How to backfill partitioned data in BigQuery? - google-cloud-platform

Question

I am trying to backfill data from GCP billing export table to another table say T1.
Both tables are partitioned.
Below scheduled query runs everyday to get yesterday’s data.
SELECT * FROM gcp_billing_export_v1 WHERE DATE(_PARTITIONTIME) = DATE_ADD(CURRENT_DATE(), INTERVAL -1 DAY)
Now I need to backfill the data , say for 15th May - how do I do that ?
I tried the backfill feature with the below query - expecting the backfill utility to take the past date i.e. May 15th as a param for the #run_date but that didn’t help.
SELECT * FROM gcp_billing_export_v1 WHERE DATE(_PARTITIONTIME) = #run_date
The data is pulled for 15th May from the source table(gcp_billing_export_v1) but is populated against current date in the destination table i.e May 15th May data is populated against June 22nd in the destination table T1. Where am I going wrong ?
Any guidance ?

Answer 1

Looks like you're using ingestion partitioning.
You would need to create a new table with the partitioning you want ie EventDate and populate that new table with historical and new daily data - as you can't overwrite an existing partition.
Link here: https://cloud.google.com/bigquery/docs/querying-partitioned-tables#query_an_ingestion-time_partitioned_table

Answer 2

As #Lemon already pointed out that you're using Ingestion time partitioned tables(both source and dest), you need to understand how it works. Ingestion time partitioned tables are different from the Regular partitioned tables.
From the Documentation-
When you create a table partitioned by ingestion time,BigQuery automatically assigns rows to partitions based on the time when BigQuery ingests the data.
This type of table has a pseudo-column named _PARTITIONTIME.The value of this column is the ingestion time for each row.
Since you are using the SELECT * FROM gcp_billing_export_v1 you are getting all the data but without any _PARTITIONTIME column. And when you are saving the same result into the destination table , it is updating the _PARTITIONTIME column as per destintaion table's data ingestion-time.
Thus you have old data with the current date in _PARTITIONTIME
To avoid this your destination table needs to be either a normal table or regular partitioned table.
Also you need to have an extra column to hold the Datetime value from the source's_PARTITIONTIME column.You can create a regular partition on this new column.
Then to get _PARTITIONTIME in your result set , in your quer you need to mention the column name specifically in your query.
SELECT *,_PARTITIONTIME AS ingestionTime
FROM gcp_billing_export_v1
WHERE DATE(_PARTITIONTIME) = #run_date
The above query will return all the data from the gcp_billing_export_v1 table with 1 extra column ingestionTime.
Now you can backfill the data for 15th,May and save it to the new table.
You can also tweak around this below query to achieve the same
SELECT *,_PARTITIONTIME AS ingestionTime
FROM gcp_billing_export_v1
WHERE DATE(_PARTITIONTIME) = DATE_ADD(#run_date, INTERVAL -1 DAY)
It will run daily as per your need .Now if you want to pull data for 15th,May then you have to schedule the backfill for 16th,May(as per the where clause)