Why does SQL Server use index scan instead of index seek?
We can see that the latter uses the index seek and key lookup because it has checked the value of variable at execution time, and the most appropriate plan for that specific value is chosen.
What does it mean to scan an index?
An index scan means that all the leaf-level of the index was searched to find the information for the query: When the index is a clustered index, this is the same as scanning the entire table. With only a few exceptions, this isn’t good; we need to try to turn scans into seeks, which means retrieving the data by just using the index tree.
Which is better seek predicate or index seek?
Based on the description, it is very clear that Seek Predicate is better than Predicate as it searches indexes whereas in Predicate, the search is on non-key columns – which implies that the search is on the data in page files itself.
What’s the difference between a full scan and an index scan?
For a non-covering index, the difference between a full table scan and an execution plan based on a full index scan is basically the difference between sequential reads and random reads: it can be close if you have fast storage or it can be very different if you have slow storage. A full index scan can become interesting when the index is covering.
Is there a way to reduce the cost of an index scan?
Basically it says that if there was an index on the CityName column that also included the LastReportedPopulation column, the cost of the query could be reduced by about 95%! That’s significant!. Before we change anything though, let’s actually run this query from ApexSQL Plan to see if the actual statistics match the estimates.
Can you run a full index scan in Percona?
Let’s take the employees database, and slightly modify the employees tables: This query can of course by executed by running a full table scan, but we could also take advantage of the idx_first index, which will translate to a full index scan. Surprising?