Oracle® Database Administrator's Guide 10g Release 2 (10.2) Part Number B14231-02 |
|
|
View PDF |
This chapter describes the management of views, sequences, and synonyms and contains the following topics:
This section describes aspects of managing views, and contains the following topics:
A view is a logical representation of another table or combination of tables. A view derives its data from the tables on which it is based. These tables are called base tables. Base tables might in turn be actual tables or might be views themselves. All operations performed on a view actually affect the base table of the view. You can use views in almost the same way as tables. You can query, update, insert into, and delete from views, just as you can standard tables.
Views can provide a different representation (such as subsets or supersets) of the data that resides within other tables and views. Views are very powerful because they allow you to tailor the presentation of data to different types of users.
See Also:
Oracle Database Concepts for a more complete description of viewsTo create a view, you must meet the following requirements:
To create a view in your schema, you must have the CREATE VIEW
privilege. To create a view in another user's schema, you must have the CREATE ANY VIEW
system privilege. You can acquire these privileges explicitly or through a role.
The owner of the view (whether it is you or another user) must have been explicitly granted privileges to access all objects referenced in the view definition. The owner cannot have obtained these privileges through roles. Also, the functionality of the view depends on the privileges of the view owner. For example, if the owner of the view has only the INSERT
privilege for Scott's emp
table, then the view can be used only to insert new rows into the emp
table, not to SELECT
, UPDATE
, or DELETE
rows.
If the owner of the view intends to grant access to the view to other users, the owner must have received the object privileges to the base objects with the GRANT OPTION
or the system privileges with the ADMIN OPTION
.
You can create views using the CREATE VIEW
statement. Each view is defined by a query that references tables, materialized views, or other views. As with all subqueries, the query that defines a view cannot contain the FOR UPDATE
clause.
The following statement creates a view on a subset of data in the emp
table:
CREATE VIEW sales_staff AS SELECT empno, ename, deptno FROM emp WHERE deptno = 10 WITH CHECK OPTION CONSTRAINT sales_staff_cnst;
The query that defines the sales_staff
view references only rows in department 10. Furthermore, the CHECK OPTION
creates the view with the constraint (named sales_staff_cnst
) that INSERT
and UPDATE
statements issued against the view cannot result in rows that the view cannot select. For example, the following INSERT
statement successfully inserts a row into the emp
table by means of the sales_staff
view, which contains all rows with department number 10:
INSERT INTO sales_staff VALUES (7584, 'OSTER', 10);
However, the following INSERT
statement returns an error because it attempts to insert a row for department number 30, which cannot be selected using the sales_staff
view:
INSERT INTO sales_staff VALUES (7591, 'WILLIAMS', 30);
The view could have been constructed specifying the WITH READ ONLY
clause, which prevents any updates, inserts, or deletes from being done to the base table through the view. If no WITH
clause is specified, the view, with some restrictions, is inherently updatable.
You can also create views that specify more than one base table or view in the FROM
clause. These are called join views. The following statement creates the division1_staff
view that joins data from the emp
and dept
tables:
CREATE VIEW division1_staff AS SELECT ename, empno, job, dname FROM emp, dept WHERE emp.deptno IN (10, 30) AND emp.deptno = dept.deptno;
An updatable join view is a join view where UPDATE
, INSERT
, and DELETE
operations are allowed. See "Updating a Join View" for further discussion.
When a view is created, Oracle Database expands any wildcard (*) in a top-level view query into a column list. The resulting query is stored in the data dictionary; any subqueries are left intact. The column names in an expanded column list are enclosed in quote marks to account for the possibility that the columns of the base object were originally entered with quotes and require them for the query to be syntactically correct.
As an example, assume that the dept
view is created as follows:
CREATE VIEW dept AS SELECT * FROM scott.dept;
The database stores the defining query of the dept
view as:
SELECT "DEPTNO", "DNAME", "LOC" FROM scott.dept;
Views created with errors do not have wildcards expanded. However, if the view is eventually compiled without errors, wildcards in the defining query are expanded.
If there are no syntax errors in a CREATE VIEW
statement, the database can create the view even if the defining query of the view cannot be executed. In this case, the view is considered "created with errors." For example, when a view is created that refers to a nonexistent table or an invalid column of an existing table, or when the view owner does not have the required privileges, the view can be created anyway and entered into the data dictionary. However, the view is not yet usable.
To create a view with errors, you must include the FORCE
clause of the CREATE VIEW
statement.
CREATE FORCE VIEW AS ...;
By default, views with errors are created as INVALID
. When you try to create such a view, the database returns a message indicating the view was created with errors. If conditions later change so that the query of an invalid view can be executed, the view can be recompiled and be made valid (usable). For information changing conditions and their impact on views, see "Managing Object Dependencies".
To replace a view, you must have all of the privileges required to drop and create a view. If the definition of a view must change, the view must be replaced; you cannot use an ALTER VIEW
statement to change the definition of a view. You can replace views in the following ways:
You can drop and re-create the view.
Caution:
When a view is dropped, all grants of corresponding object privileges are revoked from roles and users. After the view is re-created, privileges must be regranted.You can redefine the view with a CREATE VIEW
statement that contains the OR REPLACE
clause. The OR REPLACE
clause replaces the current definition of a view and preserves the current security authorizations. For example, assume that you created the sales_staff
view as shown earlier, and, in addition, you granted several object privileges to roles and other users. However, now you need to redefine the sales_staff
view to change the department number specified in the WHERE
clause. You can replace the current version of the sales_staff
view with the following statement:
CREATE OR REPLACE VIEW sales_staff AS SELECT empno, ename, deptno FROM emp WHERE deptno = 30 WITH CHECK OPTION CONSTRAINT sales_staff_cnst;
Before replacing a view, consider the following effects:
Replacing a view replaces the view definition in the data dictionary. All underlying objects referenced by the view are not affected.
If a constraint in the CHECK OPTION
was previously defined but not included in the new view definition, the constraint is dropped.
All views dependent on a replaced view become invalid (not usable). In addition, dependent PL/SQL program units may become invalid, depending on what was changed in the new version of the view. For example, if only the WHERE
clause of the view changes, dependent PL/SQL program units remain valid. However, if any changes are made to the number of view columns or to the view column names or data types, dependent PL/SQL program units are invalidated. See "Managing Object Dependencies" for more information on how the database manages such dependencies.
To issue a query or an INSERT,
UPDATE
, or DELETE
statement against a view, you must have the SELECT,
INSERT,
UPDATE,
or DELETE
object privilege for the view, respectively, either explicitly or through a role.
Views can be queried in the same manner as tables. For example, to query the Division1_staff
view, enter a valid SELECT
statement that references the view:
SELECT * FROM Division1_staff; ENAME EMPNO JOB DNAME ------------------------------------------------------ CLARK 7782 MANAGER ACCOUNTING KING 7839 PRESIDENT ACCOUNTING MILLER 7934 CLERK ACCOUNTING ALLEN 7499 SALESMAN SALES WARD 7521 SALESMAN SALES JAMES 7900 CLERK SALES TURNER 7844 SALESMAN SALES MARTIN 7654 SALESMAN SALES BLAKE 7698 MANAGER SALES
With some restrictions, rows can be inserted into, updated in, or deleted from a base table using a view. The following statement inserts a new row into the emp
table using the sales_staff
view:
INSERT INTO sales_staff VALUES (7954, 'OSTER', 30);
Restrictions on DML operations for views use the following criteria in the order listed:
If a view is defined by a query that contains SET
or DISTINCT
operators, a GROUP
BY
clause, or a group function, then rows cannot be inserted into, updated in, or deleted from the base tables using the view.
If a view is defined with WITH
CHECK
OPTION
, a row cannot be inserted into, or updated in, the base table (using the view), if the view cannot select the row from the base table.
If a NOT
NULL
column that does not have a DEFAULT
clause is omitted from the view, then a row cannot be inserted into the base table using the view.
If the view was created by using an expression, such as DECODE(deptno, 10, "SALES", ...)
, then rows cannot be inserted into or updated in the base table using the view.
The constraint created by WITH
CHECK
OPTION
of the sales_staff
view only allows rows that have a department number of 30 to be inserted into, or updated in, the emp
table. Alternatively, assume that the sales_staff
view is defined by the following statement (that is, excluding the deptno
column):
CREATE VIEW sales_staff AS SELECT empno, ename FROM emp WHERE deptno = 10 WITH CHECK OPTION CONSTRAINT sales_staff_cnst;
Considering this view definition, you can update the empno
or ename
fields of existing records, but you cannot insert rows into the emp
table through the sales_staff
view because the view does not let you alter the deptno
field. If you had defined a DEFAULT
value of 10 on the deptno
field, then you could perform inserts.
When a user attempts to reference an invalid view, the database returns an error message to the user:
ORA-04063: view 'view_name' has errors
This error message is returned when a view exists but is unusable due to errors in its query (whether it had errors when originally created or it was created successfully but became unusable later because underlying objects were altered or dropped).
An updatable join view (also referred to as a modifiable join view) is a view that contains more than one table in the top-level FROM
clause of the SELECT
statement, and is not restricted by the WITH READ ONLY
clause.
The rules for updatable join views are shown in the following table. Views that meet these criteria are said to be inherently updatable.
Rule | Description |
---|---|
General Rule | Any INSERT , UPDATE , or DELETE operation on a join view can modify only one underlying base table at a time. |
UPDATE Rule |
All updatable columns of a join view must map to columns of a key-preserved table. See "Key-Preserved Tables" for a discussion of key-preserved tables. If the view is defined with the WITH CHECK OPTION clause, then all join columns and all columns of repeated tables are not updatable. |
DELETE Rule |
Rows from a join view can be deleted as long as there is exactly one key-preserved table in the join. The key preserved table can be repeated in the FROM clause. If the view is defined with the WITH CHECK OPTION clause and the key preserved table is repeated, then the rows cannot be deleted from the view. |
INSERT Rule |
An INSERT statement must not explicitly or implicitly refer to the columns of a non-key-preserved table. If the join view is defined with the WITH CHECK OPTION clause, INSERT statements are not permitted. |
There are data dictionary views that indicate whether the columns in a join view are inherently updatable. See "Using the UPDATABLE_ COLUMNS Views" for descriptions of these views.
Note:
There are some additional restrictions and conditions that can affect whether a join view is inherently updatable. Specifics are listed in the description of theCREATE VIEW
statement in the Oracle Database SQL Reference.
If a view is not inherently updatable, it can be made updatable by creating an INSTEAD
OF
trigger on it. This is described in Oracle Database Application Developer's Guide - Fundamentals.
Additionally, if a view is a join on other nested views, then the other nested views must be mergeable into the top level view. For a discussion of mergeable and unmergeable views, and more generally, how the optimizer optimizes statements that reference views, see the Oracle Database Performance Tuning Guide.
Examples illustrating the rules for inherently updatable join views, and a discussion of key-preserved tables, are presented in following sections. The examples in these sections work only if you explicitly define the primary and foreign keys in the tables, or define unique indexes. The following statements create the appropriately constrained table definitions for emp
and dept
.
CREATE TABLE dept ( deptno NUMBER(4) PRIMARY KEY, dname VARCHAR2(14), loc VARCHAR2(13)); CREATE TABLE emp ( empno NUMBER(4) PRIMARY KEY, ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2), FOREIGN KEY (DEPTNO) REFERENCES DEPT(DEPTNO));
You could also omit the primary and foreign key constraints listed in the preceding example, and create a UNIQUE INDEX
on dept (deptno)
to make the following examples work.
The following statement created the emp_dept
join view which is referenced in the examples:
CREATE VIEW emp_dept AS SELECT emp.empno, emp.ename, emp.deptno, emp.sal, dept.dname, dept.loc FROM emp, dept WHERE emp.deptno = dept.deptno AND dept.loc IN ('DALLAS', 'NEW YORK', 'BOSTON');
The concept of a key-preserved table is fundamental to understanding the restrictions on modifying join views. A table is key-preserved if every key of the table can also be a key of the result of the join. So, a key-preserved table has its keys preserved through a join.
Note:
It is not necessary that the key or keys of a table be selected for it to be key preserved. It is sufficient that if the key or keys were selected, then they would also be keys of the result of the join.The key-preserving property of a table does not depend on the actual data in the table. It is, rather, a property of its schema. For example, if in the emp
table there was at most one employee in each department, then deptno
would be unique in the result of a join of emp
and dept
, but dept
would still not be a key-preserved table.
If you select all rows from emp_dept
, the results are:
EMPNO ENAME DEPTNO DNAME LOC ---------- ---------- ------- -------------- ----------- 7782 CLARK 10 ACCOUNTING NEW YORK 7839 KING 10 ACCOUNTING NEW YORK 7934 MILLER 10 ACCOUNTING NEW YORK 7369 SMITH 20 RESEARCH DALLAS 7876 ADAMS 20 RESEARCH DALLAS 7902 FORD 20 RESEARCH DALLAS 7788 SCOTT 20 RESEARCH DALLAS 7566 JONES 20 RESEARCH DALLAS 8 rows selected.
In this view, emp
is a key-preserved table, because empno
is a key of the emp
table, and also a key of the result of the join. dept
is not a key-preserved table, because although deptno
is a key of the dept
table, it is not a key of the join.
The general rule is that any UPDATE
, DELETE
, or INSERT
statement on a join view can modify only one underlying base table. The following examples illustrate rules specific to UPDATE
, DELETE
, and INSERT
statements.
The following example shows an UPDATE
statement that successfully modifies the emp_dept
view:
UPDATE emp_dept SET sal = sal * 1.10 WHERE deptno = 10;
The following UPDATE
statement would be disallowed on the emp_dept
view:
UPDATE emp_dept SET loc = 'BOSTON' WHERE ename = 'SMITH';
This statement fails with an error (ORA-01779
cannot modify a column which maps to a non key-preserved table
), because it attempts to modify the base dept
table, and the dept
table is not key-preserved in the emp_dept
view.
In general, all updatable columns of a join view must map to columns of a key-preserved table. If the view is defined using the WITH CHECK OPTION
clause, then all join columns and all columns taken from tables that are referenced more than once in the view are not modifiable.
So, for example, if the emp_dept
view were defined using WITH CHECK OPTION
, the following UPDATE
statement would fail:
UPDATE emp_dept SET deptno = 10 WHERE ename = 'SMITH';
The statement fails because it is trying to update a join column.
See Also:
Oracle Database SQL Reference for syntax and additional information about theUPDATE
statementYou can delete from a join view provided there is one and only one key-preserved table in the join. The key-preserved table can be repeated in the FROM
clause.
The following DELETE
statement works on the emp_dept
view:
DELETE FROM emp_dept WHERE ename = 'SMITH';
This DELETE
statement on the emp_dept
view is legal because it can be translated to a DELETE
operation on the base emp
table, and because the emp
table is the only key-preserved table in the join.
In the following view, a DELETE
operation is permitted, because although there are two key-preserved tables, they are the same table. That is, the key-preserved table is repeated. In this case, the delete statement operates on the first table in the FROM
list (e1
, in this example):
CREATE VIEW emp_emp AS SELECT e1.ename, e2.empno, e2.deptno FROM emp e1, emp e2 WHERE e1.empno = e2.empno;
If a view is defined using the WITH CHECK OPTION
clause and the key-preserved table is repeated, rows cannot be deleted from such a view.
CREATE VIEW emp_mgr AS SELECT e1.ename, e2.ename mname FROM emp e1, emp e2 WHERE e1.mgr = e2.empno WITH CHECK OPTION;
See Also:
Oracle Database SQL Reference for syntax and additional information about theDELETE
statementThe following INSERT
statement on the emp_dept
view succeeds:
INSERT INTO emp_dept (ename, empno, deptno) VALUES ('KURODA', 9010, 40);
This statement works because only one key-preserved base table is being modified (emp
), and 40 is a valid deptno
in the dept
table (thus satisfying the FOREIGN KEY
integrity constraint on the emp
table).
An INSERT
statement, such as the following, would fail for the same reason that such an UPDATE
on the base emp
table would fail: the FOREIGN KEY
integrity constraint on the emp
table is violated (because there is no deptno
77).
INSERT INTO emp_dept (ename, empno, deptno) VALUES ('KURODA', 9010, 77);
The following INSERT
statement would fail with an error (ORA-01776
cannot modify more than one base table through a join view
):
INSERT INTO emp_dept (empno, ename, loc) VALUES (9010, 'KURODA', 'BOSTON');
An INSERT
cannot implicitly or explicitly refer to columns of a non-key-preserved table. If the join view is defined using the WITH CHECK OPTION
clause, then you cannot perform an INSERT
to it.
See Also:
Oracle Database SQL Reference for syntax and additional information about theINSERT
statementViews that involve outer joins are modifiable in some cases. For example:
CREATE VIEW emp_dept_oj1 AS SELECT empno, ename, e.deptno, dname, loc FROM emp e, dept d WHERE e.deptno = d.deptno (+);
The statement:
SELECT * FROM emp_dept_oj1;
Results in:
EMPNO ENAME DEPTNO DNAME LOC ------- ---------- ------- -------------- ------------- 7369 SMITH 40 OPERATIONS BOSTON 7499 ALLEN 30 SALES CHICAGO 7566 JONES 20 RESEARCH DALLAS 7654 MARTIN 30 SALES CHICAGO 7698 BLAKE 30 SALES CHICAGO 7782 CLARK 10 ACCOUNTING NEW YORK 7788 SCOTT 20 RESEARCH DALLAS 7839 KING 10 ACCOUNTING NEW YORK 7844 TURNER 30 SALES CHICAGO 7876 ADAMS 20 RESEARCH DALLAS 7900 JAMES 30 SALES CHICAGO 7902 FORD 20 RESEARCH DALLAS 7934 MILLER 10 ACCOUNTING NEW YORK 7521 WARD 30 SALES CHICAGO 14 rows selected.
Columns in the base emp
table of emp_dept_oj1
are modifiable through the view, because emp
is a key-preserved table in the join.
The following view also contains an outer join:
CREATE VIEW emp_dept_oj2 AS SELECT e.empno, e.ename, e.deptno, d.dname, d.loc FROM emp e, dept d WHERE e.deptno (+) = d.deptno;
The following statement:
SELECT * FROM emp_dept_oj2;
Results in:
EMPNO ENAME DEPTNO DNAME LOC ---------- ---------- --------- -------------- ---- 7782 CLARK 10 ACCOUNTING NEW YORK 7839 KING 10 ACCOUNTING NEW YORK 7934 MILLER 10 ACCOUNTING NEW YORK 7369 SMITH 20 RESEARCH DALLAS 7876 ADAMS 20 RESEARCH DALLAS 7902 FORD 20 RESEARCH DALLAS 7788 SCOTT 20 RESEARCH DALLAS 7566 JONES 20 RESEARCH DALLAS 7499 ALLEN 30 SALES CHICAGO 7698 BLAKE 30 SALES CHICAGO 7654 MARTIN 30 SALES CHICAGO 7900 JAMES 30 SALES CHICAGO 7844 TURNER 30 SALES CHICAGO 7521 WARD 30 SALES CHICAGO OPERATIONS BOSTON 15 rows selected.
In this view, emp
is no longer a key-preserved table, because the empno
column in the result of the join can have nulls (the last row in the preceding SELECT
statement). So, UPDATE
, DELETE
, and INSERT
operations cannot be performed on this view.
In the case of views containing an outer join on other nested views, a table is key preserved if the view or views containing the table are merged into their outer views, all the way to the top. A view which is being outer-joined is currently merged only if it is "simple." For example:
SELECT col1, col2, ... FROM T;
The select list of the view has no expressions, and there is no WHERE
clause.
Consider the following set of views:
CREATE VIEW emp_v AS SELECT empno, ename, deptno FROM emp; CREATE VIEW emp_dept_oj1 AS SELECT e.*, Loc, d.dname FROM emp_v e, dept d WHERE e.deptno = d.deptno (+);
In these examples, emp_v
is merged into emp_dept_oj1
because emp_v
is a simple view, and so emp
is a key-preserved table. But if emp_v
is changed as follows:
CREATE VIEW emp_v_2 AS SELECT empno, ename, deptno FROM emp WHERE sal > 1000;
Then, because of the presence of the WHERE
clause, emp_v_2
cannot be merged into emp_dept_oj1
, and hence emp
is no longer a key-preserved table.
If you are in doubt whether a view is modifiable, then you can select from the USER_UPDATABLE_COLUMNS
view to see if it is. For example:
SELECT owner, table_name, column_name, updatable FROM USER_UPDATABLE_COLUMNS WHERE TABLE_NAME = 'EMP_DEPT_VIEW';
This returns output similar to the following:
OWNER TABLE_NAME COLUMN_NAM UPD ---------- ---------- ---------- --- SCOTT EMP_DEPT_V EMPNO NO SCOTT EMP_DEPT_V ENAME NO SCOTT EMP_DEPT_V DEPTNO NO SCOTT EMP_DEPT_V DNAME NO SCOTT EMP_DEPT_V LOC NO 5 rows selected.
The views described in the following table can assist you to identify inherently updatable join views.
View | Description |
---|---|
DBA_UPDATABLE_COLUMNS |
Shows all columns in all tables and views that are modifiable. |
ALL_UPDATABLE_COLUMNS |
Shows all columns in all tables and views accessible to the user that are modifiable. |
USER_UPDATABLE_COLUMNS |
Shows all columns in all tables and views in the user's schema that are modifiable. |
The updatable columns in view emp_dept
are shown below.
SELECT COLUMN_NAME, UPDATABLE FROM USER_UPDATABLE_COLUMNS WHERE TABLE_NAME = 'EMP_DEPT'; COLUMN_NAME UPD ------------------------------ --- EMPNO YES ENAME YES DEPTNO YES SAL YES DNAME NO LOC NO 6 rows selected.
See Also:
Oracle Database Reference for complete descriptions of the updatable column viewsYou use the ALTER VIEW
statement only to explicitly recompile a view that is invalid. If you want to change the definition of a view, see "Replacing Views".
The ALTER VIEW
statement lets you locate recompilation errors before run time. To ensure that the alteration does not affect the view or other objects that depend on it, you can explicitly recompile a view after altering one of its base tables.
To use the ALTER VIEW
statement, the view must be in your schema, or you must have the ALTER ANY TABLE
system privilege.
See Also:
Oracle Database SQL Reference for syntax and additional information about theALTER VIEW
statementYou can drop any view contained in your schema. To drop a view in another user's schema, you must have the DROP ANY VIEW
system privilege. Drop a view using the DROP VIEW
statement. For example, the following statement drops the emp_dept
view:
DROP VIEW emp_dept;
See Also:
Oracle Database SQL Reference for syntax and additional information about theDROP VIEW
statementThis section describes aspects of managing sequences, and contains the following topics:
Sequences are database objects from which multiple users can generate unique integers. The sequence generator generates sequential numbers, which can help to generate unique primary keys automatically, and to coordinate keys across multiple rows or tables.
Without sequences, sequential values can only be produced programmatically. A new primary key value can be obtained by selecting the most recently produced value and incrementing it. This method requires a lock during the transaction and causes multiple users to wait for the next value of the primary key; this waiting is known as serialization. If developers have such constructs in applications, then you should encourage the developers to replace them with access to sequences. Sequences eliminate serialization and improve the concurrency of an application.
See Also:
Oracle Database Concepts for a more complete description of sequencesTo create a sequence in your schema, you must have the CREATE SEQUENCE
system privilege. To create a sequence in another user's schema, you must have the CREATE ANY SEQUENCE
privilege.
Create a sequence using the CREATE SEQUENCE
statement. For example, the following statement creates a sequence used to generate employee numbers for the empno
column of the emp
table:
CREATE SEQUENCE emp_sequence INCREMENT BY 1 START WITH 1 NOMAXVALUE NOCYCLE CACHE 10;
Notice that several parameters can be specified to control the function of sequences. You can use these parameters to indicate whether the sequence is ascending or descending, the starting point of the sequence, the minimum and maximum values, and the interval between sequence values. The NOCYCLE
option indicates that the sequence cannot generate more values after reaching its maximum or minimum value.
The CACHE
clause preallocates a set of sequence numbers and keeps them in memory so that sequence numbers can be accessed faster. When the last of the sequence numbers in the cache has been used, the database reads another set of numbers into the cache.
The database might skip sequence numbers if you choose to cache a set of sequence numbers. For example, when an instance abnormally shuts down (for example, when an instance failure occurs or a SHUTDOWN ABORT
statement is issued), sequence numbers that have been cached but not used are lost. Also, sequence numbers that have been used but not saved are lost as well. The database might also skip cached sequence numbers after an export and import. See Oracle Database Utilities for details.
See Also:
Oracle Database SQL Reference for the CREATE SEQUENCE
statement syntax
Oracle Real Application Clusters Deployment and Performance Guide for information about how caching sequence numbers improves performance in an Oracle Real Application Clusters environment
To alter a sequence, your schema must contain the sequence, or you must have the ALTER ANY SEQUENCE
system privilege. You can alter a sequence to change any of the parameters that define how it generates sequence numbers except the sequence starting number. To change the starting point of a sequence, drop the sequence and then re-create it.
Alter a sequence using the ALTER SEQUENCE
statement. For example, the following statement alters the emp_sequence
:
ALTER SEQUENCE emp_sequence INCREMENT BY 10 MAXVALUE 10000 CYCLE CACHE 20;
See Also:
Oracle Database SQL Reference for syntax and additional information about theALTER SEQUENCE
statementTo use a sequence, your schema must contain the sequence or you must have been granted the SELECT
object privilege for another user's sequence. Once a sequence is defined, it can be accessed and incremented by multiple users (who have SELECT
object privilege for the sequence containing the sequence) with no waiting. The database does not wait for a transaction that has incremented a sequence to complete before that sequence can be incremented again.
The examples outlined in the following sections show how sequences can be used in master/detail table relationships. Assume an order entry system is partially comprised of two tables, orders_tab
(master table) and line_items_tab
(detail table), that hold information about customer orders. A sequence named order_seq
is defined by the following statement:
CREATE SEQUENCE Order_seq START WITH 1 INCREMENT BY 1 NOMAXVALUE NOCYCLE CACHE 20;
A sequence is referenced in SQL statements with the NEXTVAL
and CURRVAL
pseudocolumns; each new sequence number is generated by a reference to the sequence pseudocolumn NEXTVAL
, while the current sequence number can be repeatedly referenced using the pseudo-column CURRVAL
.
NEXTVAL
and CURRVAL
are not reserved words or keywords and can be used as pseudocolumn names in SQL statements such as SELECT
, INSERT
, or UPDATE
.
To generate and use a sequence number, reference seq_name.NEXTVAL
. For example, assume a customer places an order. The sequence number can be referenced in a values list. For example:
INSERT INTO Orders_tab (Orderno, Custno) VALUES (Order_seq.NEXTVAL, 1032);
Or, the sequence number can be referenced in the SET
clause of an UPDATE
statement. For example:
UPDATE Orders_tab SET Orderno = Order_seq.NEXTVAL WHERE Orderno = 10112;
The sequence number can also be referenced outermost SELECT
of a query or subquery. For example:
SELECT Order_seq.NEXTVAL FROM dual;
As defined, the first reference to order_seq.NEXTVAL
returns the value 1. Each subsequent statement that references order_seq.NEXTVAL
generates the next sequence number (2, 3, 4,. . .). The pseudo-column NEXTVAL
can be used to generate as many new sequence numbers as necessary. However, only a single sequence number can be generated for each row. In other words, if NEXTVAL
is referenced more than once in a single statement, then the first reference generates the next number, and all subsequent references in the statement return the same number.
Once a sequence number is generated, the sequence number is available only to the session that generated the number. Independent of transactions committing or rolling back, other users referencing order_seq.NEXTVAL
obtain unique values. If two users are accessing the same sequence concurrently, then the sequence numbers each user receives might have gaps because sequence numbers are also being generated by the other user.
To use or refer to the current sequence value of your session, reference seq_name.CURRVAL
. CURRVAL
can only be used if seq_name.NEXTVAL
has been referenced in the current user session (in the current or a previous transaction). CURRVAL
can be referenced as many times as necessary, including multiple times within the same statement. The next sequence number is not generated until NEXTVAL
is referenced. Continuing with the previous example, you would finish placing the customer's order by inserting the line items for the order:
INSERT INTO Line_items_tab (Orderno, Partno, Quantity) VALUES (Order_seq.CURRVAL, 20321, 3); INSERT INTO Line_items_tab (Orderno, Partno, Quantity) VALUES (Order_seq.CURRVAL, 29374, 1);
Assuming the INSERT
statement given in the previous section generated a new sequence number of 347, both rows inserted by the statements in this section insert rows with order numbers of 347.
CURRVAL
and NEXTVAL
can be used in the following places:
VALUES
clause of INSERT
statements
The SELECT
list of a SELECT
statement
The SET
clause of an UPDATE
statement
CURRVAL
and NEXTVAL
cannot be used in these places:
A subquery
A view query or materialized view query
A SELECT
statement with the DISTINCT
operator
A SELECT
statement with a GROUP
BY
or ORDER
BY
clause
A SELECT
statement that is combined with another SELECT
statement with the UNION,
INTERSECT
, or MINUS
set operator
The WHERE
clause of a SELECT
statement
DEFAULT
value of a column in a CREATE
TABLE
or ALTER
TABLE
statement
The condition of a CHECK
constraint
Sequence numbers can be kept in the sequence cache in the System Global Area (SGA). Sequence numbers can be accessed more quickly in the sequence cache than they can be read from disk.
The sequence cache consists of entries. Each entry can hold many sequence numbers for a single sequence.
Follow these guidelines for fast access to all sequence numbers:
Be sure the sequence cache can hold all the sequences used concurrently by your applications.
Increase the number of values for each sequence held in the sequence cache.
When an application accesses a sequence in the sequence cache, the sequence numbers are read quickly. However, if an application accesses a sequence that is not in the cache, then the sequence must be read from disk to the cache before the sequence numbers are used.
If your applications use many sequences concurrently, then your sequence cache might not be large enough to hold all the sequences. In this case, access to sequence numbers might often require disk reads. For fast access to all sequences, be sure your cache has enough entries to hold all the sequences used concurrently by your applications.
When a sequence is read into the sequence cache, sequence values are generated and stored in a cache entry. These values can then be accessed quickly. The number of sequence values stored in the cache is determined by the CACHE
parameter in the CREATE
SEQUENCE
statement. The default value for this parameter is 20.
This CREATE
SEQUENCE
statement creates the seq2
sequence so that 50 values of the sequence are stored in the SEQUENCE
cache:
CREATE SEQUENCE seq2
CACHE 50;
The first 50 values of seq2
can then be read from the cache. When the 51st value is accessed, the next 50 values will be read from disk.
Choosing a high value for CACHE
lets you access more successive sequence numbers with fewer reads from disk to the sequence cache. However, if there is an instance failure, then all sequence values in the cache are lost. Cached sequence numbers also could be skipped after an export and import if transactions continue to access the sequence numbers while the export is running.
If you use the NOCACHE
option in the CREATE
SEQUENCE
statement, then the values of the sequence are not stored in the sequence cache. In this case, every access to the sequence requires a disk read. Such disk reads slow access to the sequence. This CREATE
SEQUENCE
statement creates the SEQ3
sequence so that its values are never stored in the cache:
CREATE SEQUENCE seq3 NOCACHE;
You can drop any sequence in your schema. To drop a sequence in another schema, you must have the DROP ANY SEQUENCE
system privilege. If a sequence is no longer required, you can drop the sequence using the DROP SEQUENCE
statement. For example, the following statement drops the order_seq
sequence:
DROP SEQUENCE order_seq;
When a sequence is dropped, its definition is removed from the data dictionary. Any synonyms for the sequence remain, but return an error when referenced.
See Also:
Oracle Database SQL Reference for syntax and additional information about theDROP SEQUENCE
statementThis section describes aspects of managing synonyms, and contains the following topics:
A synonym is an alias for a schema object. Synonyms can provide a level of security by masking the name and owner of an object and by providing location transparency for remote objects of a distributed database. Also, they are convenient to use and reduce the complexity of SQL statements for database users.
Synonyms allow underlying objects to be renamed or moved, where only the synonym needs to be redefined and applications based on the synonym continue to function without modification.
You can create both public and private synonyms. A public synonym is owned by the special user group named PUBLIC
and is accessible to every user in a database. A private synonym is contained in the schema of a specific user and available only to the user and the user's grantees.
See Also:
Oracle Database Concepts for a more complete description of synonymsTo create a private synonym in your own schema, you must have the CREATE SYNONYM
privilege. To create a private synonym in another user's schema, you must have the CREATE ANY SYNONYM
privilege. To create a public synonym, you must have the CREATE PUBLIC SYNONYM
system privilege.
Create a synonym using the CREATE SYNONYM
statement. The underlying schema object need not exist, nor do you need privileges to access the object. The following statement creates a public synonym named public_emp
on the emp
table contained in the schema of jward
:
CREATE PUBLIC SYNONYM public_emp FOR jward.emp
When you create a synonym for a remote procedure or function, you must qualify the remote object with its schema name. Alternatively, you can create a local public synonym on the database where the remote object resides, in which case the database link must be included in all subsequent calls to the procedure or function.
See Also:
Oracle Database SQL Reference for syntax and additional information about theCREATE SYNONYM
statementYou can successfully use any private synonym contained in your schema or any public synonym, assuming that you have the necessary privileges to access the underlying object, either explicitly, from an enabled role, or from PUBLIC
. You can also reference any private synonym contained in another schema if you have been granted the necessary object privileges for the private synonym.
You can only reference another user's synonym using the object privileges that you have been granted. For example, if you have the SELECT
privilege for the jward
.emp
synonym, then you can query the jward
.emp
synonym, but you cannot insert rows using the jward
.emp
synonym.
A synonym can be referenced in a DML statement the same way that the underlying object of the synonym can be referenced. For example, if a synonym named emp
refers to a table or view, then the following statement is valid:
INSERT INTO emp (empno, ename, job) VALUES (emp_sequence.NEXTVAL, 'SMITH', 'CLERK');
If the synonym named fire_emp
refers to a standalone procedure or package procedure, then you could execute it with the command
EXECUTE Fire_emp(7344);
You can drop any private synonym in your own schema. To drop a private synonym in another user's schema, you must have the DROP ANY SYNONYM
system privilege. To drop a public synonym, you must have the DROP PUBLIC SYNONYM
system privilege.
Drop a synonym that is no longer required using DROP SYNONYM
statement. To drop a private synonym, omit the PUBLIC
keyword. To drop a public synonym, include the PUBLIC
keyword.
For example, the following statement drops the private synonym named emp
:
DROP SYNONYM emp;
The following statement drops the public synonym named public_emp
:
DROP PUBLIC SYNONYM public_emp;
When you drop a synonym, its definition is removed from the data dictionary. All objects that reference a dropped synonym remain. However, they become invalid (not usable). For more information about how dropping synonyms can affect other schema objects, see "Managing Object Dependencies".
See Also:
Oracle Database SQL Reference for syntax and additional information about theDROP SYNONYM
statementThe following views display information about views, synonyms, and sequences:
View | Description |
---|---|
DBA_VIEWS
|
DBA view describes all views in the database. ALL view is restricted to views accessible to the current user. USER view is restricted to views owned by the current user. |
DBA_SYNONYMS
|
These views describe synonyms. |
DBA_SEQUENCES
|
These views describe sequences. |
DBA_UPDATABLE_COLUMNS
|
These views describe all columns in join views that are updatable. |
See Also:
Oracle Database Reference for complete descriptions of these views