Reducers stopped working at 66.68% while running HIVE Join query

Question

Trying to join 6 tables which are having 5 million rows approximately in each table. Trying to join on account number which is sorted in ascending order on all tables. Map tasks are successfully finished and reducers stopped working at 66.68%. Tried options like increasing number of reducers and also tried other options set hive.auto.convert.join = true; and set hive.hashtable.max.memory.usage = 0.9; and set hive.smalltable.filesize = 25000000L; but the result is same. Tried with small number of records (like 5000 rows) and the query works really well.

Please suggest what can be done here to make it work.

Answer 1

Reducers at 66% start doing the actual reduce (0-33% is shuffle, 33-66% is sort). In a join with hive, the reducer is performing a Cartesian product between the two data sets.

I'm going to guess that there is at least one foreign key that is appearing frequently in all of the data sets. Watch for NULL and default values.

For example, in a join, imagine the key "abc" appears ten times in each of the six tables (10^6). That's a million output records for that one key. If "abc" appears 1000 times in one table, 1000 in another, 1000 in another, then twice in the other three tables, you get 8 billion records (1000^3 * 2^3). You can see how this gets out of hand. I'm guessing there is at least one key that is resulting in a massive number of output records.

This is general good practice to avoid in RDBMS outside of Hive as well. Doing multiple inner joins between many-to-many relationships can get you in a lot of trouble.