dbms_space.space_usage tips

Question:  What is the dbms_space.space_usage used for?  I want to get the space used for a segment, and I need to understand how the dbms_space.space_usage computes used space.  Can dbms_space.space_usage count-up actual row space used?

Answer:  The dbms_space.space_usage procedure is designed to show the space usage of data blocks under the High Water Mark for that segment.  Bitmap blocks, segment header, and extent map blocks are not accounted for by dbms_space.space_usage. 

The dbms_space.space_usage procedure can only be used on tablespaces that are created with auto segment space management.

The dbms_space package's space_usage procedure can be used to get the number of blocks in a partition:

set serveroutput on
declare
l_fs1_bytes number;
l_fs2_bytes number;
l_fs3_bytes number;
l_fs4_bytes number;
l_fs1_blocks number;
l_fs2_blocks number;
l_fs3_blocks number;
l_fs4_blocks number;
l_full_bytes number;
l_full_blocks number;
l_unformatted_bytes number;
l_unformatted_blocks number;
v_segname varchar2(500);
begin

see code depot for full script

dbms_space.space_usage(
segment_owner => 'SCOTT,
segment_name => 'EMP,
segment_type => 'TABLE',
fs1_bytes => l_fs1_bytes,
fs1_blocks => l_fs1_blocks,
fs2_bytes => l_fs2_bytes,
fs2_blocks => l_fs2_blocks,
fs3_bytes => l_fs3_bytes,
fs3_blocks => l_fs3_blocks,
fs4_bytes => l_fs4_bytes,
fs4_blocks => l_fs4_blocks,
full_bytes => l_full_bytes,
full_blocks => l_full_blocks,
unformatted_blocks => l_unformatted_blocks,
unformatted_bytes => l_unformatted_bytes
);
dbms_output.enable;
dbms_output.put_line('=============================================');
dbms_output.put_line('total blocks = '||to_char(l_fs1_blocks + l_fs2_blocks +
l_fs3_blocks + l_fs4_blocks + l_full_blocks)|| ' || total bytes = '||
to_char(l_fs1_bytes + l_fs2_bytes +
l_fs3_bytes + l_fs4_bytes + l_full_bytes));
end;

The output from dbms_space.space_usage is a count of how many blocks have certain amounts of free space.  Through the use of dbms_space.space_usage, you can determine which blocks can be recovered. 

There is additional useful information on the dbms_space.space_usage function available.

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