Oracle加入表时选择了错误的索引

Question

I have a long query, but similar to the short version here:

select * from table_a a

left join table_b b on

b.id = a.id and
b.name = 'CONSTANT';

There are 2 indexes on table_b for both id and name , idx_id has fewer duplicates and idx_name has a lot of duplicates. This is quite a large table (20M+ records). And the join is taking 10min+.

A simple explain plan shows a lot of memory uses on the join part, and it shows it uses the index for name as opposed to id .

How to solve this issue? How to force using the idx_id index?

I was thinking of putting b.name='CONSTANT' to where clause, but this is a left join and where will remove all the record that exists in table_a.

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Updated explain plan. Sorry cannot paste the whole plan.

Explain plan with b.name='CONSTANT' :

Explain plan when commenting b.name clause:

Answer 1

Add an optimizer hint to your query.

Without knowing your 'long' query, it's difficult to know if Oracle is using the wrong one, or if your interpretation that indexb < indexa so therefore must be quicker for query z is correct.

To add a hint the syntax is

select /*+ index(table_name index_name) */ * from ....;

Answer 2

What is the size of TABLE_A relative to TABLE_B? It wouldn't make sense to use the ID index unless TABLE_A had significantly less rows than TABLE_B.

Index range scans are generally only useful when they access a small percentage of the rows in a table. Oracle reads the index one-block-at-a-time, and then still has to pull the relevant row from the table. If the index isn't very selective, that process can be slower than a multi-block full table scan.

Also, it might help if you can post the full explain plan using this text format:

explain plan for select ... ;
select * from table(dbms_xplan.display);