Indexes- What?  Optional structures associated with tables  Provides a quick access path to table data  You can create indexes on one or more columns of a table  Avoids the need for large-table, full-table scans and disk sorts

Indexes - Types  B-tree – default index, works well on large tables and columns with distinct values, updates relatively inexpensive.  Bitmap indexes – used on columns which contains many repeated values on a large table. create bitmap index city_ind on emp (city);  Function-based indexes – Built on expressions create index salcomm_ind on emp (sal+comm); For Queries with SAL+COMM in where SALCOMM_IND index will be used

Indexes – When to use and on what columns  Access less than 5% of the rows in the table.  Don't index tables that are frequently updated.  Tables that don't have duplicate values on the columns usually selected in WHERE clauses.  Columns frequently specified in WHERE clauses  Columns that have unique values  Columns that are commonly used to join tables

Indexes – How to use it?  Some indexes are created part of creating constraints eg: PrimaryKey and UniqueKey  Can be created using the graphical utilities of Oracle Enterprise Manager  Using CREATE INDEX command CREATE INDEX emp_hiredate_idx ON employees (hire_date); CREATE INDEX empdept_ind on emp (empno,deptno); - Composite ALTER INDEX emp_hiredate_id RENAME TO emp_hire_date_idx; DROP INDEX emp_hire_date_idx;

Indexes – How to view them?  DBA_INDEXES - indexes on all tables in the database  ALL_INDEXES - indexes on all tables accessible to the user  USER_INDEXES - restricted to indexes owned by the user  Columns of indexes on tables DBA_IND_COLUMNS,ALL_IND_COLUMNS, USER_IND_COLUMNS  Function-based indexes on tables. DBA_IND_EXPRESSIONS,ALL_IND_EXPRESSIONS,USER_IND_EXPRESSIONS

Indexes- Rebuilding  Rebuilding an index based on an existing data source removes intra-block fragmentation.  Compared to dropping the index and using the CREATE INDEX statement, re-creating an existing index offers better performance.  The following statement rebuilds the existing index emp_name: ALTER INDEX emp_name REBUILD;

Indexes - Clustering  Co-locate related tables for faster access and for less storage. create cluster emp_dept (deptno number(2)); create index on cluster emp_dept; create table dept (deptno number(2),name varchar2(20),loc varchar2(20)) cluster emp_dept (deptno); create table emp (empno number(5),name varchar2(20), sal number(10,2), deptno number(2)) cluster emp_dept (deptno)  Not good if frequent updates are expected

Indexes – refresher  Assuming index on S# created, will the index be used?  WHERE S# > YES  WHERE S# <> NO  WHERE S#= 100 – YES  Assume index on S_NMAE  WHERE S_NAME like 'S%‘ - YES  WHERE S_NAME like '%S‘- NO  WHERE S_NAME is NULL - NO:

Views – What?  SQL statement that is stored in memory for re- use  CREATE VIEW view_emp AS SELECT empid FROM emp; SELECT * FROM view_emp WHERE empid BETWEEN 500 AND 1000; Oracle will transform the query into this: SELECT * FROM (select empid from emp) WHERE empid BETWEEN 500 AND 1000;

Views - Benefits  Common set of SQL, less likely to require parsing  Security: hide tables that actually contain the data you are querying.  Restrict Rows/Columns that a given user has access to.  Logical data independence. - Same object name is interpreted differently by different users eg: Table S can be supplier table for user1 whereas it means restricted view s_view for user2 with user2.S as synonym for user1.s_view

Views - Commands  Basic Views representing entire table: CREATE OR REPLACE VIEW AS SELECT FROM ;  Restrictive Views: CREATE OR REPLACE VIEW AS SELECT FROM WHERE = ;  Multi table views – CREATE OR REPLACE VIEW explan_four AS SELECT DISTINCT s.srvr_id FROM servers s, serv_inst i WHERE s.srvr_id = i.srvr_id;

Views – Commands contd.  Drop view - DROP VIEW date_view;  Updating views - will update underlying base table. UPDATE upd_view SET person_id = person_id * 10;  Read only views - CREATE OR REPLACE VIEW person_ro_view AS SELECT first_name, last_name, ssn FROM personWITH READ ONLY;  Views with Check option - CREATE OR REPLACE VIEW checkoption_view AS SELECT person_id, first_name, last_name, dob, ssn FROM person WHERE person_id < 10 WITH CHECK OPTION;

Views – Data Dictionary  data dictionary views are divided into three groups: USER, ALL, and DBA.  USER : information about objects owned by the current SQL user  USER TABLES all tables with their name, no. of columns, storage and statistical information etc. (TABS)  USER_CATALOG tables, views, and synonyms (CAT)  USER_COL_COMMENTS comments on columns  USER_CONSTRAINTS constraint definitions for tables  USER_INDEXES_all information about indexes created for tables (IND)  USER_OBJECTS_all database objects owned by the user (OBJ)

Views – Data Dictionary contd.  USER_TAB_COLUMNS columns of the tables and views owned by the user (COLS)  USER_TAB_COMMENTS comments on tables and views  USER_TRIGGERS triggers defined by the user  USER_USERS information about the current user  USER_VIEWS views defined by the user

Views – Materialized  Database object contains results of a query.  Local copies of data located remotely  Create summary tables based on aggregations of a table's data.  Also know as snapshots.  A materialized view can query tables, views, and other materialized views. Collectively these are called master tables

Views – Materialized contd.  FAST refresh uses the materialized view logs to send the rows that have changed from master tables to the materialized view.  COMPLETE refresh re-creates the entire materialized view.  Refresh Method - FORCE Clause When you specify a FORCE clause, Oracle will perform a fast refresh if one is possible or a complete refresh otherwise. FORCE is the default.

Views – Materialized contd.  Primary Key Materialized Views CREATE MATERIALIZED VIEW mv_emp_pk REFRESH FAST START WITH SYSDATE REFRESH FAST START WITH SYSDATE NEXT SYSDATE + 1/48 WITH PRIMARY KEY AS SELECT * FROM  Rowid Materialized Views CREATE MATERIALIZED VIEW mv_emp_rowid REFRESH WITH ROWID AS SELECT * FROM AS SELECT * FROM  Subquery Materialized Views CREATE MATERIALIZED VIEW mv_empdept AS SELECT * FROM e WHERE EXISTS (SELECT * FROM d WHERE e.dept_no = d.dept_no)