`. It contains all possible tests (that is, actual code snippets) to run part of query with or without data. Doing so helped me a lot to understand impalad codebase relative to query execution.

## Achievements ?

First of all, I actually wrote unittest that executes part of query given the query itself and data. As side effect, i did some  impalad codebase research (everything relating to query execution). There are a lot code left aside, though.

### Impalad peculiarities

 1. MPPI-oriented. Uses lots of threads and even more. Vanilla impalad when doing nothing has ~183 threads created.
 2. Some of functionality is done in JAVA via JNI.
 3. Closely coupled code, a lot of *Singletones*.