However, Oracle hints such as ORDERED, LEADING, INDEX, FULL, and the various AJ and SJ Oracle hints can tame a wild optimizer and give you optimal performance. Oracle hints are enclosed within comments to the SQL commands DELETE, SELECT or UPDATE or are designated by two dashes and a plus sign.

The no_index hint is most commonly used in cases where you have determined that a parallel full-table scan will outperform an index range scan. The no_index hint is equivalent to the full hint and is rarely used in SQL tuning. A root cause solution to using the no_index hint is to re-analyze the schema objects and for the optimizer not to use an index without the need for the no_index hint. NO_INDEX hint does not work unless you specify Table name (or alias name if used in the query) after "NO_INDEX(" and before the index name. for example, /*+ NO_INDEX(TAB1 INDEX_TAB1_C1) Once you put this, the hint will start working, Make sure if you have used alias then use alias name here. It says “The NO_INDEX hint does not applying to UNIQUE indexes. The NO_INDEX hint applies to function-based, B*-tree, bitmap, cluster, or domain indexes.” I have tested it on 9.2.0.8, 10.2.0.3 and 11.2.0.1 and I see that “no_index hint” WORKS as intended even on unique indexes. This means that the default Oracle optimization that doesn't use an optimizer hint will try to find the string inside the indexed field first and also find if the data it is a sub-string of the data in the field, then it will check that the data doesn't match another specific string. After the index is checked the other columns are then checked.

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. 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.

If this hint specifies no indexes, then the optimizer considers the cost of a scan on each available index on the table and then performs the index scan with the lowest cost. The optimizer can also choose to scan multiple indexes and merge the results, if such an access path has the lowest cost. If there is no top-level optimizer mode hint, then mode hints in referenced views are used as long as all mode hints in the views are consistent. If two or more mode hints in the referenced views conflict, then all mode hints in the views are discarded and the session mode is used, whether default or user-specified. The NO_MERGE hint causes Oracle not to merge mergeable views. This hint is most often used to reduce the number of possible permutations for a query and make optimization faster. FACT The no_index hint is most commonly used in cases where you have determined that a parallel full-table scan will outperform an index range scan. The no_index hint is equivalent to the full hint and is rarely used in SQL tuning. A root cause solution to using the no_index hint is to re-analyze the schema objects and for the optimizer not to use an index without the need for the no_index hint. This means that the default Oracle optimization that doesn't use an optimizer hint will try to find the string inside the indexed field first and also find if the data it is a sub-string of the data in the field, then it will check that the data doesn't match another specific string. After the index is checked the other columns are then checked. If this hint specifies no indexes, then the optimizer does not consider a scan on any index on the table. This behavior is the same as a NO_INDEX hint that specifies a list of all available indexes for the table. The NO_INDEX hint applies to function-based, B*-tree, bitmap, cluster, or domain indexes. 4. Re: index hint not working Jens Petersen Mar 17, 2004 9:18 PM (in response to 358284) From the manual: To ensure that Oracle will use the index rather than performing a full table scan, be sure that the value of the function is not null in subsequent queries.

Oracle does not report when a hint is mis-spelled and because hints are hint will be ignored because the table name is not specified in the index hint:.

This means that the default Oracle optimization that doesn't use an optimizer hint will try to find the string inside the indexed field first and also find if the data it is a sub-string of the data in the field, then it will check that the data doesn't match another specific string. After the index is checked the other columns are then checked. If this hint specifies no indexes, then the optimizer does not consider a scan on any index on the table. This behavior is the same as a NO_INDEX hint that specifies a list of all available indexes for the table. The NO_INDEX hint applies to function-based, B*-tree, bitmap, cluster, or domain indexes. 4. Re: index hint not working Jens Petersen Mar 17, 2004 9:18 PM (in response to 358284) From the manual: To ensure that Oracle will use the index rather than performing a full table scan, be sure that the value of the function is not null in subsequent queries.

At least one of the tables is accessed through a full table scan, or an index is accessed Oracle checks the statement for a parallel hint. The intent is to get four SQL*Loader sessions going at once, not to run four sessions one at a time.

At least one of the tables is accessed through a full table scan, or an index is accessed Oracle checks the statement for a parallel hint. The intent is to get four SQL*Loader sessions going at once, not to run four sessions one at a time. Why are Optimizer hints ignored? ▫ Specifying an index hint on a table with no indexes. Invalid hint. Invalid hint because no indexes. Oct 11, 2018 Add a first row hint, that way oracle may respond the same way as to the datagrip query. CREATE TABLE DUMMY_SCHEMA.DUMMY_TABLE  Feb 16, 2010 The Oracle hint have to append after the 'select', not before. Hibernate generated query /* + INDEX(stock idx_stock_code) */ select. Copy. Purpose: NO_INDEX hint explicitly notifies the optimizer to not to use the specified index(s). This can be used for query testing purpose without dropping the actual index. This can be used for query testing purpose without dropping the actual index. If this hint specifies no indexes, then the optimizer considers the cost of a scan on each available index on the table and then performs the index scan with the lowest cost. The optimizer can also choose to scan multiple indexes and merge the results, if such an access path has the lowest cost.

NO_INDEX hint does not work unless you specify Table name (or alias name if used in the query) after "NO_INDEX(" and before the index name. for example, /*+ NO_INDEX(TAB1 INDEX_TAB1_C1) Once you put this, the hint will start working, Make sure if you have used alias then use alias name here.

If this hint specifies no indexes, then the optimizer does not consider a scan on any index on the table. This behavior is the same as a NO_INDEX hint that specifies a list of all available indexes for the table. The NO_INDEX hint applies to function-based, B*-tree, bitmap, cluster, or domain indexes. 4. Re: index hint not working Jens Petersen Mar 17, 2004 9:18 PM (in response to 358284) From the manual: To ensure that Oracle will use the index rather than performing a full table scan, be sure that the value of the function is not null in subsequent queries. The NO_MERGE hint causes Oracle not to merge mergeable views. This hint is most often used to reduce the number of possible permutations for a query and make optimization faster. FACT NO_INDEX Documented: Explicitly disallows a set of indexes for the specified table. The NO_INDEX hint applies to function-based, B*tree, bitmap, cluster, or domain indexes. /*+ NO_INDEX([@queryblock] ) */ conn uwclass/uwclass@pdbdev set autotrace traceonly explain SELECT s.latitude FROM servers s, serv_inst i If no indexes are given as arguments for the INDEX_COMBINE hint, the optimizer uses whatever Boolean combination of bitmap indexes has the best cost estimate for the table. If certain indexes are given as arguments, the optimizer tries to use some Boolean combination of those particular bitmap indexes.

4. Re: index hint not working Jens Petersen Mar 17, 2004 9:18 PM (in response to 358284) From the manual: To ensure that Oracle will use the index rather than performing a full table scan, be sure that the value of the function is not null in subsequent queries. The NO_MERGE hint causes Oracle not to merge mergeable views. This hint is most often used to reduce the number of possible permutations for a query and make optimization faster. FACT NO_INDEX Documented: Explicitly disallows a set of indexes for the specified table. The NO_INDEX hint applies to function-based, B*tree, bitmap, cluster, or domain indexes. /*+ NO_INDEX([@queryblock] ) */ conn uwclass/uwclass@pdbdev set autotrace traceonly explain SELECT s.latitude FROM servers s, serv_inst i If no indexes are given as arguments for the INDEX_COMBINE hint, the optimizer uses whatever Boolean combination of bitmap indexes has the best cost estimate for the table. If certain indexes are given as arguments, the optimizer tries to use some Boolean combination of those particular bitmap indexes. 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; However, Oracle hints such as ORDERED, LEADING, INDEX, FULL, and the various AJ and SJ Oracle hints can tame a wild optimizer and give you optimal performance. Oracle hints are enclosed within comments to the SQL commands DELETE, SELECT or UPDATE or are designated by two dashes and a plus sign. 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. 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.

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