Help in selecting last n records

Question

Hi All
I have the following tables in my database
CREATE TABLE MACHINE_M (
ID     VARCHAR(20)     NOT NULL,
NAME     VARCHAR(20)
CREATE TABLE STG_READING (
ID     VARCHAR(20)     NOT NULL,
MACHINE_ID     VARCHAR(20),
DATE_ID     NUMBER,
TIME_ID NUMBER,
READING NUMBER
CREATE TABLE READING_DWR (
ID     VARCHAR(20)     NOT NULL,
MACHINE_ID     VARCHAR(20),
DATE_ID     NUMBER,
TIME_ID NUMBER,
READING NUMBER
CREATE TABLE DATE_DIMENSION (
DATE_ID     NUMBER     NOT NULL,
DATE_VALUE     DATE     NOT NULL,
DAY     VARCHAR(10),
DAY_OF_WEEK     INTEGER,
DAY_OF_MONTH     INTEGER,
DAY_OF_YEAR     INTEGER,
PREVIOUS_DAY     DATE,
NEXT_DAY     DATE,
WEEK_OF_YEAR     INTEGER,
MONTH     VARCHAR(10),
MONTH_OF_YEAR     INTEGER,
QUARTER_OF_YEAR     INTEGER,
YEAR     INTEGER
CREATE TABLE TIME_DIMENSION (
TIME_ID     NUMBER     NOT NULL,
HOUR     VARCHAR(3),
MINUTE     VARCHAR(3),
SECOND     VARCHAR(3),
INTERVAL     NUMBER
Referential Constrains:-
     STG_READING(DATE_ID)>>>>>DATE_DIMENSION(DATE_ID)
     STG_READING(TIME_ID)>>>>>TIME_DIMENSION(TIME_ID)
     MACHINE_M(ID)>>>>>STG_READING(MACHINE_ID)
     READING(DATE_ID)>>>>>DATE_DIMENSION(DATE_ID)
     READING(TIME_ID)>>>>>TIME_DIMENSION(TIME_ID)
     MACHINE_M(ID)>>>>>READING(MACHINE_ID)
i will be getting the reading from the machine every 1min which i will be inserting into STG_READING table. the READING column must always be progressive.
so i have to validate the reading column from the staging with the last n records in the READING_DWR table.
selecting the last n records for a particular machine from READING_DWR table every time is causing performance bottleneck.
Can some one suggest me a way to improve the performance

Answer 1

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I have the following table called sales
with sales
as
      select 1 sales_id, 1 prod_id, 1001 inv_num, 120 qty from dual
      union all
      select 2 sales_id, 1 prod_id, 1002 inv_num, 25 qty from dual
select *
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For example in the above sales table, I want to know the total quantity and number of invoice for each product.
The output should look like this
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