2026-03-12 20:23:15
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idev/dba_highwater_mark.sql
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idev/dba_highwater_mark.sql
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-- +----------------------------------------------------------------------------+
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-- | Jeffrey M. Hunter |
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-- | jhunter@idevelopment.info |
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-- | www.idevelopment.info |
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-- |----------------------------------------------------------------------------|
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-- | Copyright (c) 1998-2012 Jeffrey M. Hunter. All rights reserved. |
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-- |----------------------------------------------------------------------------|
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-- | DATABASE : Oracle |
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-- | FILE : dba_highwater_mark.sql |
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-- | CLASS : Database Administration |
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-- | PURPOSE : Determine the highwater mark of a given table. |
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-- | NOTE : As with any code, ensure to test this script in a development |
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-- | environment before attempting to run it in production. |
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-- +----------------------------------------------------------------------------+
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analyze table &owner.&table_name compute statistics
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/
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SELECT
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blocks
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FROM
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dba_segments
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WHERE
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owner = '&&owner'
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AND segment_name = '&&table_name'
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/
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SELECT
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empty_blocks
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FROM
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dba_tables
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WHERE
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owner = '&&owner'
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AND table_name = '&&table_name'
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/
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HIGHWATER_MARK = dba_segments.blocks - dba_tables.empty_blocks - 1
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----------------------------------------------------------------
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What is the High Water Mark?
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----------------------------
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All Oracle segments have an upper boundary containing the data within
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the segment. This upper boundary is called the "high water mark" or HWM.
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The high water mark is an indicator that marks blocks that are allocated
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to a segment, but are not used yet. This high water mark typically bumps
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up at 5 data blocks at a time. It is reset to "zero" (position to the start
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of the segment) when a TRUNCATE command is issued. So you can have empty
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blocks below the high water mark, but that means that the block has been
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used (and is probably empty caused by deletes). Oracle does not move the
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HWM, nor does it *shrink* tables, as a result of deletes. This is also
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true of Oracle8. Full table scans typically read up to the high water mark.
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Data files do not have a high water mark; only segments do have them.
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How to determine the high water mark
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------------------------------------
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To view the high water mark of a particular table::
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ANALYZE TABLE <tablename> ESTIMATE/COMPUTE STATISTICS;
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This will update the table statistics. After generating the statistics,
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to determine the high water mark:
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SELECT blocks, empty_blocks, num_rows
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FROM user_tables
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WHERE table_name = <tablename>;
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BLOCKS represents the number of blocks 'ever' used by the segment.
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EMPTY_BLOCKS represents only the number of blocks above the 'HIGH WATER MARK'
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.
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Deleting records doesn't lower the high water mark. Therefore, deleting
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records doesn't raise the EMPTY_BLOCKS figure.
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Let us take the following example based on table BIG_EMP1 which
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has 28672 rows (Oracle 8.0.6):
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SQL> connect system/manager
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Connected.
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SQL> SELECT segment_name,segment_type,blocks
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2> FROM dba_segments
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3> WHERE segment_name='BIG_EMP1';
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SEGMENT_NAME SEGMENT_TYPE BLOCKS EXTENTS
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----------------------------- ----------------- ---------- -------
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BIG_EMP1 TABLE 1024 2
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1 row selected.
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SQL> connect scott/tiger
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SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
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Statement processed.
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SQL> SELECT table_name,num_rows,blocks,empty_blocks
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2> FROM user_tables
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3> WHERE table_name='BIG_EMP1';
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TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
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------------------------------ ---------- ---------- ------------
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BIG_EMP1 28672 700 323
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1 row selected.
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Note: BLOCKS + EMPTY_BLOCKS (700+323=1023) is one block less than
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DBA_SEGMENTS.BLOCKS. This is because one block is reserved for the
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segment header. DBA_SEGMENTS.BLOCKS holds the total number of blocks
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allocated to the table. USER_TABLES.BLOCKS holds the total number of
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blocks allocated for data.
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SQL> SELECT COUNT (DISTINCT
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2> DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid)||
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3> DBMS_ROWID.ROWID_RELATIVE_FNO(rowid)) "Used"
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4> FROM big_emp1;
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Used
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----------
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700
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1 row selected.
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SQL> DELETE from big_emp1;
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28672 rows processed.
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SQL> commit;
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Statement processed.
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SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
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Statement processed.
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SQL> SELECT table_name,num_rows,blocks,empty_blocks
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2> FROM user_tables
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3> WHERE table_name='BIG_EMP1';
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TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
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------------------------------ ---------- ---------- ------------
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BIG_EMP1 0 700 323
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1 row selected.
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SQL> SELECT COUNT (DISTINCT
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2> DBMS_ROWID.ROWID_BLOCK_NUMBER(rowid)||
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3> DBMS_ROWID.ROWID_RELATIVE_FNO(rowid)) "Used"
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4> FROM big_emp1;
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Used
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----------
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0
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1 row selected.
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SQL> TRUNCATE TABLE big_emp1;
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Statement processed.
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SQL> ANALYZE TABLE big_emp1 ESTIMATE STATISTICS;
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Statement processed.
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SQL> SELECT table_name,num_rows,blocks,empty_blocks
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2> FROM user_tables
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3> WHERE table_name='BIG_EMP1';
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TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
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------------------------------ ---------- ---------- ------------
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BIG_EMP1 0 0 511
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1 row selected.
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SQL> connect system/manager
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Connected.
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SQL> SELECT segment_name,segment_type,blocks
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2> FROM dba_segments
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3> WHERE segment_name='BIG_EMP1';
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SEGMENT_NAME SEGMENT_TYPE BLOCKS EXTENTS
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----------------------------- ----------------- ---------- -------
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BIG_EMP1 TABLE 512 1
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1 row selected.
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NOTE:
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----
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TRUNCATE has also deallocated the space from the deleted rows.
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To retain the space from the deleted rows allocated to the table use:
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SQL> TRUNCATE TABLE big_emp1 REUSE STORAGE
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