Hopefully, I can get answers for each database server.
For an outline of how indexing works check out: How does database indexing work?
sp_helpindex is a system stored procedure which lists the information of all the indexes on a table or view. This is the easiest method to find the indexes in a table. sp_helpindex returns the name of the index, description of the index and the name of the column on which the index was created.
Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random lookups and efficient access of ordered records.
Duplicate values must be removed before a unique index can be created on the column or columns. Columns that are used in a unique index should be set to NOT NULL, because multiple null values are considered duplicates when a unique index is created.
An index column is also added to an Excel worksheet when you load it. To open a query, locate one previously loaded from the Power Query Editor, select a cell in the data, and then select Query > Edit. For more information see Create, load, or edit a query in Excel (Power Query). Select Add Column > Index Column.
The following is SQL92 standard so should be supported by the majority of RDMBS that use SQL:
CREATE INDEX [index name] ON [table name] ( [column name] )
Sql Server 2005
gives you the ability to specify a covering index. This is an index that includes data from other columns at the leaf level, so you don't have to go back to the table to get columns that aren't included in the index keys.
create nonclustered index my_idx on my_table (my_col1 asc, my_col2 asc) include (my_col3);
This is invaluable for a query that has my_col3
in the select list, and my_col1
and my_col2
in the where clause.
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