May 3, 2013

Oracle Collection - Nested tables

Nested tables

         Nested tables hold an arbitrary number of elements and use sequential numbers as the index or key to the elements. Nested tables can be stored in database tables and manipulated through SQL. They are appropriate for data relationships that must be stored persistently. Nested tables are flexible in that arbitrary elements can be deleted, rather than just removing an element from the end. Note that the order and subscripts (keys) of nested tables are not preserved as the table is stored and retrieved in the database

   Nested tables can be stored in a database column.Nested tables are initially dense, but they can become sparse (Data does not have to be stored in consecutive rows) when elements are deleted. Nested Table is available in PL/SQL as well as in SQL.

Nested tables are dense, but they can become sparse (have nonconsecutive subscripts). So, you can delete elements from a nested table using the built-in procedure DELETE. That might leave gaps in the index, but the built-in function NEXT lets you iterate over any series of subscripts

CREATE OR REPLACE TYPE nestedtable_type AS TABLE OF element_type; (PLSQL Type in SQL)

CREATE TABLE nested_table (id NUMBER, col1 nestedtable_type) NESTED TABLE col1 STORE AS col1_tab;   (Nested Table)

TYPE nestedtable_type IS TABLE OF element_type; (PLSQL Type in PLSQL)

Oracle Collection - PLSQL Table


    A collection is an ordered group of elements ,of the same type.Collection have only one dimension,we can nodel multi-dimensional arrays by creating collections whose elements are also collections.

The three type of collections are following

1) Associative Array
2) Nested tables
3) Varrays

Associative Array
TYPE array_type IS TABLE OF element_type INDEX BY key_type;
array_name array_type;
array_name(index) := value;

The “Associative Arrays” are also known as “Index-By” tables in PL/SQL.Associative arrays are single-dimensional, unbounded, sparse collections of homogeneous elements. Associative arrays can be sparse, which means not all elements between two elements need to be defined.Similar to hash table in other language
For instance you can have an element at index -29 and one at index 12 with nothing in between. Homogeneous elements mean every element must be of the same type.
Associative arrays are useful for small to medium sized lookup tables where the array can be constructed in memory ( only , which is destroyed after the session ends) each time a procedure is called or a package is initialized. There is no fixed limit on their size and their index values are more flexible- associative array keys can be negative and/or nonsequential, and associative arrays can use string values instead of numbers .The Associative Array is only available in PL/SQL

type my_tab_t is table of number index by pls_integer/ binary_integer;
type my_tab_t is table of number index by varchar2(4000);
type my_tab_t is table of tab.value%TYPE index by;
Example :
  cursor c_customer is select cust_name,cust_no,rownum from customers;
  type type_cname is table of customers% rowtype index by binary_integer;
  tab_cname type_cname;
  v_counter number:=0;


for r_customer in c_customer loop
   tab_cname(v_counter) := r_customer.cust_name ;
end loop;

forall i in tab_cname.first .. tab_cname.last

PLSQL Table attributes

DELETE - Delete rows in a table.
EXISTS - Return TRUE if the specified entry exists in the table.
COUNT - Returns the number of rows in the table .
FIRST - Returns the index of the first in the table.
LAST - Returns the index of last row in the table.
NEXT - Returns the index of the last row in the table .
PRIOR - Returns the index of previous row in the table before the specified row.

Use NOCOPY hint to reduce overhead of passing collections in and out of program units


Feb 12, 2013


          What is necessary for top-tier Web sites, according to proponents of NoSQL, is massive scalability, low latency, the ability to grow the capacity of your database on demand and an easier programming model. These, and others, are things which, according to them, SQL RDBMSes just don't provide in a cost-effective manner.

NoSQL database systems are developed to manage large volumes of data that do not necessarily follow a fixed schema. Data is partitioned among different machines (for performance reasons and size limitations) so JOIN operations are not usable.Documents are addressed in the database via a unique key that represents that document.No SQl Suuports CAP Property (Consistency, Availability, Partition tolerance)

C - Consistency
A - Availability
P - Partition tolerance
Partial partition tolerance for these databases is obtained by mirroring database clusters between multiple data centers. The advantage these databases have over a traditional RDBMS is that with the work spread over all of those machines, you can achieve ultra-low latency even when there are extremely high numbers of reads and writes, and with all those machines, you can analyze massive amounts of data quickly.This is meaningless when the database is on a single server

NoSQL implementations can be categorized by their manner of implementation:

1) Document store

Compared to relational databases, for example, collections could be considered as tables as well as documents could be considered as records.But they are different: every record in a table has the same sequence of fields, while documents in a collection could have fields that are completely different.Documents are addressed in the database via a unique key that represents that document. One of the other defining characteristics of a document-oriented database is that, beyond the simple key-document (or key-value) lookup that you can use to retrieve a document, the database will offer an API or query language that will allow you to retrieve documents based on their contents.
Having keys and values are the so-called document databases. A document, in this case, is a collection of various fields of information. Each individual document can have a different number of fields of varying lengths. These databases are useful if you have a lot of semi-structured data, and they are a good fit for object-oriented programming models

Eg :CouchDB, MongoDB

2) Graph

This kind of database is designed for data whose relations are well represented as a graph (elements interconnected with an undetermined number of relations between them). The kind of data could be social relations, public transport links, road maps or network topologies

Eg: Neo4j, InfoGrid and HyperGraphDB

3) Key-value store

Key-value stores allow the application to store its data in a schema-less way. The data could be stored in a datatype of a programming language or an object.Each piece of data that goes into the database is given a key, and when you want the data back, you use the key to get

4) XML databases

It does not use SQL as its query language.
It may not give full ACID guarantees.
It has a distributed, fault-tolerant architecture

5) Wide-column databases

It tend to draw inspiration from Google's BigTable model.

Eg: Cassandra, HBase

Advantages of NoSQL
Elastic scaling

Storage Type: Column Based NoSQL Databases, Document Based NoSQL Databases, Graph Based NoSQL Databases, Key-Value Based NoSQL Databases

License Type: AGPL NoSQL Databases, Apache NoSQL Databases, BSD NoSQL Databases, GPL NoSQL Databases, Open Source NoSQL Databases, Proprietary NoSQL Databases

Implementation Language: C NoSQL Databases, C++ NoSQL Databases, Erlang NoSQL Databases, Java NoSQL Databases, Python NoSQL Databases

Data Storage: BDB NoSQL Databases, Disk NoSQL Databases, GFS NoSQL Databases, Hadoop NoSQL Databases, Plug-in NoSQL Databases, RAM NoSQL Databases, S3 NoSQL Databases

Big Data

Big Data is all about finding a needle of value in a haystack of unstructured information.Big data refers to large datasets that are challenging to store, search, share, visualize, and analyze.

BigTable is a compressed, high performance, and proprietary data storage system built on Google File System .Big Table is Googles Database used for Google Reader,Google Maps,Google Book Search,My Search History,Google Earth,,Google Code hosting, Orkut,YouTube and Gmail.

BigTable maps two arbitrary string values (row key and column key) and timestamp (hence three dimensional mapping) into an associated arbitrary byte array. It is not a relational database

Relational databases (such as Oracle, MySQL, and SQL Server) versus newer non-relational databases (such as MongoDB, CouchDB, BigTable, and others).

Big Data is not just about volume, the approach to analysis contends with data content and structure that cannot be anticipated or predicted.

Feb 6, 2013

Connection Pool

A connection pool is a cache of database connections maintained so that the connections can be reused when future requests to the database are required. Connection pools are used to enhance the performance of executing commands on a database .

connection pooling is generally the practice of a middle tier (application server)

getting N connections to a database (say 20 connections).

These connections are stored in a pool in the middle tier, an "array" .Each connection is set to "not in use"
When a user submits a web page to the application server, it runs a piece of your code,
your code says "i need to get to the database", instead of connecting right there and
then (that takes time), it just goes to this pool and says "give me a connection please".
the connect pool software marks the connection as "in use" and gives it to you.