https://towardsdatascience.com/understanding-hyperparameters-and-its-optimisation-techniques-f0debba07568

To improve the performance of querying a large BigQuery table with filters on countryname and username, clustering the table by these fields is the most effective approach. Here's why option C is the best choice:
Clustering in BigQuery:
Clustering organizes data based on the values in specified columns. This can significantly improve query performance by reducing the amount of data scanned during query execution.
Clustering by countryname and username means that data is physically sorted and stored together based on these fields, allowing BigQuery to quickly locate and read only the relevant data for queries using these filters.
Filter Efficiency:
With the table clustered by countryname and username, queries that filter on these columns can benefit from efficient data retrieval, reducing the amount of data processed and speeding up query execution.
This directly addresses the performance issue of the dashboard queries that apply filters on these fields.
Steps to Implement:
Redesign the Table:
Create a new table with clustering on countryname and username:
CREATE TABLE project.dataset.new_table
CLUSTER BY countryname, username AS
SELECT * FROM project.dataset.customer_order;
Migrate Data:
Transfer the existing data from the original table to the new clustered table.
Update Queries:
Modify the dashboard queries to reference the new clustered table.
Reference Links:
BigQuery Clustering Documentation
Optimizing Query Performance

When you apply a BigQueryIO.Write transform in batch mode to write to a single table, Dataflow invokes a BigQuery load job. When you apply a BigQueryIO.Write transform in streaming mode or in batch mode using a function to specify the destination table, Dataflow uses BigQuery's streaming inserts Reference: https://cloud.google.com/dataflow/model/bigquery-io

Explanation: