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Dropping an Index
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There will be times when an index is redundant and should be removed (i.e., dropped) from a table. Dropping an index is simply a case of executing the DROP INDEX statement, followed by the table name and the index name.
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Note If the index is used by a primary key or unique constraint, you cannot drop it directly. In this case,
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you must use the DROP CONSTRAINT command. The removal of this constraint will also remove the index from the table.
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Try It Out: Dropping an Index in Query Editor
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1. If you want to drop the index created in the last section, all you need to do is execute the following code. This will remove the index from SQL Server and also remove any statistics associated with it. USE ApressFinancial GO DROP INDEX IX_TransactionTypes ON TransactionDetails.TransactionTypes 2. After execution, you should see that everything executed correctly:
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CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
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Command(s) completed successfully.
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3. Don t forget to re-create the index by running the following code: CREATE UNIQUE CLUSTERED INDEX IX_TransactionTypes ON TransactionDetails.TransactionTypes ( TransactionTypeId ASC ) WITH (STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = OFF) ON [PRIMARY] GO
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In the next section, we ll examine what s needed to alter an index.
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Altering an Index in Query Editor
Unlike with a table, it is not possible to use an ALTER command to change the columns contained in an index. To do this, you first have to drop the index and then re-create it. The DROP command will physically remove the index from the table; therefore, you should ensure that you know what the contents of the index are before you drop the index, if you want to re-create a similar index.
Note In Management Studio, you can add and remove columns from an index s definition without dropping and
re-creating the index, as this is all done for you behind the scenes.
This next exercise demonstrates the steps you need to take to remove an index and then re-create it. You ll learn how to do all of this in two steps, rather than the expected three steps.
Try It Out: Altering an Index in Query Editor
1. First, you want to create an index to retrieve the price of a specific share at a set point in time. The following index will do just that, as you are querying the data using the share ID and the date you want the price for: USE ApressFinancial GO CREATE UNIQUE CLUSTERED INDEX IX_SharePrices ON ShareDetails.SharePrices
CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
( ShareId ASC, PriceDate ASC ) WITH (STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = OFF) ON [PRIMARY] GO 2. However, it would be better to have the PriceDate descending, so that the latest price is at the top, because asking for this information is a query. By including this column, SQL Server would read only one row rather than an increasing number as more prices were created. It would also be advantageous to include the Price itself to avoid a second read to retrieve that column of information from the clustered index.
Note Remember, clustered indexes hold the data, and not pointers to the data. However, in this instance,
without the Price column, a second read would be performed.
CREATE UNIQUE CLUSTERED INDEX IX_SharePrices ON ShareDetails.SharePrices ( ShareId ASC, PriceDate DESC, Price ) WITH (STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = OFF, DROP_EXISTING = OFF) ON [PRIMARY]
Note If you did accidentally run the first set of code, change to DROP_EXISTING = ON.
3. Now execute the code using your chosen method, and you should see the following results:
The command(s) completed successfully.
By using the DROP_EXISTING clause of the CREATE INDEX command, you can then perform the modification in one execution rather than two. This will drop the index and re-create it.
CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
Note Take care when building indexes. It is possible to use the same columns in the same order more
than once, thus creating the same index twice, but under two different index names. This is a waste of time and will place unnecessary overhead on SQL Server. However, when including key column(s) from a clustered index in a nonclustered index, because the row pointer is actually the clustered index key, SQL is smart enough not to store the information twice in the nonclustered index, and you can explicitly define the order of the clustered index keys as they are used in the nonclustered index.
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