Why does putting a WHERE clause outside view have terrible performance

Question

Let\'s say you have a view:

CREATE VIEW dbo.v_SomeJoinedTables AS
SELECT
    a.date,
    a.Col1,
    b.Col2,
    DENSE_RANK() 
      OVER(PARTITION BY a.date, a.         


        

Answer 1

I'm not a SQL expert, so I may be voted down for my foolishness, but my guess is that in the first case SQL is fetching the results of the entire view before applying the predicate in the WHERE clause. So when you query the view, it selects all of the records, puts them in memory, and then applies the Date filter after it is done.

This seems similar to the way the entire data set specified in your joins is fetched prior to applying the filter in the WHERE (lesson here is that you should apply predicates in your ON clause when possible).

Unless views are treated differently somehow.