Strange behavior of Oracle and group by - sql

Question

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 1

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.