Strange behavior of Oracle and group by

While analyzing performance of an SQL query in Oracle, I noticed a strange behavior. I noticed that Oracle’s plan behavior changes depending on value used in query.

For example here is my table structure:

  CREATE TABLE "USAGE" 
   (    "ID" NUMBER(11,0) NOT NULL ENABLE, 
    "CREATED_DATE" TIMESTAMP (6), 
    "MODIFIED_DATE" TIMESTAMP (6),
    "PERIOD" TIMESTAMP (6) NOT NULL ENABLE, 
    "DOWNLOAD" NUMBER(19,0),
     PRIMARY KEY ("ID")
   );

  CREATE INDEX "USAGE_A0ACFA46" ON "USAGE" ("PERIOD");


  CREATE UNIQUE INDEX "USAG_PERIOD_772992E2_UNIQ" ON "USAGE" ("PERIOD");

When I fetched plan of following query, I see that table is accessed by INDEX RANGE SCAN, which is expected:

explain plan for
select usg.period, sum(usg.download)
 from usage usg
 where usg.period>=TIMESTAMP '2018-11-30 00:00:00'
    group by usg.period;

SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());

----------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                      | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                           |     1 |    18 |     3   (0)| 00:00:01 |
|   1 |  SORT GROUP BY NOSORT        |                           |     1 |    18 |     3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| USAGE                     |     1 |    18 |     3   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | USAG_PERIOD_E67F63D3_UNIQ |     1 |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------

However, when I change the value only, I noticed that table is accessed by TABLE ACCESS FULL, which is very strange for me:

select usg.period, sum(usg.download)
 from usage usg
 where usg.period>=TIMESTAMP '2017-11-30 00:00:00'
    group by usg.period;

SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());

----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |   133 |  2394 |    69   (2)| 00:00:01 |
|   1 |  HASH GROUP BY     |       |   133 |  2394 |    69   (2)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL| USAGE |  9505 |   167K|    68   (0)| 00:00:01 |
----------------------------------------------------------------------------

My question is, why does it happen? I would expect Oracle to use INDEX RANGE SCAN, no matter what the value is.

My database is Oracle 11g

Answer

the optimizer may decide whether to use or not to use an individual index depending on the amount of the data, for huge set of data the full-scan is preferred rather than the index range scan.

Your second case seems scanning a bigger data set as being the interval is longer.

As an example, try to restrict your scans for only one-month period

Q1 :

select usg.period, sum(usg.download)
  from usage usg
 where usg.period between timestamp'2017-11-01 00:00:00' and timestamp'2017-11-30 00:00:00'
 group by usg.period;

and

Q2 :

select usg.period, sum(usg.download)
 from usage usg
 where usg.period between timestamp'2018-11-01 00:00:00' and timestamp'2018-11-30 00:00:00'
    group by usg.period;

for both queries Q1 and Q2, you most probably can see a index range scan with close values of costs depending on the homogeneously populated data for the table. The indexes are mostly good for small number of rows.