Question

I have to join tables in Hbase.

I integrated HIVE and HBase and that is working well. I can query using HIVE.

But can somebody help me how to join tables in HBase without using HIVE. I think using mapreduce we can achieve this, if so can anybody share a working example that I can refer.

Please share your opinions.

I have an approach in mind. That is,

If I need to JOIN tables A x B x C;
I may use TableMapReduceUtil to iterate over A, then get Data from B and C inside the TableMapper. Then use the TableReducer to write back to another table Y.

Will this approach be a good one.

Answer 1

That is certainly an approach, but if you are doing 2 random reads per scanned row then your speed will plummet. If you are filtering the rows out significantly or have a small dataset in A that may not be an issue.

Sort-merge Join

However the best approach, which will be available in HBase 0.96, is the MultipleTableInput method. This means that it will scan table A and write it’s output with a unique key that will allow table B to match up.

E.g. Table A emits (b_id, a_info) and Table B will emit (b_id, b_info) merging together in the reducer.

This is an example of a sort-merge join.

Nested-Loop Join

If you are joining on the row key or the joining attribute is sorted in line with table B, you can have a instance of a scanner in each task which sequentially reads from table B until it finds what it’s looking for.

E.g. Table A row key = “companyId” and Table B row key = “companyId_employeeId”. Then for each Company in Table A you can get all the employees using the nest-loop algorithm.

Pseudocode:

for(company in TableA):
    for(employee in TableB):
        if employee.company_id == company.id:
            emit(company.id, employee)

This is an example of a nest-loop join.

More detailed join algorithms are here:

• http://en.wikipedia.org/wiki/Nested_loop_join
• http://en.wikipedia.org/wiki/Hash_join
• http://en.wikipedia.org/wiki/Sort-merge_join