Use of index in multiple join condition oracle - oracle

Question

I have two tables: tableA and tableB
TableA have millions of record and tableB have around 1000 records
Table A {
aid
city, (city is indexed)
state,
X,
Y
}
Table B {
bid,
city,
state
}
Now my query is
SELECT X, Y, COUNT(*) FROM A,B
WHERE A.city = B.city
and A.state=B.state
group by X,Y
This query is running very slow. However when we had join only on city everything was working very quickly.
Now my query is
SELECT X, Y, COUNT(*) FROM A,B
WHERE A.city = B.city
group by X,Y
So I went to the explain plan and in the first case(slow) the query plan is not using the index whereas in the second case it was using the city index. I tried adding state index in A table which did not help as expected. Also i tried to use the index hint like /*+ INDEX(A,city_idx) */ after select which did not help much. Can you help me out in this case?

Answer 1

Creating indexes for both tables on city and state is likely to help.

Answer 2

Create a composite index on the table A that has all the four columns: city, state, X, Y:
CREATE INDEX index_name ON table_name (city, state, X, Y);
In this way, your query won't need to access the table A, only the newly created index. Of course, the downside of yet another index -> insert/update/delete in this table will be slower.

Answer 3

TableA have millions of record and tableB have around 1000
In this case using nested loops seems like the most suited access path for the job.
you are requesting a aggregation based on two columns from table A meaning oracle will have to access pretty much all the blocks in the table anyway. In this case creating an index on the big table will be useless. creating an index on the small, inner table of the join, will make sense.
WHERE A.city = B.city and A.state=B.state
WHERE A.city = B.city
Can the same city exist in two states ? sounds unlikely... if a city cannot exists in more then one state then any index on state (in either table) will be redundant.
As #Florin Ghita noted in his comment you can use the hint USE_NL to force oracle to use nested loops but personally, I highly recommend avoiding hints (for so many reasons - mostly maintenance).
my suggestions are
gather stats on both tables to make sure oracle knows the
proportions and have sufficient data to estimate cardinality
exec dbms_stats.gather_table_stats(user,'tableX').
Test the query with parallel execution - parallel is great at
speeding NL between small and big tables by broadcasting the entire
small table to the slave process working the big table chunk (get
even further with compression on the small table).

Answer 4

Cities and states are related but the optimizer does not understand that. Oracle can probably accurately predict each condition separately but not together.
For example, assume that 10% of all states match and 10% of all cities match. When both conditions are present Oracle will estimate 0.1 * 0.1 = 0.01. The real number is probably closer to 0.1. If the city name matches the state name will almost always match.
Adding extended statistics tells Oracle about this column relationship. And these statistics can help any query, not just the current problem query.
declare
v_name varchar2(100);
begin
v_name := dbms_stats.create_extended_stats(user, 'A', '(city, state)');
v_name := dbms_stats.create_extended_stats(user, 'B', '(city, state)');
dbms_stats.gather_table_stats(user, 'A');
dbms_stats.gather_table_stats(user, 'B');
end;
/
Without the plans we can't accurately predict whether this will solve the problem or not. But giving the optimizer more accurate information usually helps and almost never hurts.