Can you force the database to use an index on a query in Oracle?

Can you force the database to use an index on a query in Oracle?

As for your question — there is no way to FORCE the optimizer to use an index, we can make it so that the use of an index is possible.

How do I force an index in SQL query?

In case the query optimizer ignores the index, you can use the FORCE INDEX hint to instruct it to use the index instead. In this syntax, you put the FORCE INDEX clause after the FROM clause followed by a list of named indexes that the query optimizer must use.

How do I create an index usable?

Altering Indexes

  1. Rebuild or coalesce an existing index.
  2. Deallocate unused space or allocate a new extent.
  3. Specify parallel execution (or not) and alter the degree of parallelism.
  4. Alter storage parameters or physical attributes.
  5. Specify LOGGING or NOLOGGING.
  6. Enable or disable key compression.
  7. Mark the index unusable.

Does Oracle have clustered indexes?

The only clustered indexes in Oracle are the Index-Organized tables (IOT) primary key indexes. You can determine if a table is an IOT by looking at the IOT_TYPE column in the ALL_TABLES view (its primary key could be determined by querying the ALL_CONSTRAINTS and ALL_CONS_COLUMNS views).

What is SQL force index?

FORCE INDEX works by only considering the given indexes (like with USE_INDEX) but in addition it tells the optimizer to regard a table scan as something very expensive. However if none of the ‘forced’ indexes can be used, then a table scan will be used anyway.

What makes an oracle index unusable?

Oracle indexes can go into a UNUSABLE state after maintenance operation on the table or if the index is marked as ‘unusable’ with an ALTER INDEX command. A direct path load against a table or partition will also leave its indexes unusable.

What is the trick in Oracle force indexing?

What is the trick in Oracle force index? Answer: Given accurate CBO statistics, the Oracle optimizer will also choose the “best” index, and it’s not always a good idea to force a specific index unless you are sure that the index will always be the fastest path to your rows. The easiest way to force index usage is with the index hint.

Why index is not being used in Oracle?

There could be many reasons for Index not being used. Even after you specify hints, there are chances Oracle optimizer thinks otherwise and decide not to use Index. You need to go through the EXPLAIN PLAN part and see what is the cost of the statement with INDEX and without INDEX. Assuming the Oracle uses CBO.

How do I force an index to use a table alias?

When forcing an index, always use the table alias whenever you have a query that specifies an alias. For example, the following query will force the use of the dept_idx index because the emp table is aliased with “e”:

Why does the optimizer ignore index for full table scan?

Assuming the Oracle uses CBO. Most often, if the optimizer thinks the cost is high with INDEX, even though you specify it in hints, the optimizer will ignore and continue for full table scan. Your first action should be checking DBA_INDEXES to know when the statistics are LAST_ANALYZED.