oracle高水位线详解Oracle高水位线详解.doc

oracle高水位线详解(Oracle高水位线详解)
First, Oracle high water level detailed explanation
First, what is the waterline (High, Wat
WHERE segment_name=，BIG EMPT :
SEGMENT NAME SEGMENT TYPE BLOCKS
BIG_EMP1 TABLE 1024 1 row selected.
())SQL>, ANALYZE, TABLE, big_empl, ESTIMATE, STATISTICS;
Statement processed.
3) SQL>, SELECT, table_ name, num_rows, blocks, empty blocks
FROM usertables
WHERE table_name='BIG_EMPl';
TABLE_NAME NUM_R0WS BLOCKS EMPTY_BLOCKS
BIG_EMP1 28672700323
1 row selected.
Be careful：
Notice that the space allocated by this table is reduced from 1024 to 512.
In order to retain the free space generated by the delete command, you can use TRUNCATE TABLE bigempl REUSE STORAGE.
With this command, the table will be the original 1024.
Four, Oracle section of the high water line HWM
In the storage of Oracle data, the storage space can be imagined as a reservoir, and data is imagined as the water in the reservoir. The position of the water in the reservoir has a line called the water level line. In Oracle, this line is called the high water line (High-warter, mark, HWM). When the database table has just been created, there is no data, so the water level is empty at this point, that is to say, HWM is the minimum. When inserting the data, the high water level will rise, but there is a characteristic, is that if you use the delete statement to delete the data, although