Know about Oracle High Water Mark

REM This is an example SQL*Plus Script to find tables fragmentated below high water mark

set heading off verify off echo off
Spool fragment.sql

REM The below queries gives information about the size of the table with respect to the High water Mark
REM note that BLOCKS*8192 is BLOCKS times the block size: 8192.  Substitue your DB blocksize.
REM SELECT BLOCKS*8192/1024/1024 FROM  DBA_TABLES WHERE  TABLE_NAME='<TABLE_NAME>'  and    owner='<owner>'   ;
REM The below queries gives the actual size in MB used by the table in terms of data .
REM SELECT NUM_ROWS*AVG_ROW_LEN/1024/1024 FROM  DBA_TABLES WHERE TABLE_NAME='<TABLE_NAME>' and  owner='<owner'
REM
REM You can use the difference of the two sql statements specified above to get the table which
REM has fragementation below high water mark prompt Enter name(s) of schema for which you want to find
REM fragemented object.
PROMPT Please enter the schema name

SELECT TABLE_NAME ,  (BLOCKS *8192 / 1024/1024 ) - (NUM_ROWS*AVG_ROW_LEN/1024/1024)
"Data lower than HWM in MB"   FROM  DBA_TABLES WHERE  UPPER(owner) =UPPER('&OWNER') order by 2 desc;

Spool off

Goal

This article explains, with examples, how to view the high water mark and when the high water mark is reset. The queries given in this article applies when the segment , whose high water mark has to be determined ,  is in one datafile and is not spawned across multiple datafiles .

Solution

The high water mark is the boundary between used and unused space in a segment. As requests for new free blocks that cannot be satisfied by existing free lists are received, the block to which the high water mark points to becomes a used block, and the high water mark is advanced to the next block. In other words, the segment space to the left of the high water mark is used, and the space to the right of it is unused.

The high-water mark is the level at which blocks have never been formatted to receive data.

When a table is created in a tablespace, some initial number of blocks / extents are allocated to the table. Later, as the number of rows inserted increases, extents are allocated accordingly.

To find out how many blocks / extents are allocated to the table, query DBA_SEGMENTS for ‘blocks’ and ‘extents’.

For example:

SQL>create table test1 (num number) tablespace tbsp1;

Table created

SQL>select blocks, extents from dba_segments where segment_name=’TEST1′

BLOCKS EXTENTS
——– ————-
8 1

Now, to view the high water mark, perform an:

SQL> analyze table test1 compute statistics;

Querying dba_tables for ‘Blocks’ and ‘Empty_blocks’ should give the high water mark.

Blocks — > Number blocks that has been formatted to recieve data
Empty_blocks —> Among the allocated blocks, the blocks that were never used

SQL> select blocks,empty_blocks,num_rows from dba_tables where table_name=’TEST1′

BLOCKS EMPTY_BLOCKS NUM_ROWS
————– ————————– ——————–
0 7 0

If you insert some rows, then the output of the above query returns:

BLOCKS EMPTY_BLOCKS NUM_ROWS
————– ————————– ——————–
1 6 8

blocks+Empty_blocks=1+6=7 (but not 8) because 1 block is for segment header.

Insert some more rows into table ‘TEST1’ to increase the number of extents allocated so that
DBA_SEGMENTS will show :

BLOCKS EXTENTS
————– —————
32 4

And dba_tables (after analyze table) shows:

BLOCKS EMPTY_BLOCKS NUM_ROWS
————– ————————– ——————–
28 3 14338

Deleting the records doesn’t lower the high water mark. Therefore, deleting the records doesn’t raise the ‘Empty_blocks’. After deleting the records, if you query dba_segments or dba_tables, there would be no change. Even an ‘Alter table test1 deallocate unused;’ will not bring the high water mark down.

To determine the exact number of blocks that contain data i.e. space used by table below the high water mark, query rowid and get the first and last used block from the rowid.

SQL> select count (distinct dbms_rowid.rowid_block_number(rowid)) “used blocks” from TEST1;

This works fine if only one file is used for the segment. If more files, we need to include the file number in some way, for instance:

SQL> select count (distinct dbms_rowid.rowid_block_number(rowid)||’-‘||dbms_rowid.rowid_relative_fno (rowid)) “used blocks” from TEST1

USED_BLOCKS
———————–
22

From this we can conclude that for table ‘TEST1’, 32 blocks are allocated out of which 28 blocks are formatted to receive data but only 22 blocks contain the actual data.

The high water mark can be reset with a truncate table or if the table is moved to another tablespace.  Additionally, in 10g the following option to shrink a segment was introduced to reset the high water mark. eg. ALTER TABLE <tablename> SHRINK SPACE;

When the table is created with CTAS from another table, the high water mark of the latter table is not reflected in the new table. If the tablespace is moved back to the same tablespace, the high water mark is reset. In this case, query on obj#, dataobj# of obj$. Obj# remains the same but dataobj# changes.

 

PURPOSE
This article describes how to find out how many blocks are really being
used within a table ie. are not empty. Please note that this article does
not cover what to do when chaining is taking place.

SCOPE & APPLICATION
For DBA’s needing to determine how many blocks within a table are
empty blocks.

How many blocks contain data (are not empty)
——————————————–
Each row in the table has pseudocolumn called ROWID.
This pseudo contains information about physical location
of the row in format
block_number.row.file

If the table is stored in a tablespace which has one
datafile, all we have to do is to get DISTINCT
number of block_number from ROWID column of this table.

But if the table is stored in a tablespace with more than one
datafile then you can have the same block_number but in
different datafiles so we have to get DISTINCT number of
block_number+file from ROWID.

The SELECT statements which give us the number of “really used”
blocks is below. They are different for ORACLE 7 and ORACLE 8
because of different structure of ROWID column in these versions.

For ORACLE 7:

SELECT COUNT(DISTINCT SUBSTR(rowid,15,4)||
SUBSTR(rowid,1,8)) “Used”
FROM schema.table;

For ORACLE 8+:

SELECT COUNT (DISTINCT
DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid)||
DBMS_ROWID.ROWID_RELATIVE_FNO(rowid)) “Used”
FROM schema.table;
or

SELECT COUNT (DISTINCT SUBSTR(rowid,1,15)) “Used”
FROM schema.table;

You could ask why the above information could not be determined
by using the ANALYZE TABLE command. The ANALYZE TABLE command only
identifies the number of ‘ever’ used blocks or the high water mark
for the table.

What is the High Water Mark?
—————————-
All Oracle segments have an upper boundary containing the data within
the segment. This upper boundary is called the “high water mark” or HWM.
The high water mark is an indicator that marks blocks that are allocated
to a segment, but are not used yet. This high water mark typically bumps
up at 5 data blocks at a time. It is reset to “zero” (position to the start
of the segment) when a TRUNCATE command is issued. So you can have empty
blocks below the high water mark, but that means that the block has been
used (and is probably empty caused by deletes). Oracle does not move the
HWM, nor does it *shrink* tables, as a result of deletes. This is also
true of Oracle8. Full table scans typically read up to the high water mark.

Data files do not have a high water mark; only segments do have them.

How to determine the high water mark
————————————
To view the high water mark of a particular table::

ANALYZE TABLEESTIMATE/COMPUTE STATISTICS;

This will update the table statistics. After generating the statistics,
to determine the high water mark:

SELECT blocks, empty_blocks, num_rows
FROM user_tables
WHERE table_name =;

BLOCKS represents the number of blocks ‘ever’ used by the segment.
EMPTY_BLOCKS represents only the number of blocks above the ‘HIGH WATER MARK’
.
Deleting records doesn’t lower the high water mark. Therefore, deleting
records doesn’t raise the EMPTY_BLOCKS figure.

Let us take the following example based on table BIG_EMP1 which
has 28672 rows (Oracle 8.0.6):

SQL> connect system/manager
Connected.

SQL> SELECT segment_name,segment_type,blocks
2> FROM dba_segments
3> WHERE segment_name=’BIG_EMP1′;
SEGMENT_NAME SEGMENT_TYPE BLOCKS EXTENTS
—————————– —————– ———- ——-
BIG_EMP1 TABLE 1024 2
1 row selected.

SQL> connect scott/tiger

SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
Statement processed.

SQL> SELECT table_name,num_rows,blocks,empty_blocks
2> FROM user_tables
3> WHERE table_name=’BIG_EMP1′;
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
—————————— ———- ———- ————
BIG_EMP1 28672 700 323
1 row selected.

Note: BLOCKS + EMPTY_BLOCKS (700+323=1023) is one block less than
DBA_SEGMENTS.BLOCKS. This is because one block is reserved for the
segment header. DBA_SEGMENTS.BLOCKS holds the total number of blocks
allocated to the table. USER_TABLES.BLOCKS holds the total number of
blocks allocated for data.

SQL> SELECT COUNT (DISTINCT
2> DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid)||
3> DBMS_ROWID.ROWID_RELATIVE_FNO(rowid)) “Used”
4> FROM big_emp1;
Used
———-
700
1 row selected.

SQL> DELETE from big_emp1;
28672 rows processed.

SQL> commit;
Statement processed.

SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
Statement processed.

SQL> SELECT table_name,num_rows,blocks,empty_blocks
2> FROM user_tables
3> WHERE table_name=’BIG_EMP1′;
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
—————————— ———- ———- ————
BIG_EMP1 0 700 323
1 row selected.

SQL> SELECT COUNT (DISTINCT
2> DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid)||
3> DBMS_ROWID.ROWID_RELATIVE_FNO(rowid)) “Used”
4> FROM big_emp1;
Used
———-
0
1 row selected.

SQL> TRUNCATE TABLE big_emp1;
Statement processed.

SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
Statement processed.

SQL> SELECT table_name,num_rows,blocks,empty_blocks
2> FROM user_tables
3> WHERE table_name=’BIG_EMP1′;
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
—————————— ———- ———- ————
BIG_EMP1 0 0 511
1 row selected.

SQL> connect system/manager
Connected.

SQL> SELECT segment_name,segment_type,blocks
2> FROM dba_segments
3> WHERE segment_name=’BIG_EMP1′;
SEGMENT_NAME SEGMENT_TYPE BLOCKS EXTENTS
—————————– —————– ———- ——-
BIG_EMP1 TABLE 512 1
1 row selected.

Note: TRUNCATE has also deallocated the space from the deleted rows.
To retain the space from the deleted rows allocated to the table use:
TRUNCATE TABLE big_emp1 REUSE STORAGE