Answer / gourvendra singh

Highwatermark means the largest amount of data that was
placed in the table. for ex you are having a table which is
having 100 records, now ur highwatermark is 100, now you
add 10000 record more in the table ur highwatermark will
raise to 10000, nd if you delete all the records ur
highwatermark will remain on 10000, thats why in these case
query takes some times as it scan upto 10000. that why we
should use truncate in case of deleting all the records. as
the truncate cammand will remove the highwatermark and ur
processing will be faster.

for any query pls contact:
Gourvendra Singh
9873367485.

Answer / suresh kumar somayajula

High Watermark : It's an indicator or Porinter upto which
a Table or Index has ever contained
data .

Answer / p.rajasekar

High-water mark is an indicator or pointer up to which
table or index has ever contain data. Let us illustrate
this statement as Suppose we create an empty table , the
high-water mark would be at the beginning of the table
segment.
After inserting data in the table segment , the HWM moves
and point the position up to which the data is in the
segment.By inserting more data in the table segment, HWM
moves further to point the position up to which the data
is in the segment.
------- DATA-----------------
Un-Used Blocks





Now let us delete the data and see the pointer of HWM

DATA
------- Empty Blocks Un-Used Blocks



Full Table
Scan

As you seen above by deleting the data , HWM does
not move. The main disadvantage of this is that oracle
always read the blocks up to high water mark in case of
full table scan . You may have ever notice that doing a
count(*) on empty table , takes time to show you 0 rows.
The reason for delay is setting of HWM at higher position.

Now the question arises in front of us , how we set the
high-water mark at lower position ?
The only way to set the HWM is to truncate a table.

Let us see how truncate set the HWM.

No data in the segment



HWM is reset now , after truncating data.

Table where lots of deletion or insertion takes place ,
probably has High HWM. If the HWM is high , it is better
to rebuild table segment for performance sake.

3. Select sum(a) sum_1,max(a) max_1 ,count(a) count_1 from ( ( select 1 a from dual union all Select to_number(‘2011’) a from dual union all select 1 a from dual) union select 2 b from dual);

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