Why not indexing on every column ?

Question

If Indexing gives very quick search result, why we should not create index on each and every column of the table? What is limitation of the indexes in the database?

Answer 1

> If I have only one index on the primary key (say table is EMPLOYEE and
EMP_ID as primary key), then also there will be a separate table?
Not necessarily. More on why a PK does not need to be an index below.
Some comments first.
There are different types of indexing methods in Oracle. B+tree indexes, bitmap indexes, function indexes, etc. So even though indexes may seem like a "table" - they are not. They use space like tables, but have very different internal structures. Also, it is not the space used by indexes that is the primary "problem" with using indexes.
Indexes are "good" for searching/reading data. Indexes are "bad" for creating/changing data. When an INSERT/UPDATE/DELETE is done, all these indexes need to be updated. This requires I/O on the indexes to make the changes. I/O is the most expensive operation on a database. And indexes cause more I/O as they need to be maintained. Indexes are therefore expensive things to use.. and thus needs to be used correctly to balance the "good" versus the "bad" of indexing.
As for primary key issue. There are also, like indexes, different structures that can be used for tables.
The standard structure is called a heap table. There is also one called index organised table (IOT). This is essentially a merge of a table with a B+tree index.. so your data and index structures are stored together.
An IOT is a single structure that "contains" both the PK index and the data - for the price of storing the PK columns once. In a heap table the PK index would have been a separate structure and would have duplicated the PK column values.
IOTs have however limitations due to the physical structure used for an IOT.
I suggest that you read the [url http://download.oracle.com/docs/cd/B19306_01/server.102/b14220/toc.htm]Oracle® Database Concepts guide - it goes into these in a lot better and clearer detail.