What is extent for..

Extent is a specific number of contignous data blocks, but I don't understand sense in it. When we read data from the table - we ask for blocks, when we update/insert/delete - we also operate on block. So what is extent for?
Thx for help.
Aliq.

Well we shall try just to see what are extents in a more simple way.Just take the oracle table as a home.You make a home by adding lots of walls.The walls are actualy you wont bother anytime whenever you will talk about your home right?But they are very important.So you make walls.The walls are nothing but your different extents for the home.Now wall is no thing.Its made up by collection of bricks(blocks).Now when we talk about a home, neither it is physicla,nor the wall.The only real thing is the blocks. So same is the case of table itself.There is no such thing called table.Please note I am saying it ust for the reference purpose.Table is a logical grouping that is given to us by oracle/db for storing our data in a more structured way and this gives us more control and prciseness when we refer our data eg. show me all employees.Ok no issues, see we have this table called Employees.Its much better than to say that we have blocks from file 1 to 11 and from block no 1 to 1000 containing employees and our dear scott user is in block no 237.Isnt it?So the real thing is blocks.Well to be precise again they are not real but still lets not go to O/s blocks and all that.
Now you are very right in saying that when we do read /write/select data , we select in the terms of blocks.But assume a situation when we are doing a big full table scan and we need to scan some 4kb data in one chunk as our o/s supports that. Now it would be very difficult to come and start looking all the blocks in a file which are just lying there.But if they are mapped in a grouping like extent,we know that we need to read thse many extents to complete our IO.So extents are basically the logical grouping for the blocks which helps in contegous read to happen. Its just a mapping that Oracle has made for easier and better management for the IO operations.
Example given is not so good but I hope you would get some idea.
Just my 2 cents.
Aman....

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