I’ve been out in industry for over 10 years now doing DB work most of that time. Lately I’ve been doing some work that is starting to challenge my long held assumptions about the internal workings of SQL Server, specifically what exactly is happening when I do joins and where clauses.
I was explaining to someone that when doing a join putting in a where clause limits the amount of records that are available to do the join on thereby making the query faster. My thought was if a table has 100MM records but only 50MM of those meet the WHERE criteria then the JOIN clause wouldn’t try matching the 50MM records that don’t meet the WHERE clause.
I did some work that afternoon and the empirical results didn’t match the expected outcome of a query I had written in terms of execution time. I don’t think my understanding of what goes on during a JOIN is correct. Can someone educate me?