Oracle is designed for high performance, and oracle does not clean up as rows are added and deleted, thus leaving a honeycomb of free space within the physical data blocks. So basically if the oracle table block is flat structure, it is right most block in the structure. To reduce high water mark we can use one of below three steps step1. How to adjust the high watermark in oracle 10g alter. By default high water mark starts at the first block for new tables. Show the hwm for a datafile and whether they can be resized. The low high water mark lhwm is similar to the older high water mark in releases of oracle before 9i, in that all of the blocks below it have been formated for use by oracle. How to find high water mark in oracle 11g database blogger. The pctfree and pctused parameters have nothing to do with the hwm.
In oracle 10g onwards you can also use the shrink command to reorganize. This upper boundary is called the high water mark or hwm. The only way to reduce the hwm is to rebuild the able or use. Prior to oracle 10g, truncating a table was the only way to reduce the high water mark value.
First, lower the high water mark through extent movement with the following statement. On a recent post about reducing the high water mark in a table by shrinking it someone asked if you could tell how much space there was in a table that could be recovered by compressing it. This high water mark typically bumps up at 5 data blocks at a time. When table gets data via insertupdate more blocks being used high water mark moves forward. The mark a logical mark between free and used space can be called as high water mark hwm.
Assm was introduced in oracle 9ir2 and it was made the default for tablespaces in oracle 10gr2. So you can think of the total number of extents blocks used as the high water mark. Moving the high water mark in normal db operations, the high water mark only moves upwards, not downwards. The issue with the high water mark is that fulltable scans will always read up to the high water mark, even thought oracle may be reading through many empty blocks that were allocated to the table, used for rows, and then deleted. Oracle keeps track of the high water mark for a segment in the segment header. High water mark high water mark and its significance in a full table scan. In oracle 10g, there is a new option to get oracle to lower the hwm, but this command can be timeconsuming as rows of data will have to move around. To explain the movement of high water mark how does hwm affect full table scan. During a full table scan, all blocks in the table that are under the high water mark are scanned. This is because the table high water mark is not reset after delete operations.
Even though you deleted almost half of the rows, the above shows that table high water mark is up to 154 blocks, and to perform any query for full table scan, oracle will go up to 154 blocks to search the data. Basically the high water mark hwm has a function that comes into play with. The pctfree parameter instructs oracle to leave that percentage of the block free. Shrink space command was introduced in oracle 10g to perform online segment shrinks for tables, lobs and iot overflow segments. How do i find the high water mark for sessions on o racle 9i. The high water mark is an indicator that marks blocks that are allocated to a segment, but are not used yet. Oracle still has the same number of extents but many of the blocks are empty. Alter tablespace ts lower high water mark next, you would reduce the size of tablespace alter tablespace ts reduce.
High water mark oracle oracle plsql technology blog. Table high water mark tips burleson oracle consulting. You can use it to consolidate inuse extents below the high water mark and return unused extents in your table space to the system for reuse with table spaces created before db2 v9. Fix high water mark in oracle database gourangas oracle. To fix you export the table, drop it and import it back in. What are the different ways that i can lower the table highwater mark and. How can i find the high water mark the historical maximum number of concurrent users in an oracle database 9i. Reducing datafile size to recover free space oracle. We now can have unformatted blocks in the middle of a segment.
Reducing datafile size to recover free space oracle database 10g. Also, this trick would not work if you want to drop one of multiple datafiles from a tablespace, since oracle tends to spread the data over all files available. How to find the high water mark in oracle database. High water mark in oracle database is used to show the blocks which has been. The main disadvantage of this is that oracle always read the blocks up to high water mark in case of full table scan. On deleting records, oracle does not move back hwm. Hw, segment high water mark contention % description. Table high water mark and how empty the table is april 30, 2012 posted by mwidlake in performance, statistics. In oracle 9i, 10g and 11g there is actually two hwm a lower high water mark lhwm and a higher high water mark hhwm. Dbas often do not document the patches they install. Table 143 hints for changing the query optimizer goal.
The main benefit of assm is it automates the task of freelist management in order to handle the concurrent dml on the table. What are the different ways that i can lower the table high water mark and release the disk space to be reused by another table. All oracle segments have an upper boundary containing the data within the segment. How do i find the high water mark for sessions on oracle 9i. To reduce the high water mark in a table use exportdatapump utility and. High water mark in oracle is a term used in oracle table to show the blocks which has been ever used in the life cycle of the oracle table. High water mark this is a term used with table segments stored in the database. Freelists are used to allocate new blocks when data is added to the table. Developers and dbas get help from oracle experts on. This lock is acquired when a segments high water mark is moved, which typically is the case during heavy inserts. You may have ever notice that doing a count on empty table, takes time to show you 0 rows.
Just like after a flood you can never reduce the high water mark on a building, unless you rip the building down and build a new one. The reason for delay is setting of hwm at higher position. As the amount of data grows due to row inserts and updates, the segments high water mark grows accordingly. What actually it means and how to interprate when it high or low. Articles oracle 8i oracle 9i oracle 10g oracle 11g oracle 12c oracle c oracle 18c oracle 19c miscellaneous plsql sql oracle rac oracle apps weblogic linux mysql.
Segment shrink reclaims unused space both above and below the high water mark. To lower the high water mark and reduce the container size with dms table space is a twostep operation. If you are accessing many mapped tables, increase the high water mark. But as row deletes and updates shrink the amount of data in the object, the high water mark is not altered to reflect the segments new characteristics.
You are correct, when rows are deleted from a tables, oracle does not release the free space for other tables, and you must explicitly lower the table high water mark. This limit is known as the describe cache high water mark. When data get deleted, high water mark remain same even though blocks doesnt have any data. Therefore, we need to check them before we process them. Full table scans scan all the way to the tables high water mark. It acts as the boundary between used and unused spaceblocks. Larger amounts of memory allocated for sorts or hash joins reduce the optimizer cost of these operations. This process consolidates fragmented free space below the high water mark. Heterogeneous services initialization parameters oracle. The cache contains descriptions of the mapped tables that heterogeneous services reuses so that it does not have to reaccess the non oracle data store. Show the hwm for a datafile and whether they can be.
When a object is created, it will be created physically in the datafile and will be allocated a block. I even think that it was my first contribution to oracle. The hw enqueue is used to serialize the allocation of space above the high water mark in an object. When you run a query against the table oracle will scan through all the blocks including empty ones looking for data. Oracle database high water mark hwm boundary between. If there is a lot of free space below the high water mark, one might consider to use alter table move statements. When you specify compact, oracle database defragments the segment space and compacts the table rows but postpones the resetting of the high water mark and the deallocation of the space until a future time. If you envision a table, for example, as a flat structure or as a series of blocks laid one after the other in a line from left to right, the high water mark hwm would be the rightmost block that ever contained data, as illustrated in figure 101. For performance reasons, oracle keeps the high water mark for a. In oracle 9i, 10g and 11g there is actually two hwm a lower high water. How to reduce tablespace high water mark dba to dba. As a table undergoes deletes and updates, rows shrink and table data blocks become empty. How to find high water mark in oracle 11g database for multiple tables set linesize 300 set serveroutput on set verify off.
The function of the high water mark in oracle blogger. Create table as select ctas this copies the table rows into a clean area, lowering the high water mark, packing the rows densely as dictated by pctfree and releasing free space. Oracle leaves the high water mark alone after rows are deleted, and you can reclaim space at the table level with these techniques, all of which lower the high water mark for the table, thereby freeingup the space. Improving performance for an oracle database lyris. The function of the high water mark in oracle the database wizard.
247 1651 384 1518 1289 616 713 807 771 114 1541 1579 1086 1325 367 549 1069 389 638 214 1456 652 1649 1518 1531 476 711 355 75 1464 1054 224 1023 288 1378 1472 1073 88 47 808 283 608 506