Is an Oracle tablespace the same as disk space, conceptually?

Question

Oracle has the concept of a tablespace. Does a tablespace resemble actual physical storage space on the disk? If this is true, and I delete a million records from the database, will the space on the disk be freed up immediately?

Answer 1

No and no.

A tablespace is made using one or more datafiles. A datafile comes closest to a disk. The data in a tablespace is striped over the datafiles. If you delete rows from a table, there will be free space in that table. If you then shrink that table, the free space in the tablespace grows and the same in the supporting tablespaces.

If you are lucky and your table data was in the end of the datafile[s], you might be able to also shrink the datafile, after which you end up with more free space on the OS.

Answer 2

Yes a tablespace represents actual physical storage space on the disks.
No, if you delete a million records then no space will be freed.

A tablespace is a collection of datafiles on the disk that are pre-sized; by removing data from one of these datafiles you don't free up any space because you haven't reduced the size of the tablespace. It has a predetermined size that has not changed.

You can manually resize a datafile but only if the amount of data stored in that file (not the tablespace) is less than the size you want to reduce it to. To use Oracle's example syntax:

ALTER DATABASE DATAFILE '/u02/oracle/rbdb1/stuff01.dbf'
   RESIZE 100M;

This is not something to be playing around with. There are often good reasons for the size of datafiles and tablespaces. You should talk to your DBA before doing anything.