Question

I have a table MYTABLE with a date column SDATE which is the primary key of the table and has a unique index on it.

When I run this query:

SELECT MIN(SDATE) FROM MYTABLE

it gives answer instantly. The same happens for:

SELECT MAX(SDATE) FROM MYTABLE

But, if I query both together:

SELECT MIN(SDATE), MAX(SDATE) FROM MYTABLE

it takes much more time to execute. I analyzed the plans and found when one of min or max is queried, it uses INDEX FULL SCAN(MIN/MAX) but when both are queried at the same time, it does a FULL TABLE SCAN.

why?

Test Data:

version 11g

create table MYTABLE
(
  SDATE  DATE not null,
  CELL   VARCHAR2(10),
  data NUMBER
)
tablespace CHIPS
  pctfree 10
  pctused 40
  initrans 1
  maxtrans 255
  storage
  (
    initial 64K
    minextents 1
    maxextents unlimited
  );

alter table MYTABLE
  add constraint PK_SDATE primary key (SDATE)
  using index 
  tablespace SYSTEM
  pctfree 10
  initrans 2
  maxtrans 255
  storage
  (
    initial 64K
    minextents 1
    maxextents unlimited
  );

Load table:

declare 
  i integer;
begin
  for i in 0 .. 100000 loop
     insert into MYTABLE(sdate, cell, data)
     values(sysdate - i/24, 'T' || i, i);     
     commit;
  end loop;
end;

Gather stats:

begin
  dbms_stats.gather_table_stats(tabname => 'MYTABLE', ownname => 'SYS');
end;

Plan1:

Plan2:

Answer 1

The Index Full Scan can only visit one side of the index. When you are doing

SELECT MIN(SDATE), MAX(SDATE) FROM MYTABLE

you are requesting to visit 2 sides. Therefore, if you want both the minimum and the maximum column value, an Index Full Scan is not viable.

A more detailed analyze you can find here.