Implement a low-level HASH-JOIN dataset

[jjeffcaii]

The HASH-JOIN dataset API could be similar with below codes:

func HashJoin(left,right Dataset, joinColumns, ...other options) Dataset {
    //  ...
}

The HASH-JOIN should contain two phases:

1. build hash chunk from the left dataset, a chunk looks like a hash map: key=hash(values of join_columns), value=rows
2. probe each row in right dataset, compute the hash key, then check if the row matched one by one

some docs:

• https://www.luozhiyun.com/archives/631
• https://www.modb.pro/db/68477
• https://www.infoq.cn/article/6xgx92fyq45cmxpj2mgz

A tiny example, we have two datasets, and we want to execute SQL like select foo.id,bar.id from foo join bar on foo.x = bar.y

--- Dataset foo

id	x
a	5
b	6
c	7

--- Dataset bar

id	y
j	5
k	8

1. build from foo: a hash map by hash method x -> x%2, we got a map like { 0: [b-6], 1: [a-5,c-7] }
2. probe from bar: j-5 will check the chunk[key=1] and the k-8 will check the chunk[key=0]
3. finally, a-5 and j-5, bingo!

[wang1309]

pls assign to me