CHAPTER 9Views, Synonyms, and Sequences

Views, Synonyms, and Sequences• Views are used extensively in reporting applications and

also to present subsets of data to applications.• Synonyms provide a means to transparently allow users

to display and use other users’ objects.• Sequences are often used to generate unique numbers

that are used to populate primary-key and foreign-key values.

Uses for Views• Create an efficient method of storing a SQL query for

reuse.• Provide an interface layer between an application and

physical tables.• Hide the complexity of a SQL query from an application.• Report to a user about only a subset of columns and/or

rows.

Creating a View• Create views on tables, views, or materialized views.• Use the CREATE VIEW statement.

create table sales(

sales_id number primary key

,amnt number

,state varchar2(2)

);

• If the view already exists, consider using CREATE OR REPLACE VIEW statement.

Views With Check Option• You can issue UPDATE statements against views which result in

updating the underlying tables referenced in the view SQL query.• WITH CHECK OPTION specifies that the UPDATE statement can

only affect rows that are selected by the SQL the view is based upon.

create or replace view sales_rockies as

select

sales_id

,amnt

,state

from sales

where state in ('CO','UT','WY','ID','AZ')

with check option;

Creating Read-Only Views• If you don’t want a user to be able to perform INSERT, UPDATE, or

DELETE operations on a view, then don’t grant those object privileges on the underlying table(s) to users with access to the view.

• Also, specify READ ONLY when creating the view.

create or replace view sales_rockies as

select

sales_id

,amnt

,state

from sales

where state in ('CO','UT','WY','ID','AZ')

with read only;

Updatable Join Views• If you have multiple tables defined in the FROM clause of

the SQL query on which the view is based, it’s still possible to update the underlying tables.

• The DML statement must modify only one underlying table.

• The view must be created without the READ ONLY clause.

• The column being updated belongs to the key-preserved table in the join view (there is only one key-preserved table in a join view).

Creating an INSTEAD OF Trigger• An INSTEAD OF trigger on a view instructs Oracle to

execute PL/SQL code instead of using the DML statement.

create or replace trigger emp_dept_v_updt

instead of update on emp_dept_v

for each row

begin

update emp set emp_name=UPPER(:new.emp_name)

where emp_id=:old.emp_id;

end;

/

Implementing an Invisible Column in a View

• New starting with Oracle Database 12c

create or replace view sales_co

(sales_id, amnt, state, sales_person_id invisible)

as

select

sales_id, amnt, state, sales_person_id

from sales

where state = 'CO';

Modifying a View Definition• CREATE OR REPLACE allows you to modify the view

definition without dropping the object.• Advantage is that you don’t have to reissue any grants on

the view.• Most DBAs/Developers almost always use the CREATE

OR REPLACE, and not dropping and recreating the view.

Displaying the SQL Used to Create a View

• Troubleshooting issues with views often begins with viewing the SQL that the view is based upon.

select

view_name

,text

from dba_views

where owner = upper('&owner')

and view_name like upper('&view_name');

Oracle Internal View Definitions• You may occasionally need the definition of an Oracle

internal view.• Sometimes you may wonder how Oracle is retrieving

information; you might be troubleshooting a complex database issue or bug.

SQL> select view_definition from v$fixed_view_definition where view_name='V$BH';

• Gives you insight into Oracle’s internal workings.

Renaming a View• You may want to change the name so that it better

conforms to standard.• You may want to rename a view before dropping it so that

you can better determine whether it’s in use.

SQL> rename inv to inv_old;

Dropping a View• In general, for objects not being used, it’s better to drop

them so as to not clutter up your environment and make it more difficult to maintain and manage.

• Drop views that aren’t being used.

SQL> drop view inv_v;

Uses for Synonyms• Provide transparent access to other users’ database

objects• Eliminates the need to hard code schema name into

application code

Synonyms are Pointers to Database Objects

• Tables• Views or object views• Other synonyms• Remote objects via a database link• PL/SQL packages, procedures, and functions• Materialized views• Sequences• Java class schema object• User-defined object types

Creating a Synonym• Use the CREATE SYNONYM statement to create a

synonym.

SQL> select * from inv;

• Query from DBA/USER/ALL_SYNONYMS to view details.

Public Synonyms• Public synonyms allow you to create access to the

synonym on a database wide basis.• This simplifies some aspects of the DBA’s job.• However, it has a few drawbacks:

• Troubleshooting can be problematic if you’re not aware of globally defined (public) synonyms.

• Applications that share one database can have collisions on object names if multiple applications use public synonyms that aren’t unique within the database.

• Security should be administered as needed, not on a wholesale basis.

• You’ll run into DBAs who often use public synonyms, and other DBAs who never use them.

Dynamically Generating Synonyms• Use SQL to generate SQL.

CONNECT &&master_user/&&master_pwd.@&&tns_alias

--

SET LINESIZE 132 PAGESIZE 0 ECHO OFF FEEDBACK OFF

SET VERIFY OFF HEAD OFF TERM OFF TRIMSPOOL ON

--

SPO gen_syns_dyn.sql

--

select 'create or replace synonym ' || table_name ||

' for ' || '&&master_user..' ||

table_name || ';'

from user_tables;

--

SPO OFF;

--

SET ECHO ON FEEDBACK ON VERIFY ON HEAD ON TERM ON;

Displaying Synonym Metadata

select synonym_name, table_owner, table_name, db_linkfrom user_synonymsorder by 1;

select owner,object_name,object_type,statusfrom dba_objectswhere object_name like upper('&object_name%');

Renaming a Synonym• Conform to naming standards.• Determine if the synonym is being used.

SQL> rename inv_s to inv_st;

Dropping a Synonym• In general, you should drop objects that are not being

used.

SQL> drop synonym inv;

Sequences• Object that guarantees SELECT access to a unique

integer for the given sequence.• Used often for PK and FK values.

Creating a Sequence

SQL> create sequence inv_seq;

SQL> create sequence inv2 start with 10000 maxvalue 1000000;

Sequence Pseudo-columns• Two pseudo column values used to access a sequence:

• NEXTVAL• CURRVAL

SQL> select inv_seq.nextval from dual;

SQL> select inv_seq.currval from dual;

Autoincrementing Columns• Starting with 12c, Oracle does have autoincrementing

columns• If using 11g or older, manually simulate this with a trigger:

create or replace trigger inv_bu_tr

before insert on inv

for each row

begin

select inv_seq.nextval into :new.inv_id from dual;

end;

/

Interesting Uses of Sequences

SQL> create sequence inv_seq_odd start with 1 increment by 2;

SQL> create sequence inv_seq_even start with 2 increment by 2;

SQL> create sequence inv_seq_odd_dwn start with -1 increment by -2;

SQL> create sequence inv_seq_even_dwn start with -2 increment by -2;

Using One Sequence or Many• Usually there are not performance issues with accessing

one sequence.• Usually there are not issues with using large integer

values for populating NUMBER columns.

Viewing Sequence Metadata

select

sequence_name

,min_value

,max_value

,increment_by

from user_sequences;

SQL> select dbms_metadata.get_ddl('SEQUENCE','INV_SEQ') from dual;

Renaming a Sequence• Conform to naming standards.• Rename the object before you drop it.

SQL> rename inv_seq to inv_seq_old;

Dropping a Sequence• Drop objects that aren’t being used.

SQL> drop sequence inv;

Resetting a Sequence• UNDEFINE seq_name• UNDEFINE reset_to• PROMPT "sequence name" ACCEPT '&&seq_name'• PROMPT "reset to value" ACCEPT &&reset_to• COL seq_id NEW_VALUE hold_seq_id• COL min_id NEW_VALUE hold_min_id• --• SELECT &&reset_to - &&seq_name..nextval - 1 seq_id• FROM dual;• --• SELECT &&hold_seq_id - 1 min_id FROM dual;• --• ALTER SEQUENCE &&seq_name INCREMENT BY &hold_seq_id MINVALUE

&hold_min_id;• --• SELECT &&seq_name..nextval FROM dual;• --• ALTER SEQUENCE &&seq_name INCREMENT BY 1;

Summary• Views, Synonyms, and Sequences are used extensively

within database applications.• Views are stored SQL queries that allow you to

encapsulate SQL logic and present it as an object to the user.

• Synonyms provide a transparent mechanism to enable access to another user’s objects.

• Sequences are counters that are often used to guarantee unique values are generated for use when populating primary and foreign key columns.