A company currently has a database cluster setup in Redshift. They also have an AWS RDS PostgreSQL database setup in place. A table has been setup in PostgreSQL which stores data based on a timestamp. The requirement now is to ensure that the data from the PostgreSQL table gets stored into the Redshift database. For this a staging table has been setup in Redshift. It needs to be ensured that the data lag between the staging and PostgreSQL tables is not greater than 4 hours. Which of the following is the most efficient implementation step you would use for this requirement?

Answer options:

A.Create a trigger in the PostgreSQL table to send new data to a Kinesis stream. Ensure the data is transferred from the Kinesis Stream to the staging table in Redshift.
B.Create a SQL query that is run every hour to check for new data. Use the query results to send the new data to the staging table.
C.Use the extensions available in PostgreSQL and use the dblink facility
D.Create a trigger in the PostgreSQL table to send new data to a Kinesis Firehose stream. Ensure the data is transferred from the Kinesis Firehose Stream to the staging table in Redshift

Answer – C
Such a use case is clearly mentioned in the documentation
########
For further reading, check out Bob Strahan’s blog post Query Routing and Rewrite: Introducing pgbouncer-rr for Amazon Redshift and PostgreSQL post.
RDS PostgreSQL includes two extensions to execute queries remotely. The first extension is the PostgreSQL foreign-data wrapper, postgres_fdw. The postgres_fdw module enables the creation of external tables. External tables can be queried in the same way as a local native table, However, the query is not currently executed entirely on the remote side because postgres_fdw doesn’t push down aggregate functions and limit clauses. When you perform an aggregate query through an external table, all the data is pulled into PostgreSQL for an aggregation step. This is unacceptably slow for any meaningful number of rows.
The second extension is dblink, which includes a function also called dblink. The dblink function allows the entire query to be pushed to Amazon Redshift. This lets Amazon Redshift do what it does best—query large quantities of data efficiently and return the results to PostgreSQL for further processing.
########
Since this is clearly mentioned in the documentation, all other options are invalid.
For more information on this use case, please refer to the below URL
https://aws.amazon.com/blogs/big-data/join-amazon-redshift-and-amazon-rds-postgresql-with-dblink/