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Chicago Fowler Gary Granger Granger Hammond Hammond Valparaiso BLU001 SNE001 BLU002 WIL001 WIL003 WIS002 WIL002 WIS001
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Figure 7-2 Using an index for a column
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The primary goal of database performance tuning is to speed up queries One method that is often used for this is to create an index for columns in the table Just like the index in a book, a column index lists just the data elements in that column, and references the record where that value is located This is shown in Figure 7-2 Now, instead of having to read the entire Customer table looking for all the customers located in Hammond, PostgreSQL can scan the much smaller City index to look for the desired column value Each record in the index file only contains the key value and a pointer to the location in the table that contains the key value When the values are found in the index, PostgreSQL then knows immediately which records to include in the output result set based on the related primary key values However, there is a trade-off for this feature When an index exists on a column, every time a new record is created, PostgreSQL must also update the index file, adding overhead to every INSERT, UPDATE, and DELETE command For applications that do lots of data manipulation, this overhead may outweigh the performance increase for queries
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6 showed how to create one type of index when using the CREATE TABLE command Part of creating a table is often defining a column to use as the primary key The primary key is a special index that causes PostgreSQL to automatically create an index for the primary key column You can see the index created using the \d meta-command:
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test=> \d store"Customer" Table "storeCustomer" Column | Type | Modifiers + + CustomerID | character varying | not null LastName | character varying | FirstName | character varying | Address | character varying | City | character varying | State | character(2) | Zip | character(5) | Phone | character(8) | Indexes: "CustomerKey" PRIMARY KEY, btree ("CustomerID") test=>
The CustomerKey index was automatically created for the CustomerID column when the primary key was defined You can manually create indexes for any column within the table that might be used in a query To create an additional index, you must use the CREATE INDEX command:
CREATE ( [ [ [ UNIQUE ] INDEX name ON table [USING method ] ( column | ( expression ) ) [ opclass ] [,] ) TABLESPACE tablespace ] WHERE condition ]
When creating an index, you have the option of specifying several things First off, if you specify the UNIQUE keyword, each entry in the index file must be unique, so every record must have unique data for the specified column While using a UNIQUE index is allowed in PostgreSQL, the preferred method of forcing a column value to be unique is to add a constraint to the column (see Creating a Table in 6) One or more columns or expressions that use columns are specified as the value to create the index on You can specify as an index value not only a column, but also a function that uses a column The classic example of this is the lower() string function (described in 8) This function converts a string to all lowercase letters By default, the index on a string value is case sensitive String values in uppercase are considered different from string values in lowercase In some applications this can be a problem, as you never know when a customer will enter data values such as names in upper- or lowercase By creating an index using the lower() function, you can eliminate this problem by converting all string data values to lowercase for the index Picking which columns to create indexes for is often a science in itself For each index you define, there is overhead associated with it Every time a new record is added to the table, PostgreSQL must automatically determine where in the index file to place the indexed column value The trick is balancing which indexes will improve your query
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