Question

I have a query:

select min(timestamp) from table

This table has 60+million rows, and daily I delete a few off the end. To determine whether or not there is any data old enough do delete I run the query above. There is an index on timestamp ascending, containing only one column, and the query plan in oracle causes this to be a full index scan. Should this not be the definition of a seek?

edit including plan:

| Id  | Operation                  | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
|   2 |   INDEX FULL SCAN (MIN/MAX)| NEVENTS_I2 |     1 |     8 |     4   (100)| 00:00:01 |
|   1 |  SORT AGGREGATE            |            |     1 |     8 |            |          |
|   0 | SELECT STATEMENT           |            |     1 |     8 |     4   (0)| 00:00:01 |

Answer 1

At first I thought that the index would only be used if the column is declared NOT NULL. I tested with the following setup:

SQL> CREATE TABLE my_table (ts TIMESTAMP);

Table created

SQL> INSERT INTO my_table
  2  SELECT systimestamp + ROWNUM * INTERVAL '1' SECOND 
  3    FROM dual CONNECT BY LEVEL <= 100000;

100000 rows inserted

SQL> CREATE INDEX ix ON my_table(ts);

Index created

SQL> EXPLAIN PLAN FOR SELECT MIN(ts) FROM my_table;

Explained

SQL> SELECT * FROM TABLE(dbms_xplan.display);

--------------------------------------------------------------------------------
| Id  | Operation                  | Name | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |      |     1 |    13 |    69   (2)| 00:00:0
|   1 |  SORT AGGREGATE            |      |     1 |    13 |            |
|   2 |   INDEX FULL SCAN (MIN/MAX)| IX   | 90958 |  1154K|            |
--------------------------------------------------------------------------------

Here we notice that the index is used, but all rows from the index are read. If we specify that the column is not null we get a much better plan:

SQL> ALTER TABLE my_table MODIFY ts NOT NULL;

Table altered

SQL> EXPLAIN PLAN FOR SELECT MIN(ts) FROM my_table;

Explained

SQL> SELECT * FROM TABLE(dbms_xplan.display);

--------------------------------------------------------------------------------
| Id  | Operation                  | Name | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |      |     1 |    13 |     2   (0)| 00:00:0
|   1 |  SORT AGGREGATE            |      |     1 |    13 |            |
|   2 |   INDEX FULL SCAN (MIN/MAX)| IX   | 90958 |  1154K|     2   (0)| 00:00:0
--------------------------------------------------------------------------------

In fact this is the same plan that is also used if we add a WHERE clause (Oracle will read a single row from the index):

SQL> EXPLAIN PLAN FOR SELECT MIN(ts) FROM my_table WHERE ts IS NOT NULL;

Explained

SQL> SELECT * FROM TABLE(dbms_xplan.display);

--------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    13 |     2   (0)| 00:00:
|   1 |  SORT AGGREGATE             |      |     1 |    13 |            |
|   2 |   FIRST ROW                 |      | 90958 |  1154K|     2   (0)| 00:00:
|   3 |    INDEX FULL SCAN (MIN/MAX)| IX   | 90958 |  1154K|     2   (0)| 00:00:
--------------------------------------------------------------------------------

This last plan shows (line 2) that Oracle is indeed performing a “seek”.