What are the best way to find out in impala if table a is a subset of table b?

Question

I have two parquet based external tables in Impala, like to know if one is the subset of another, what would be the best way to get that?

The two tables has same schema with dozen or even hundred fields

Thank you.

Answer 1

Assuming no duplicates in either table, then A would be a subset of B if the count from:

select count(*)
from B;

is the same as the count from:

select count(*)
from ((select * from a) union
      (select * from b)
     ) ab;

That is, adding the rows of A to B and eliminating duplicates does not add any more rows.

This is not a strict subsetting relationship, because "A" could be equivalent to "B". For a strict subsetting relationship, add the condition that :

select count(*)
from A

is strictly less than the count of B .

This assumes that the types and columns in A and B are compatible -- a reasonable assumption if you are asking about one being a subset of the other.