Index creation guide
Indexes should be considered on all columns that are frequently accessed by the WHERE, ORDER BY, GROUP BY, TOP, and DISTINCT clauses.
Only add indexes if you know that they will be used by the queries run against the table
As a rule of thumb, every table should have at least a clustered index. Generally, but not always, the clustered index should be on a column that monotonically increases--such as an identity column, or some other column where the value is increasing--and is unique. In many cases, the primary key is the ideal column for a clustered index
Static tables (those tables that change very little, or not at all) can be more heavily indexed that dynamic tables (those that are subject to many INSERTS, UPDATES, or DELETES) without negative effect.
If you will be creating an index to speed the retrieval of a single record, you may want to consider making it a non-clustered index, and saving the clustering index (you can only have one) for a more complex query
Don't over index your OLTP tables, as every index you add increases the time it takes to perform INSERTS, UPDATES, and DELETES
Don't add the same index twice on a table.
Whether an index on a foreign key has either high or low selectivity, an index on a foreign key can be used by the Query Optimizer to perform a merge join on the tables in question. A merge join occurs when a row from each table is taken and then they are compared to see if they match the specified join criteria. If the tables being joined have the appropriate indexes (no matter the selectivity), a merge join can be performed, which is generally much faster than a join to a table with a foreign key that does not have an index.
If you have two or more tables that are frequently joined together, then the columns used for the joins should have an appropriate index.
When creating indexes, try to make them unique indexes if at all possible. SQL Server can often search through a unique index faster than a non-unique index because in a unique index, each row is unique, and once the needed record is found, SQL Server doesn't have to look any further.
Ways boost the performance of a query that includes an AND operator in the WHERE clause, consider the following:
- Of the search criterions in the WHERE clause, at least one of them should be based on a highly selective column that has an index.
- If at least one of the search criterions in the WHERE clause is not highly selective, consider adding indexes to all of the columns referenced in the WHERE clause.
If none of the column in the WHERE clause are selective enough to use an index on their own, consider creating a covering index for this query. - The Query Optimizer will always perform a table scan or a clustered index scan on a table if the WHERE clause in the query contains an OR operator and if any of the referenced columns in the OR clause are not indexed (or does not have a useful index). Because of this, if you use many queries with OR clauses, you will want to ensure that each referenced column in the WHERE clause has an index.
- The Query Optimizer converts the Transact-SQL IN clause to the OR operator when parsing your code. Because of this, keep in mind that if the referenced column in your query doesn't include an index, then the Query Optimizer will perform a table scan or clustered index scan on the table.
- Create composite indexes with the most restrictive column first.
Don’t add the index under the following cases
- If it is not used by the query optimizer. Use Query Analyzer's "Show Execution Plan" option to see if your queries against a particular table use an index or not. If the table is small, most likely indexes will not be used.
- If the column values exhibit low selectivity, often less than 90%-95% for non-clustered indexes.
- If the column(s) to be indexed are very wide.
- If the column(s) are defined as TEXT, NTEXT, or IMAGE data types.
- If the table is rarely queried.
