Oracle 21c

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create table t (
  id not null primary key,
  val
) as
select level,
  dbms_random.string('x', 1)
from dual
connect by level < 1000000
999999 rows affected

-- Set results caching to be manually applied via hints.
ALTER SESSION SET RESULT_CACHE_MODE=MANUAL;
DECLARE
  v_ts   TIMESTAMP := SYSTIMESTAMP;
  v_data SYS.ODCINUMBERLIST;
BEGIN
  -- First execution.
SELECT 1
  BULK COLLECT INTO v_data
    FROM t
    WHERE id = 121;
  
  DBMS_OUTPUT.PUT_LINE(SYSTIMESTAMP - v_ts);
END;
/
1 rows affected

dbms_output:
+000000000 00:00:00.005619000

alter table t modify
PARTITION BY RANGE(id)
	( PARTITION P1 VALUES LESS THAN (1000),
	  PARTITION P2 VALUES LESS THAN (MAXVALUE))
999999 rows affected

DECLARE
  v_ts   TIMESTAMP := SYSTIMESTAMP;
  v_data SYS.ODCINUMBERLIST;
BEGIN
  -- First execution.
SELECT 1
  BULK COLLECT INTO v_data
    FROM t
    WHERE id = 121;
  
  DBMS_OUTPUT.PUT_LINE(SYSTIMESTAMP - v_ts);
END;
/
1 rows affected

dbms_output:
+000000000 00:00:00.006601000