How to recreate old snapshot using field history table in Bigquery - database

Question

I'm currently working on an interesting problem. I am trying to recreate state of table as it was on a given previous date. I have 2 tables
Table A: consists of live data, gets refreshed on an hourly basis.
Table A_field_history: consists of changes made to the fields in Table A.
Following image consists of current state, where Table A has live updated data and Table A_field_history only captures changes made to the fields on table A.
I am trying to recreate Table A as of particular given date. Following image consists of table state as it was on 06/30/2020.
The requirement is to have capability to recreate state of Table A based on any given date.

Answer 1

I actually identified a way to rollback (virtually, not on actual table) all the updates made after given specific date. Following are the steps followed:
Create dummy tables:
WITH
Table_A AS
(
SELECT 1 As ID, '2020-6-28' as created_date, 10 as qty, 100 as value
Union ALL
SELECT 2 As ID, '2020-5-29' as created_date, 20 as qty, 200 as value),
Table_A_field_history AS
(
SELECT 'xyz' id,'2020-07-29' created_date,'12345' created_by,'qty' field,'10' new_value,'200' old_value,'1' A_id
UNION ALL
SELECT 'abc' id,'2020-07-24' created_date,'12345' created_by,'qty' field,'20' new_value,'10' old_value,'2' A_id
UNION ALL
SELECT 'xyz' id,'2020-07-29' created_date,'12345' created_by,'value' field,'100' new_value,'2000' old_value,'1' A_id
UNION ALL
SELECT 'abc' id,'2020-07-24' created_date,'12345' created_by,'value' field,'200' new_value,'5000' old_value,'2' A_id
UNION ALL
SELECT 'xyz' id,'2020-06-29' created_date,'12345' created_by,'qty' field,'200' new_value,'' old_value,'1' A_id
UNION ALL
SELECT 'abc' id,'2020-05-30' created_date,'12345' created_by,'qty' field,'10' new_value,'' old_value,'2' A_id
UNION ALL
SELECT 'xyz' id,'2020-06-29' created_date,'12345' created_by,'value' field,'2000' new_value,'' old_value,'1' A_id
UNION ALL
SELECT 'abc' id,'2020-05-30' created_date,'12345' created_by,'value' field,'5000' new_value,'' old_value,'2' A_id
),
Step 1. Create date cte to filter data based on given date:
`date_spine
AS
(
SELECT * FROM UNNEST(GENERATE_DATE_ARRAY('2020-01-01', CURRENT_DATE(), INTERVAL 1 Day)) AS as_of_date
),`
Step 2. Above created date cte can be used as a Spine for our query, cross join to map as_of_date with all the changes made in the history table.
date_changes
AS
(
SELECT DISTINCT
date.as_of_date,
hist.A_id
FROM Table_A_field_history hist CROSS JOIN date_spine date
),
Step 3. Now we have as_of_date mapped to all historical transactions, now we can get max of change date.
most_recent_changes AS (
SELECT
dc.as_of_date,
dc.A_id ,
MAX(fh.created_date) AS created_date,
FROM date_changes dc
LEFT JOIN Table_A_field_history AS fh
ON dc.A_id = fh.A_id
WHERE CAST(fh.created_date AS DATE) <= dc.as_of_date
GROUP BY dc.as_of_date,
dc.A_id
),
Step 4. Now mapping max change date with actual created_date and history table
past_changes AS (
SELECT
mr.as_of_date,
mr.A_id,
mr.created_date,
a.id AS entry_id,
a.created_by AS created_by_id,
CASE WHEN a.field='qty' THEN a.new_value ELSE '' END AS qty,
CASE WHEN a.field='value' THEN a.new_value ELSE '' END AS value,
FROM most_recent_changes AS mr
LEFT JOIN Table_A_field_history AS a
ON mr.A_id = a.A_id
AND mr.created_date = a.created_date
WHERE a.id IS NOT NULL
)
Step 5. Now we can use as_of_date to get historical state of Table A.
Select *
From past_changes x
WHERE x.as_of_date = '2020-07-29'