What if you wanted to select "top 100 most expensive products" or number of products between $0.01 and $10, $10.01 and $100, $100.01 and $1000? Sure you could do a full table scan on your products table on both queries but an index on price would speed both queries up a lot if you have a lot of products. Of course you have to determine if the index would be used enough to make up for the extra time on index update when the price changes or products are added or deleted.
Cheap solution, sure, add an index. But you're asking an OLAP question question of an OLTP system. Questions like that are best asked at least of an out-of-production read replica or better an analytics db.
In general just avoiding mixed types of load. Predictable, audited application queries in a user request shouldn’t be mixed with potentially extremely expensive long running analytics queries. Different replica sets isolates customers from potential performance impacts caused by data analytics.
You stream CDC events to have a 1 to 1 read replica in something like Snowflake/Databricks where you can run all kinds of OLAP workflows on this analytics db replica.
Oh, sure, but wouldn't the whole website be served out of a read-friendly database? Why would you have a separate "analytics" database to the main database(s) driving the site?
