SQL Query Performance
Posted on 2014-03-05
A few questions on SQL performance. Query will be run against millions of records so performance is key.
1. If I have the following condition: where t1.date < getdate()
Would it improve performance to run getdate() once at the beginning of the query, and save it to a variable? Then do: where t1.date < variable? This way each record is checked against a variable instead of having to run getdate() a million times. I'm assuming it's run for every record -- one record at a time? At least that's how I think SQL parses queries.
2. Does it matter what order the conditions are placed in the where clause? For example, certain conditions eliminate 50% of all records. Should such conditions be placed at the beginning of the where clause, and conditions that eliminate only 5% of records placed towards the end?