Question:  I placed a hint in my SQL but when I check the explain plan the hint is bring ignored.  I was taught that hints could not be ignored, so why does Oracle SQL ignore my hint?

Answer:  First, remember that hints are a last resort measure and you should only use the "good hints" like ordered and ordered_predicates, hints that give the SQL optimizer information that it cannot easily gather from the dbms_stats statistics. 

Also, see my tips on using hints properly.

You are correct that a SQL hint is an "optimizer directive" and that, in theory, hints cannot be ignored, but in the real world, hints are ignored frequently.  Oracle even has a hidden parameter called _optimizer_ignore_hints to make the optimizer ignore valid hints.

There are three main reasons why an invalid hint attempt is being ignored, syntax errors, semantic errors and invalid arguments:

Syntax error hints are ignored

 Oracle does not report when a hint is mis-spelled and because hints are embedded into comments, syntax errors are common, and your hint attempt will be ignored.  If the hint name spelled incorrectly, then the hint will not be ignored. Here we see a query with a misspelled hint name:

select /*+ indrex(erp, dept_idx) */ * from emp;

Semantic error hints are ignored

If a hint argument is spelled incorrectly, and not found in the data dictionary then a semantic hint error will cause the hint will not be ignored. Here we see a query with a misspelled table name, erp instead of emp:

select /*+ index(erp, dept_idx) */ * from emp;

A hint will also be ignored if it is specified improperly, with missing arguments.  For example, the table name is mandatory for an index hint. For example, the following hint will be ignored because the table name is not specified in the index hint:

select /*+ index(dept_idx) */ * from emp;

Incompatible hints are ignored

 Incongruent hints are ignored, such as a hint that does not "make sense".  In the context of the query, an incompatible is indeed ignored, such as a query that specifies incompatible access plans.  For example, the following hints are inconsistent and one of the hints will be ignored:

Ignored:  Parallel hint with index hint

Ignored:  Full hint with index hint

Hints with bad parameters are ignored

It's possible to have a valid hint (syntax and semantics) that is ignored because of a lack of server resources.  For example, the use_hash hint is dependent upon there being enough RAM (as defined by pga_aggregate_target or hash_area_size).  If these settings prohibit the optimizer from invoking a hash join, the use_hash hint will be ignored.

select /*+ use_hash(e,b) parallel(b, 15) */
   e.ename,
   hiredate,
   b.comm
from
   smalltab e,
   largetab b
where
   e.ename = b.ename;

Here are other cases where an invalid hint will be ignored:

Hint	When Ignored
cluster	When used with a noncluster table
hash	When used with a noncluster table
hash_aj	When no subquery exists
index	When the specified index does not exist
index_combine	When no bitmapped indexes exist
merge_aj	When no subquery exists
parallel	When a plan other than TABLE ACCESS FULL is invoked
push_subq	When no subquery exists
star	When improper indexes exists on the fact table
use_concat	When no multiple or conditions exist in the where clause
use_nl	When indexes do not exist on the tables

For complete steps on using hints and other examples of hints that are ignored, see my book "Oracle Tuning: The Definitive Reference". 

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