30-01-2006

When the data is read from the hard it is called “Physical Read.”

When the data is read from the memory it is called “Logical Read.”

When the data is read from the Buffer Cache (Memory) + Roll back segment it is called “Consistent Read.”

In case of consistent read the o/p contains original data.

There are two important concepts in Oracle:

1. Data Consistency.
2. Data Integrity.

# Data Consistency: Data consistency is defined as the value of the data as it exits at the time of request.

EG: Suppose a request is made by a client c1 at 11:00 AM and in case it takes 5 min to reach to server (exaggerated). If at time say 11:00 AM the data which c1 demanded was changed by another client c2 at 11:02 AM then client c1 will still get the original data on account of Data Consistency provided server has kept the original data intact.

# Data Integrity: It implies to validity of data with respect to natural and business rules.

EG: The Loan given by Bank will never be more than the loan sanctioned by the bank.

Roll Back reading corresponds to Consistent reading.

*: 18000 brains have worked so far on Oracle in 30 years.

When ever a SQL or PL-SQL statement is fired by a user, it involves approximately following processes:

Statement à Parsing à Planning à Execution

# Parsing: Parsing is a process which resolve the references to object, data types, privileges, syntax etc.

In short in parsing checking of syntax and validity of different parameters is made. Checking is made by making Parsed Tree.

Optimizer makes execution plans. There are two types of optimizers:

1. Cost based.
2. Rule based.

Most times same set of commands are fires