[Solved] Oracle doesn\'t use index on varchar2 column in case of of order by clause

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Oracle doesn\'t use index on varchar2 column in case of of order by clause

Database type:

Here's my table with several millions of rows:

  CREATE TABLE "TEST_TABLE"
  ( "ID" NUMBER(*,0),
    "TYPE" NUMBER(*,0),
    "NAME" VARCHAR2(10 BYTE)
  );

and a couple of indexes:

   CREATE INDEX TEST_INDEX_T_N ON TEST_TABLE (TYPE, NAME);
   CREATE INDEX TEST_INDEX_T_I ON TEST_TABLE (TYPE, ID);

In case of following query

   SELECT * FROM TEST_TABLE WHERE TYPE = 1 ORDER BY ID

I got following plan 

   |   0 | SELECT STATEMENT |                |   886K|    21M|  2192   (1)| 00:00:27 |
   |*  1 |  INDEX RANGE SCAN| TEST_INDEX_T_I |   886K|    21M|  2192   (1)| 00:00:27 |

However, in case of:

   SELECT NAME FROM TEST_TABLE WHERE TYPE = 1 ORDER BY NAME;

I got

   |   0 | SELECT STATEMENT  |                |   886K|    16M|       |  7826   (1)| 00:01:34 |
   |   1 |  SORT ORDER BY    |                |   886K|    16M|    23M|  7826   (1)| 00:01:34 |
   |*  2 |   INDEX RANGE SCAN| TEST_INDEX_T_N |   886K|    16M|       |  2415   (1)| 00:00:29 |

So, as far as I understand, oracle doesn't use index for sorting varchar column (cause there's extra SORT ORDER BY operation within explain plan), but everything is fine in case of INT or DATE types. How it is possible to "use index for sorting" varchar columns?

How to optimize this SQL query?

The following recommendations will help you in your SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
  2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
Optimal indexes for this query:
CREATE INDEX test_table_idx_type_id ON TEST_TABLE (TYPE,ID);
The optimized query:
SELECT
        * 
    FROM
        TEST_TABLE 
    WHERE
        TEST_TABLE.TYPE = 1 
    ORDER BY
        TEST_TABLE.ID

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* original question posted on StackOverflow here.