如何减少Oracle数据库存储？

Question

I have a problem with hard disk space lack. so I delete all records from a big table in my oracle database. But my hard disk space is not changed. How can I fix it?

Answer 1

Try ALTER TABLE table_name SHRINK SPACE CASCADE; and rebuild your indexes.

Answer 2

Deleting all rows from a table does not reset the high water mark and does not free up any space. This is because with DML operations Oracle assumes the space will be re-used later and doesn't bother to release it.

You probably want to run a TRUNCATE statement instead. It's also much faster than a DELETE as it doesn't generate any REDO or UNDO. But beware that truncating a table is a DDL statement that cannot be rolled back.

TRUNCATE ing tables releases space to the tablespace and that space is available to other objects. But it still doesn't reduce the amount of operating system space used. Lowering disk space requires shrinking datafiles.

Shrinking datafiles can be tricky. Datafiles can only be shrunk to the last extent. It's possible for a datafile to have a ton of empty space but not be shrinkable, if a single block of data is at the end of the file. In that case the datafile must be defragmented. That can be done by Oracle Enterprise Manager and you can find many other scripts to do it. They usually involve moving or rebuilding all the objects.

The simplest way to save some space is to try to shrink every file. This won't free up all available space, but in practice most of the free space is is usually after the last extent and can be reclaimed.

Run the below script, it will generate a bunch of alter statements to try shrink the datafiles by a large amount. Most of the statements will fail but that's OK. Cut the number in half and try again, repeatedly. This is a "dumb" way to do it, but it can usually free up some space quickly.

with decrease_size as (select 64 gb from dual)
select 'alter database datafile '''||file_name||''' resize '||to_char(round(bytes/1024/1024/1024)-(select gb from decrease_size))||'g;'
from dba_data_files
where bytes > (select gb from decrease_size) * 1024*1024*1024
order by 1 desc;