Database Programming

Sections 12 – Sequences, Indexes, and Synonymns

What is a sequence?

A SEQUENCE is a shareable object used to automatically generate unique numbers.

Can be used by multiple users Often used to create primary-key values Incremented or decremented by an

internal ORACLE routine Reduces amount of code you need to

write.

Marge Hohly 2

Marge Hohly 3

The Syntax for Creating a Sequence CREATE SEQUENCE sequence_name

[INCREMENT BY n][START WITH n][{MAXVALUE n | NOMAXVALUE}][{MINVALUE n | NOMINVALUE}][{CYCLE | NOCYCLE}][{CACHE n | NOCACHE}];

Sequence_name – the name of sequence generator (object) INCREMENT BY n – interval between sequence numbers where n is an integer (if omitted n is 1) START WITH n – specifies the first sequence number to be generated (if omitted start with 1) MAXVALUE n – specifies the maximum value the sequence can generate NOMAXVALUE – specifies a maximum value of 10^27 for ascending and -1 for descending MINVALUE n – specifies the minimum value the sequence can generate NOMINVALUE – specifies a minimum value of 1 for ascending and -10^27 for descending CYCLE – whether the sequence continues to generate values after reaching its max or min value NOCYCLE – the default if CYCLE is not specified CACHE n – specifies how many values the Oracle Server preallocates and keeps in memory

(default is 20) if the sequence values are cached, they will be lost if there is a system failure NOCACHE – does not cache any values

The syntax for creating a sequence

CREATE SEQUENCE sequence_name[INCREMENT BY n][START WITH n][{MAXVALUE n | NOMAXVALUE}][{MINVALUE n | NOMINVALUE}][{CYCLE | NOCYCLE}][{CACHE n | NOCACHE}];

Marge Hohly 4

Marge Hohly 5

Example of a Sequence

CREATE SEQUENCE emp_emp_id_seqINCREMENT BY 10START WITH 300MAXVALUE 9999NOCACHENOCYCLE;

300 310 320 330 340 350 360 .....99999 ask for NEXTVAL = 300 – it becomes CURRVAL the number

just generated

in HTMLDB once you return the NEXTVAL from the sequence you no longer have the “session” and the database no longer knows what’s the CURVAL

Confirm sequence

SELECT sequence_name, min_value, max_value, Increment_by, last_numberFROM user_sequences;

Marge Hohly 6

Marge Hohly 7

NEXTVAL and CURRVAL NEXTVAL is a pseudocolumn used to return the

next available sequence value CURRVAL is a pseudocolumn used to obtain the

last-used sequence value NEXTVAL must be issued before CURRVAL

contains a value NEXTVAL and CURRVAL must be qualified with

a sequence name: emp_emp_id_seq.nextval

NEXTVAL & CURRVAL

INSERT INTO departments (department_id, department_name, location_id)VALUES (departments_seq.NEXTVAL, ‘Support’, 2500);

Inserts a new department into the department table

Uses DEPARTMENT_SEQ sequence to generate new department number

Marge Hohly 8

Marge Hohly 9

Using a Sequence to INSERT

INSERT INTO employees VALUES(emp_emp_id_seq.NEXTVAL, ‘Kramer’, ‘Wilson’, ‘KWILSON’, ‘803.245.4642’, ’11-FEB-87’, ‘AD_ASST’, 5000, NULL, 101, 10);

Using a SEQUENCE

You can use NEXTVAL and CURRVAL in the following contexts: The SELECT list of the SELECT statement

that is not part of the subquery The SELECT list of a subquery is an INSERT

statement The VALUES clause of an INSERT

statement The SET calused of an UPDATE statement

Marge Hohly 10

Using a SEQUENCE 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 The DEFAULT expression in a CREATE TABLE

or ALTER TABLE statement

Marge Hohly 11

Marge Hohly 12

Modifying & Deleting a Sequence

ALTER SEQUENCE emp_emp_id_seqINCREMENT BY 5MAXVALUE 9999NOCACHENOCYCLE:

DROP SEQUENCE emp_emp_id_seq;

Marge Hohly 13

Sequence Gaps Gaps (nonsequential numbers) can

be generated by: rolling back a statement containing a

sequence, the number is lost a system crash. If the sequence caches

values into the memory and the system crashes, these values are lost.

the same sequence being used for multiple tables. If you do so, each table can contain gaps in the sequential numbers

Modifying a Sequence

Use the ALTER SEQUENCE statement ALTER SEQUENCE runner_id_seq

INCREMENT BY 1 MAXVALUE 999999 NOCACHE NOCYCLE;

Some validation occurs: Example if MAXVALUE is less than current sequence, it cannot execute.

Error codeMarge Hohly 14

ALTER SEQUENCE guidelines

You must be the owner or have ALTER privilege for the sequence

Only future sequence numbers are affected by the ALTER SEQUENCE statement

The START WITH option cannot be changed using the ALTER SEQUENCE statement.

Marge Hohly 15

Marge Hohly 16

What is an Index? A schema object that can speed up the retrieval of rows

by using a POINTER (isles in a grocery store) If you do not have an index on the column you’re

selecting, then a full table scan occurs Unique Index – Automatically created when you define

a column in a table to have a PRIMARY KEY or a UNIQUE KEY constraint.

Non-Unique Index – An index that a user can create to speed up access to the rows For example, 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.

Marge Hohly 17

Example of an INDEX WHEN TO CREATE AN INDEX

The column contains a wide range of values A column contains a large number of null values One 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. WHEN NOT TO CREATE AN INDEX

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 in the table The table is updated frequently – DML required index updates The indexed columns are referenced as part of an expression

Marge Hohly 18

Example of an INDEX CREATE INDEX index_name

ON table_name(column….,column);

CREATE INDEX d_cds_name_email_idxON d_clients(last_name, email);

DROP INDEX d_cds_name_email_idx;

When to Create an INDEX

Marge Hohly 19

WHEN NOT to create an index

Marge Hohly 20

When NOT to create an index

Usually not worth creating when: Table is small Columns are not often used as a

condition in the query Most queries are expected to retrieve

more than 2-4% of the rows in table Table is updated frequently Indexed columns are referenced as part

of an expression

Marge Hohly 21

Confirming indexes

SELECT ic.index_name,ic.column_name, ic.column_position col_pos, ix.uniquenessFROM user_indexes ix, user_ind_columns icWHERE ic.index_name = ix.index_name AND ic.table_name = ‘Employees’;

Marge Hohly 22

Function-based indexes

CREATE INDEX upper_last_name_idx ON employees (UPPER(last_name));

SELECT *FROM employeesWHERE UPPER(last_name)=‘King’;

Marge Hohly 23

Removing an INDEX

You can not modify indexes To change them you must delete and

create again To DROP an index you must be the

owner or have DROP ANY INDEX privilege.

If you drop a table it automatically drops constrains and indexes, but views and sequences remain.

Marge Hohly 24

Marge Hohly 25

Example of a SYNONYM CREATE [PUBLIC] SYNONYM

synonym_nameFOR object;

CREATE SYNONYM empFOR ussc_bhs_sql01_s02.employees; PUBLIC: creates a synonym accessible to all users (we

don’t have the privilege to use PUBLIC in HTML_DB) synonym_name: is the name of the synonym to be created object: identifies the object for which the synonym is

created -A private synonym name must be distinct from all other

objects owned by the same user.

Remove synonym

DROP [PUBLIC] SYNONYMname_of_synonym;

DROP SYNONYM dj_titles; Guidelines:

Object cannot be contained in a package A private synonym name must be

distinct from all other objects owned by the same user.

Marge Hohly 26