What are Materialized View & Materialized View Logs?

A materialized view is a database object that contains the results of a query same like a table. They are local copies of data located on remote site. Also, used to create summary tables based on aggregations of a table's data, which store data based on remote tables are also, referred as Snapshots.

A materialized view can query tables, views, and other materialized views. Collectively these are called master tables (a replication term) or detail tables (a data warehouse term).

For replication purposes, materialized views allow you to maintain copies of remote data on your local server as read-only data.If we want to update the local copies of data, we have to use the Advanced Replication features in oracle. We can select data from a materialized view same as local table or view.

Materialized Views refer local data from Materialized View Logs (Snapshot Logs), which need to create on Local tables.

CREATE MATERIALIZED VIEW LOG ON emp;

Primary Key Materialized Views which are created with "WITH PRIMARY KEY" clause.

CREATE MATERIALIZED VIEW mv_emp_pk
REFRESH FAST START WITH SYSDATE 
NEXT  SYSDATE + 1/48
WITH PRIMARY KEY 
AS SELECT * FROM emp@remote;

Rowid Materialized Views which are created with "ROWID" clause.

CREATE MATERIALIZED VIEW mv_emp_rowid
REFRESH WITH ROWID 
AS SELECT * FROM emp@remote;

Subquery Materialized Views which are created by using subquery.

CREATE MATERIALIZED VIEW  mv_empdept
AS SELECT * FROM emp@remote e
WHERE EXISTS
     (SELECT * FROM dept@remote d
     WHERE e.deptid = d.deptid)

For data warehousing purposes, materialized views are the aggregate views, single-table aggregate views and join views.

CREATE MATERIALIZED VIEW  mv_empdept_dwh
AS SELECT * FROM emp@remote a, dept@remote b
WHERE a.deptid = b.deptid
and a.deptid < 1000;

COMPLETE refreshes by recalculating the defining query of the materialized view.

FAST refreshes by incrementally applying changes to the materialized view as per materialized view logs. For local materialized views, it chooses the refresh method which is estimated by optimizer to be most efficient.

FORCE Attempts a FAST refresh. If that is not possible, it does a complete refresh. For local materialized views, it chooses the refresh method which is estimated by optimizer to be most efficient. The refresh methods considered are log based FAST and COMPLETE.

NULLS LAST & NULLS FIRST in Select Statement.

In modern DBMS like Oracle, PostgreSQL, DB2, Firebird, Apache Derby, HSQLDB , H2, etc. you can specify NULLS LAST or NULLS FIRST in a select statement.

Used when you required sorted data in output having required column contains NULL values in it. It shows NULL values per below clauses.

NULLS LAST -  To sort data in which NULLS are showing in the last records.
NULLS FIRST - To sort data to the beginning - Default clause of a sql select statement.

select * from emp order by emp_name DESC NULLS LAST; 

select * from emp order by emp_name DESC NULLS FIRST; 

What are Global Temporary Tables in Oracle?

Global Temporary Tables, introduced by Oracle since 8i and mainly they are the private tables which stored data inserted by a session only and only accessed by that session itself.
Widely used in large applications make extensive use of temporary data storage which preserves data for the whole session or just for current transaction.

CREATE GLOBAL TEMPORARY TABLE GTT_TEMP  
(
AAAA  Number,
BBBB  Varchar2(10),
CCCC  Number
)
ON COMMIT [DELETE | PRESERVE] ROWS;

ON COMMIT DELETE ROWS : It sets the life of the data contained by the table to a single TRANSACTION.
The data is automatically flushed away after each COMMIT/ROLLBACK is executed - such tables are Transaction-specific Temporary tables.

ON COMMIT PRESERVE ROWS: It restricts the life of the data to a single SESSION.
Data is preserved in the table for a session only - such tables are Session-specific Temporary tables.

Below are the major features due to that it is used in large applications:

• If the TRUNCATE statement is issued against a temporary table, only the session specific data is trucated. There is no affect on the data of other sessions.
• Data in temporary tables is automatically deleted at the end of the database session, even if it ends abnormally.
• Indexes can be created on temporary tables. The content of the index and the scope of the index is the same as the database session.
• Views can be created against temporary tables and combinations of temporary and permanent tables.
• Triggers can alo be associated with such tables.
• Export and Import utilities can be used to transfer the table definitions, but no data rows are processed.
• Table Statistics of temporary tables are common to all sessions.
• Foreign key constraints are not applicable in case of Temporary tables.
• Global Temporary tables cannot be partitioned.
• Data in Global Temporary Tables is stored in temp segments in the temp tablespace only.
• In Oracle 8i,9i & 10g, TABLESPACE cannot be defined for GTT i.e. GTT segments were created in user’s default tablespace. 
• After Oracle 11g, GTT segments can be created on other Temporary Tablespaces too, it must be create on a TEMP Tablespace not any other.
• Use of LOB_STORAGE_CLAUSE, LOGGING/ NOLOGGING, MONITORING/ NOMONITORING, LOB_INDEX_CLAUSE is restricted in GTT definition. 

PIVOT & UNPIVOT functions in Oracle 11g

PIVOT Function is used in oracle 11g to produce query output in matrix style same as what we are doing in xls or xlsx.

in oracle 10g,

select deptid,jobid,sum(sal) from emp
group by deptid,jobid
order by deptno,jobid;  

for to get ouput of a query like in Excel Pivot funcationality.

select deptid, 
sum(decode(jobid, 101, sal)) "Analyst",
sum(decode(jobid, 102, sal)) "Programmer",
sum(decode(jobid, 103, sal)) "DBA",
sum(decode(jobid, 104, sal)) "Developer"
from emp
group by deptid
order by deptid;

Using the same output in oracle 11g with below query.

select * from(select deptid,jobid,sal from emp)
PIVOT (sum(sal) FOR jobid 
in (
101 "Analyst",
102, "Programmer",
103, "DBA",
104, "Developer"
))order by deptid;

same way we can achieve the first query output using UNPIVOT function.

select * from(select deptid,jobid,sal from emp)
UNPIVOT (sum(sal) FOR jobid 
in (
101, "Analyst",
102, "Programmer",
103, "DBA",
104, "Developer",
))order by deptid;