Creating Indexes

15-2 Database Systems Objectives Distinguish between the indexes that are created automatically and those that are created manually. Identify the uses for indexes. Explain the index structure and why it improves query speed. Create a non-unique index. Remove an index from the data dictionary. Evaluate guidelines for creating and using indexes.

15-3 Database Systems What Is an Index? Database object Used by the Oracle Server to speed up the retrieval of rows by using a pointer Reduces disk I/O by using rapid path access method to locate the data quickly Independent of the table it indexes Automatically used and maintained by the Oracle Server

15-4 Database Systems How Are Indexes Created? Automatically A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition. Manually Users can create non-unique indexes on columns to speed up access time to the rows.

15-5 Database Systems Create an index on one or more columns. Example Improve the speed of query access on the ENAME column in the EMP table. Creating an Index: Syntax CREATE INDEX index ON table (column[, column]...); CREATE INDEX index ON table (column[, column]...); SQL> CREATE INDEX emp_ename_idx 2 ON emp(ename); 2 ON emp(ename); Index created. SQL> CREATE INDEX emp_ename_idx 2 ON emp(ename); 2 ON emp(ename); Index created.

15-6 Database Systems When to Create an Index The column is used frequently in the WHERE clause or in a join condition. The column contains a wide range of values. The column contains a large number of null values. Two or more columns are frequently used together in a WHERE clause or a join condition. The table is large and most queries are expected to retrieve less than 2–4% of the rows. More indexes do not always speed up queries.

15-7 Database Systems When Not to Create an Index Do not create an index if The table is small. The columns are not often used as a condition in the query. Most queries are expected to retrieve more than 2–4% of the rows. The table is updated frequently.

15-8 Database Systems Confirming Indexes The USER_INDEXES data dictionary view contains the name of the index and its uniqueness. The USER_IND_COLUMNS view contains the index name, the table name, and the column name. SQL> SELECTic.index_name, ic.column_name, 2 ic.column_position col_pos,ix.uniqueness 2 ic.column_position col_pos,ix.uniqueness 3 FROMuser_indexes ix, user_ind_columns ic 3 FROMuser_indexes ix, user_ind_columns ic 4 WHEREic.index_name = ix.index_name 4 WHEREic.index_name = ix.index_name 5 ANDic.table_name = 'EMP'; 5 ANDic.table_name = 'EMP'; SQL> SELECTic.index_name, ic.column_name, 2 ic.column_position col_pos,ix.uniqueness 2 ic.column_position col_pos,ix.uniqueness 3 FROMuser_indexes ix, user_ind_columns ic 3 FROMuser_indexes ix, user_ind_columns ic 4 WHEREic.index_name = ix.index_name 4 WHEREic.index_name = ix.index_name 5 ANDic.table_name = 'EMP'; 5 ANDic.table_name = 'EMP';

15-9 Database Systems Removing an Index Remove an index from the data dictionary. To drop an index, you must be the owner of the index or have the DROP ANY INDEX privilege. SQL> DROP INDEX emp_ename_idx; Index dropped. SQL> DROP INDEX emp_ename_idx; Index dropped.

15-10 Database Systems Summary Indexes are database objects that are used to improve query retrieval speed. Some unique indexes are created automatically. Users can create indexes by issuing the CREATE INDEX command. The definition of the index is in the USER_INDEXES data dictionary table.

15-11 Database Systems Practice Overview Creating non-unique indexes Displaying data dictionary information about the index Dropping indexes

15-12 Database Systems Practice 1 Create a sequence to be used with the primary key column of the DEPARTMENT table. The sequence should start at 60 and have a maximum value of 200. Have your sequence increment by ten numbers. Name the sequence DEPT_ID_SEQ. Write a script to display the following information about your sequence: sequence name, maximum value, increment size, and last number. Execute your script.

15-13 Database Systems Practice 2 Write an interactive script to insert a row into the DEPARTMENT table. Be sure to use the sequence you created for the ID column. Create a customized prompt to enter the department name. Execute the script and add two departments named Education and Administration. Confirm your additions.

15-14 Database Systems Practice 3 Create a non-unique index on the foreign key column in the EMPLOYEE table. Display the indexes and uniqueness that exist in the data dictionary for the EMPLOYEE table.