Question

I was wondering if there is a performance gain between a SELECT query with a not very specific WHERE clause and another SELECT query with a more specific WHERE clause.

For instance is the query:

SELECT * FROM table1 WHERE first_name='Georges';

slower than this one:

SELECT * FROM table1 WHERE first_name='Georges' AND nickname='Gigi';

In other words is there a time factor that is link to the precision of the WHERE clause ?

I’m not sure to be very understandable and even if my question takes into account all the components that are involved in database query (MYSQL in my case)

My question is related to the Django framework because I would like to cache an evaluated queryset, and on a next request, take back this cached-evaluated queryset, filter it more, and evaluate it again.

Answer 1

There is no hard and fast rule about this.

There can be either an increase or decrease in performance by adding more conditions to the WHERE clause, as it depends on, among other things, the:

• indexing
• schema
• data quantity
• data cardinality
• statistics
• intelligence of the query engine

You need to test with your data set and determine what will perform the best.