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Is it bad to do an index seek?
An index seek operation isn’t necessarily good, nor is an index scan inherently bad. To cast judgment, dig deeper. Its execution plan does a clustered index seek – it’s going to jump to what happens to be the first row in the table (Id -1) and read through all of the rows in the entire table, looking for ones who have a reputation < 0:
What does it mean when an index has zero seeks?
An index that is scanned heavily with zero or small number of seeks means that the index is badly used and should be replaced with more optimal one. An index with large number of Lookups means that we need to optimize the index by adding the frequently looked up columns to the existing index non-key columns using the INCLUDE clause.
What does it mean when an index is not being used?
It means the index tree is being used to go directly to the records we need. Index Scan: Not so good as an index seek, so it could be better. However, sometimes even an index scan is good, a non clustered index scan means the pages of that index are smaller an better for a scan than the pages of the clustered index.
Which is the first category to gather information about the indexes?
The first category of information that we need to gather about the indexes is the list of available indexes in our database, the list of columns participating in each index and the properties of these indexes.
How can index fragmentation impact query plan choice?
Let’s look at an example. The first thing that we need to do is build an appropriate test database and data set to use for examining how index fragmentation can impact query plan choice in SQL Server. The following script will create a database with two tables with identical data, one heavily fragmented and one minimally fragmented.
What is the impact of fragmentation on execution plans?
Among other downsides from doing this, the accumulated costs of query execution will catch up to you, with query plan shifts because the query optimizer is a cost-based optimizer and so rightly deems those fragmented indexes as more expensive to utilize.
What does index scan mean in SQL Server?
Seek means SQL Server knows what specific row it’s going to start at. That’s it. It might have been performed multiple times (hover your mouse over the operator in the plan and look at “number of executions” for an indication) Scan means SQL Server is going to start at either end of the index (min or max) and start reading from there. That’s it.