In hiveql, what is the most elegant/performatic way of calculating an average value if some of the data is implicitly not present? - hadoop

Question

In Hiveql, what is the most elegant and performatic way of calculating an average value when there are 'gaps' in the data, with implicit repeated values between them? i.e. Considering a table with the following data:
+----------+----------+----------+
| Employee | Date | Balance |
+----------+----------+----------+
| John | 20181029 | 1800.2 |
| John | 20181105 | 2937.74 |
| John | 20181106 | 3000 |
| John | 20181110 | 1500 |
| John | 20181119 | -755.5 |
| John | 20181120 | -800 |
| John | 20181121 | 1200 |
| John | 20181122 | -400 |
| John | 20181123 | -900 |
| John | 20181202 | -1300 |
+----------+----------+----------+
If I try to calculate a simple average of the november rows, it will return ~722.78, but the average should take into account the days that are not shown have the same balance as the previous register. In the above data, John had 1800.2 between 20181101 and 20181104, for example.
Assuming that the table always have exactly one row for each date/balance and given that I cannot change how this data is stored (and probably shouldn't since it would be a waste of storage to write rows for days with unchanged balances), I've been tinkering with getting the average from a select with subqueries for all the days in the queried month, returning a NULL for the absent days, and then using case to get the balance from the previous available date in reverse order. All of this just to avoid writing temporary tables.

Answer 1

Step 1: Original Data
The 1st step is to recreate a table with the original data. Let's say the original table is called daily_employee_balance.
daily_employee_balance
use default;
drop table if exists daily_employee_balance;
create table if not exists daily_employee_balance (
employee_id string,
employee string,
iso_date date,
balance double
);
Insert Sample Data in original table daily_employee_balance
insert into table daily_employee_balance values
('103','John','2018-10-25',1800.2),
('103','John','2018-10-29',1125.7),
('103','John','2018-11-05',2937.74),
('103','John','2018-11-06',3000),
('103','John','2018-11-10',1500),
('103','John','2018-11-19',-755.5),
('103','John','2018-11-20',-800),
('103','John','2018-11-21',1200),
('103','John','2018-11-22',-400),
('103','John','2018-11-23',-900),
('103','John','2018-12-02',-1300);
Step 2: Dimension Table
You will need a dimension table where you will have a calendar (table with all the possible dates), call it dimension_date. This is a normal industry standard to have a calendar table, you could probably download this sample data over the internet.
use default;
drop table if exists dimension_date;
create external table dimension_date(
date_id int,
iso_date string,
year string,
month string,
month_desc string,
end_of_month_flg string
);
Insert some sample data for entire month of Nov 2018:
insert into table dimension_date values
(6880,'2018-11-01','2018','2018-11','November','N'),
(6881,'2018-11-02','2018','2018-11','November','N'),
(6882,'2018-11-03','2018','2018-11','November','N'),
(6883,'2018-11-04','2018','2018-11','November','N'),
(6884,'2018-11-05','2018','2018-11','November','N'),
(6885,'2018-11-06','2018','2018-11','November','N'),
(6886,'2018-11-07','2018','2018-11','November','N'),
(6887,'2018-11-08','2018','2018-11','November','N'),
(6888,'2018-11-09','2018','2018-11','November','N'),
(6889,'2018-11-10','2018','2018-11','November','N'),
(6890,'2018-11-11','2018','2018-11','November','N'),
(6891,'2018-11-12','2018','2018-11','November','N'),
(6892,'2018-11-13','2018','2018-11','November','N'),
(6893,'2018-11-14','2018','2018-11','November','N'),
(6894,'2018-11-15','2018','2018-11','November','N'),
(6895,'2018-11-16','2018','2018-11','November','N'),
(6896,'2018-11-17','2018','2018-11','November','N'),
(6897,'2018-11-18','2018','2018-11','November','N'),
(6898,'2018-11-19','2018','2018-11','November','N'),
(6899,'2018-11-20','2018','2018-11','November','N'),
(6900,'2018-11-21','2018','2018-11','November','N'),
(6901,'2018-11-22','2018','2018-11','November','N'),
(6902,'2018-11-23','2018','2018-11','November','N'),
(6903,'2018-11-24','2018','2018-11','November','N'),
(6904,'2018-11-25','2018','2018-11','November','N'),
(6905,'2018-11-26','2018','2018-11','November','N'),
(6906,'2018-11-27','2018','2018-11','November','N'),
(6907,'2018-11-28','2018','2018-11','November','N'),
(6908,'2018-11-29','2018','2018-11','November','N'),
(6909,'2018-11-30','2018','2018-11','November','Y');
Step 3: Fact Table
Create a fact table from the original table. In normal practice, you ingest the data to hdfs/hive then process the raw data and create a table with historical data where you keep inserting in increment manner. You can look more into data warehousing to get the proper definition but I call this a fact table - f_employee_balance.
This will re-create the original table with missing dates and populate the missing balance with earlier known balance.
--inner query to get all the possible dates
--outer self join query will populate the missing dates and balance
drop table if exists f_employee_balance;
create table f_employee_balance
stored as orc tblproperties ("orc.compress"="SNAPPY") as
select q1.employee_id, q1.iso_date,
nvl(last_value(r.balance, true) --initial dates to be populated with 0 balance
over (partition by q1.employee_id order by q1.iso_date rows between unbounded preceding and current row),0) as balance,
month, year from (
select distinct
r.employee_id,
d.iso_date as iso_date,
d.month, d.year
from daily_employee_balance r, dimension_date d )q1
left outer join daily_employee_balance r on
(q1.employee_id = r.employee_id) and (q1.iso_date = r.iso_date);
Step 4: Analytics
The query below will give you the true average for by month:
select employee_id, monthly_avg, month, year from (
select employee_id,
row_number() over (partition by employee_id,year,month) as row_num,
avg(balance) over (partition by employee_id,year,month) as monthly_avg, month, year from
f_employee_balance)q1
where row_num = 1
order by year, month;
Step 5: Conclusion
You could have just combined step 3 and 4 together; this would save you from creating extra table. When you are in the big data world, you don't worry much about wasting extra disk space or development time. You can easily add another disk or node and automate the process using workflows. For more information, please look into data warehousing concept and hive analytical queries.