WL#3258: Push down join into NDB kernel

Proposal:
Have TC send SCAN_FRAG_REQ to DBSPJ instead of DBLQH.
Let attr/key info contain description of join.
One could also "save" join in DBSPJ then it would in practice be a view.

CREATE TABLE PERSON (PERSON_ID);
CREATE TABLE ADDRESS (ADDRESS_ID, PERSON_ID, CITY_ID);
CREATE TABLE CITY (CITY_ID)

Phase 1: [ RANGE ] SCAN + [ PK ] LOOKUP
SELECT * FROM ADRESS, PERSON
WHERE PERSON.PERSON_ID = ADRESS.ADDRESS_ID
  [ FILTER ON PERSON ]
  [ FILTER ON ADDRESS ]
  [ RANGE ON ADDRESS ]

I.e API decides join order.
This will be implemented as (range) scan on ADRESS
  followed by (pk) lookup on PERSON

Phase 2: [ RANGE ] SCAN + RANGE SCAN
SELECT * FROM PERSON, ADDRESS
WHERE PERSON.PERSON_ID = ADRESS.ADDRESS_ID
  [ FILTER ON PERSON ]
  [ FILTER ON ADDRESS ]
  [ RANGE ON PERSON ]

This will be impl. as (range) scan on PERSON
  followed by range scan on ADDRESS(PERSON_ID)

Phase 3: More than 2-way join
SELECT * FROM PERSON, ADDRESS, CITY
WHERE PERSON.PERSON_ID = ADRESS.ADDRESS_ID
  AND ADRESS.CITY_ID = CITY.CITY_ID
  [ FILTER ON PERSON ]
  [ FILTER ON ADDRESS ]
  [ FILTER ON CITY ]
  [ RANGE ON PERSON ]