Database Objects

Editor's Notes

• #2: Schedule:TimingTopic 20 minutesLecture 20 minutesPractice 40 minutesTotal
• #3: Lesson Aim In this lesson, you learn how to create and maintain some of the other commonly used database objects. These objects include sequences, indexes, and synonyms.
• #4: Database Objects Many applications require the use of unique numbers as primary key values. You can either build code into the application to handle this requirement or use a sequence to generate unique numbers. If you want to improve the performance of some queries, you should consider creating an index. You can also use indexes to enforce uniqueness on a column or a collection of columns. You can provide alternative names for objects by using synonyms.
• #5: What Is a Sequence? A sequence is a user created database object that can be shared by multiple users to generate unique integers. A typical usage for sequences is to create a primary key value, which must be unique for each row. The sequence is generated and incremented (or decremented) by an internal Oracle routine. This can be a time-saving object because it can reduce the amount of application code needed to write a sequence-generating routine. Sequence numbers are stored and generated independently of tables. Therefore, the same sequence can be used for multiple tables.
• #6: Creating a Sequence Automatically generate sequential numbers by using the CREATE SEQUENCE statement. In the syntax: sequenceis the name of the sequence generator INCREMENT BY nspecifies the interval between sequence numbers where n is an integer (If this clause is omitted, the sequence increments by 1.) START WITH nspecifies the first sequence number to be generated (If this clause is omitted, the sequence starts with 1.) MAXVALUE nspecifies the maximum value the sequence can generate NOMAXVALUEspecifies a maximum value of 10^27 for an ascending sequence and –1 for a descending sequence (This is the default option.) MINVALUE nspecifies the minimum sequence value NOMINVALUEspecifies a minimum value of 1 for an ascending sequence and – (10^26) for a descending sequence (This is the default option.) CYCLE | NOCYCLEspecifies whether the sequence continues to generate values after reaching its maximum or minimum value (NOCYCLE is the default option.) CACHE n | NOCACHEspecifies how many values the Oracle server preallocates and keep in memory (By default, the Oracle server caches 20 values.)
• #7: Creating a Sequence (continued) The example on the slide creates a sequence named DEPT_DEPTID_SEQ to be used for the DEPARTMENT_ID column of the DEPARTMENTS table. The sequence starts at 120, does not allow caching, and does not cycle. Do not use the CYCLE option if the sequence is used to generate primary key values, unless you have a reliable mechanism that purges old rows faster than the sequence cycles. For more information, see Oracle9i SQL Reference, “CREATE SEQUENCE.” Note: The sequence is not tied to a table. Generally, you should name the sequence after its intended use; however the sequence can be used anywhere, regardless of its name. Instructor Note If the INCREMENT BY value is negative, the sequence descends. Also, ORDER | NOORDER options are available. The ORDER option guarantees that sequence values are generated in order. It is not important if you use the sequence to generate primary key values. This option is relevant only with the Parallel Server option.If sequence values are cached, they will be lost if there is a system failure.
• #8: Confirming Sequences Once you have created your sequence, it is documented in the data dictionary. Since a sequence is a database object, you can identify it in the USER_OBJECTS data dictionary table. You can also confirm the settings of the sequence by selecting from the USER_SEQUENCES data dictionary view. Instructor Note Demo: 12_dd.sql Purpose: To illustrate the USER_SEQUENCES data dictionary view and its contents.
• #9: Using a Sequence After you create your sequence, it generates sequential numbers for use in your tables. Reference the sequence values by using the NEXTVAL and CURRVAL pseudocolumns. NEXTVAL and CURRVAL Pseudocolumns The NEXTVAL pseudocolumn is used to extract successive sequence numbers from a specified sequence. You must qualify NEXTVAL with the sequence name. When you reference sequence.NEXTVAL, a new sequence number is generated and the current sequence number is placed in CURRVAL. The CURRVAL pseudocolumn is used to refer to a sequence number that the current user has just generated. NEXTVAL must be used to generate a sequence number in the current user’s session before CURRVAL can be referenced. You must qualify CURRVAL with the sequence name. When sequence.CURRVAL is referenced, the last value returned to that user’s process is displayed.
• #10: Rules for Using NEXTVAL and CURRVAL You can use NEXTVAL and CURRVAL in the following contexts: The SELECT list of a SELECT statement that is not part of a subquery The SELECT list of a subquery in an INSERT statement The VALUES clause of an INSERT statement The SET clause of an UPDATE statement You cannot use NEXTVAL and CURRVAL in the following contexts: The SELECT list of a view A SELECT statement with the DISTINCT keyword A SELECT statement with GROUP BY, HAVING, or ORDER BY clauses A subquery in a SELECT, DELETE, or UPDATE statement The DEFAULT expression in a CREATE TABLE or ALTER TABLE statement For more information, see Oracle9i SQL Reference, “Pseudocolumns” section and “CREATE SEQUENCE.” Instructor Note Be sure to point out the rules listed on this page.
• #11: Using a Sequence The example on the slide inserts a new department in the DEPARTMENTS table. It uses the DEPT_DEPTID_SEQ sequence for generating a new department number as follows: You can view the current value of the sequence: SELECT dept_deptid_seq.CURRVAL FROM dual; Suppose now you want to hire employees to staff the new department. The INSERT statement to be executed for all new employees can include the following code: INSERT INTO employees (employee_id, department_id, ...) VALUES (employees_seq.NEXTVAL, dept_deptid_seq .CURRVAL, ...); Note: The preceding example assumes that a sequence called EMPLOYEE_SEQ has already been created for generating new employee numbers.
• #12: Caching Sequence Values Cache sequences in memory to provide faster access to those sequence values. The cache is populated the first time you refer to the sequence. Each request for the next sequence value is retrieved from the cached sequence. After the last sequence value is used, the next request for the sequence pulls another cache of sequences into memory. Gaps in the Sequence Although sequence generators issue sequential numbers without gaps, this action occurs independent of a commit or rollback. Therefore, if you roll back a statement containing a sequence, the number is lost. Another event that can cause gaps in the sequence is a system crash. If the sequence caches values in the memory, then those values are lost if the system crashes. Because sequences are not tied directly to tables, the same sequence can be used for multiple tables. If you do so, each table can contain gaps in the sequential numbers. Viewing the Next Available Sequence Value without Incrementing It If the sequence was created with NOCACHE, it is possible to view the next available sequence value without incrementing it by querying the USER_SEQUENCES table. Instructor Note Frequently used sequences should be created with caching to improve efficiency. For cached sequences, there is no way to find out what the next available sequence value will be without actually obtaining, and using up, that value. It is recommended that users resist finding the next sequence value. Trust the system to provide a unique value each time a sequence is used in an INSERT statement.
• #13: Altering a Sequence If you reach the MAXVALUE limit for your sequence, no additional values from the sequence are allocated and you will receive an error indicating that the sequence exceeds the MAXVALUE. To continue to use the sequence, you can modify it by using the ALTER SEQUENCE statement. Syntax ALTER SEQUENCEsequence [INCREMENT BY n] [{MAXVALUE n | NOMAXVALUE}] [{MINVALUE n | NOMINVALUE}] [{CYCLE | NOCYCLE}] [{CACHE n | NOCACHE}]; In the syntax: sequence is the name of the sequence generator For more information, see Oracle9i SQL Reference, “ALTER SEQUENCE.”
• #14: Guidelines for Modifying Sequences You must be the owner or have the ALTER privilege for the sequence in order to modify it. Only future sequence numbers are affected by the ALTER SEQUENCE statement. The START WITH option cannot be changed using ALTER SEQUENCE. The sequence must be dropped and re-created in order to restart the sequence at a different number. Some validation is performed. For example, a new MAXVALUE that is less than the current sequence number cannot be imposed. ALTER SEQUENCE dept_deptid_seq INCREMENT BY 20 MAXVALUE 90 NOCACHE NOCYCLE; ALTER SEQUENCE dept_deptid_seq * ERROR at line 1: ORA-04009: MAXVALUE cannot be made to be less than the current value
• #15: Removing a Sequence To remove a sequence from the data dictionary, use the DROP SEQUENCE statement. You must be the owner of the sequence or have the DROP ANY SEQUENCE privilege to remove it. Syntax DROP SEQUENCE sequence; In the syntax: sequence is the name of the sequence generator For more information, see Oracle9i SQL Reference, “DROP SEQUENCE.”
• #16: Indexes An Oracle server index is a schema object that can speed up the retrieval of rows by using a pointer. Indexes can be created explicitly or automatically. If you do not have an index on the column, then a full table scan occurs. An index provides direct and fast access to rows in a table. Its purpose is to reduce the necessity of disk I/O by using an indexed path to locate data quickly. The index is used and maintained automatically by the Oracle server. Once an index is created, no direct activity is required by the user. Indexes are logically and physically independent of the table they index. This means that they can be created or dropped at any time and have no effect on the base tables or other indexes. Note: When you drop a table, corresponding indexes are also dropped. For more information, see Oracle9i Concepts, “Schema Objects” section, “Indexes” topic. Instructor Note The decision to create indexes is a global, high-level decision. Creation and maintenance of indexes is often a task for the database administrator. Reference the column that has an index in the predicate WHERE clause without modifying the indexed column with a function or expression.A ROWID is a hexadecimal string representation of the row address containing block identifier, row location in the block, and the database file identifier. The fastest way to access any particular row is by referencing its ROWID.
• #17: Types of Indexes Two types of indexes can be created. One type is a unique index: the Oracle server automatically creates this index when you define a column in a table to have a PRIMARY KEY or a UNIQUE key constraint. The name of the index is the name given to the constraint. The other type of index is a nonunique index, which a user can create. For example, you can create a FOREIGN KEY column index for a join in a query to improve retrieval speed. Note: You can manually create a unique index, but it is recommended that you create a unique constraint, which implicitly creates a unique index.
• #18: Creating an Index Create an index on one or more columns by issuing the CREATE INDEX statement. In the syntax: indexis the name of the index tableis the name of the table columnis the name of the column in the table to be indexed For more information, see Oracle9i SQL Reference, “CREATE INDEX.” Instructor Note To create an index in your schema, you must have the CREATE TABLE privilege. To create an index in any schema, you need the CREATE ANY INDEX privilege or the CREATE TABLE privilege on the table on which you are creating the index. Another option in the syntax is the UNIQUE keyword. Emphasize that you should not explicitly define unique indexes on tables. Instead define uniqueness in the table as a constraint. The Oracle server enforces unique integrity constraints by automatically defining a unique index on the unique key.
• #19: More Is Not Always Better More indexes on a table does not mean faster queries. Each DML operation that is committed on a table with indexes means that the indexes must be updated. The more indexes you have associated with a table, the more effort the Oracle server must make to update all the indexes after a DML operation. When to Create an Index Therefore, you should create indexes only if: The column contains a wide range of values The column contains a large number of null values One or more columns are frequently used together in a WHERE clause or join condition The table is large and most queries are expected to retrieve less than 2–4% of the rows Remember that if you want to enforce uniqueness, you should define a unique constraint in the table definition. Then a unique index is created automatically. Instructor Note A composite index (also called a concatenated index) is an index that you create on multiple columns in a table. Columns in a composite index can appear in any order and need not be adjacent in the table. Composite indexes can speed retrieval of data for SELECT statements in which the WHERE clause references all or the leading portion of the columns in the composite index.
• #20: Instructor Note Null values are not included in the index.To optimize joins, you can create an index on the FOREIGN KEY column, which speeds up the search to match rows to the PRIMARY KEY column.The optimizer does not use an index if the WHERE clause contains the IS NULL expression.
• #21: Confirming Indexes Confirm the existence of indexes from the USER_INDEXES data dictionary view. You can also check the columns involved in an index by querying the USER_IND_COLUMNS view. The example on the slide displays all the previously created indexes, with the names of the affected column, and the index’s uniqueness, on the EMPLOYEES table.
• #22: Function-Based Index Function-based indexes defined with the UPPER(column_name) or LOWER(column_name) keywords allow case-insensitive searches. For example, the following index: CREATE INDEX upper_last_name_idx ON employees (UPPER(last_name)); Facilitates processing queries such as: SELECT * FROM employees WHERE UPPER(last_name) = 'KING'; To ensure that the Oracle server uses the index rather than performing a full table scan, be sure that the value of the function is not null in subsequent queries. For example, the following statement is guaranteed to use the index, but without the WHERE clause the Oracle server may perform a full table scan: SELECT * FROM employees WHERE UPPER (last_name) IS NOT NULL ORDER BY UPPER (last_name);
• #23: Function-Based Index (continued) The Oracle server treats indexes with columns marked DESC as function-based indexes. The columns marked DESC are sorted in descending order. Instructor Note Let students know that to create a function-based index in your own schema on your own table, you must have the CREATE INDEX and QUERY REWRITE system privileges. To create the index in another schema or on another schema's table, you must have the CREATE ANY INDEX and GLOBAL QUERY REWRITE privileges. The table owner must also have the EXECUTE object privilege on the functions used in the function-based index.
• #24: Removing an Index You cannot modify indexes. To change an index, you must drop it and then re-create it. Remove an index definition from the data dictionary by issuing the DROP INDEX statement. To drop an index, you must be the owner of the index or have the DROP ANY INDEX privilege. In the syntax: indexis the name of the index Note: If you drop a table, indexes and constraints are automatically dropped, but views and sequences remain.
• #25: Creating a Synonym for an Object To refer to a table owned by another user, you need to prefix the table name with the name of the user who created it followed by a period. Creating a synonym eliminates the need to qualify the object name with the schema and provides you with an alternative name for a table, view, sequence, procedure, or other objects. This method can be especially useful with lengthy object names, such as views. In the syntax: PUBLICcreates a synonym accessible to all users synonymis the name of the synonym to be created objectidentifies the object for which the synonym is created Guidelines The object cannot be contained in a package. A private synonym name must be distinct from all other objects owned by the same user. For more information, see Oracle9i SQL Reference, “CREATE SYNONYM.”
• #26: Creating a Synonym for an Object (continued) The slide example creates a synonym for the DEPT_SUM_VU view for quicker reference. The database administrator can create a public synonym accessible to all users. The following example creates a public synonym named DEPT for Alice’s DEPARTMENTS table: CREATE PUBLIC SYNONYM dept FOR alice.departments; Synonym created. Removing a Synonym To drop a synonym, use the DROP SYNONYM statement. Only the database administrator can drop a public synonym. DROP PUBLIC SYNONYM dept; Synonym dropped. For more information, see Oracle9i SQL Reference, “DROP SYNONYM.” Instructor Note In the Oracle server, the DBA can specifically grant the CREATE PUBLIC SYNONYM privilege to any user, and that user can create public synonyms.
• #27: Summary In this lesson you should have learned about some of the other database objects including sequences, indexes, and views. Sequences The sequence generator can be used to automatically generate sequence numbers for rows in tables. This can save time and can reduce the amount of application code needed. A sequence is a database object that can be shared with other users. Information about the sequence can be found in the USER_SEQUENCES table of the data dictionary. To use a sequence, reference it with either the NEXTVAL or the CURRVAL pseudocolumns. Retrieve the next number in the sequence by referencing sequence.NEXTVAL. Return the current available number by referencing sequence.CURRVAL. Indexes Indexes are used to improve query retrieval speed. Users can view the definitions of the indexes in the USER_INDEXES data dictionary view. An index can be dropped by the creator, or a user with the DROP ANY INDEX privilege, by using the DROP INDEX statement. Synonyms Database administrators can create public synonyms and users can create private synonyms for convenience, by using the CREATE SYNONYM statement. Synonyms permit short names or alternative names for objects. Remove synonyms by using the DROP SYNONYM statement.
• #28: Practice 12 Overview In this practice, you create a sequence to be used when populating your table. You also create implicit and explicit indexes.
• #29: Practice 12 1.Create a sequence to be used with the primary key column of the DEPT table. Thesequence should start at 200 and have a maximum value of 1000. Have your sequence incrementby ten numbers. Name the sequence DEPT_ID_SEQ. 2.Write a query in a script to display the following information about your sequences: sequence name, maximum value, increment size, and last number. Name the script lab12_2.sql. Run the statement in your script. 3.Write a script to insert two rows into the DEPT table. Name your script lab12_3.sql. Be sure to use the sequence that you created for the ID column. Add two departments named Education and Administration. Confirm your additions. Run the commands in your script. 4.Create a nonunique index on the foreign key column (DEPT_ID) in the EMP table. 5.Display the indexes and uniqueness that exist in the data dictionary for the EMP table.Save the statement into a script named lab12_5.sql.