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Tablespace utilization in Oracle
A DBA must maintain adequate space in database tablespaces in order to have smooth functioning. All the tablespaces must be below threshold value and they all must be online (In case if no tablespace is put offline). Below query gives the tablespace utilization percentages.
Query to Check Tablespace Utilization
set colsep |
set linesize 100 pages 100 trimspool on numwidth 14
col name format a25
col owner format a15
col "Used (GB)" format a15
col "Free (GB)" format a15
col "(Used) %" format a15
col "Size (M)" format a15
SELECT d.status "Status", d.tablespace_name "Name",
TO_CHAR(NVL(a.bytes / 1024 / 1024 /1024, 0),'99,999,990.90') "Size (GB)",
TO_CHAR(NVL(a.bytes - NVL(f.bytes, 0), 0)/1024/1024 /1024,'99999999.99') "Used (GB)",
TO_CHAR(NVL(f.bytes / 1024 / 1024 /1024, 0),'99,999,990.90') "Free (GB)",
TO_CHAR(NVL((a.bytes - NVL(f.bytes, 0)) / a.bytes * 100, 0), '990.00') "(Used) %"
FROM sys.dba_tablespaces d,
(select tablespace_name, sum(bytes) bytes from dba_data_files group by tablespace_name) a,
(select tablespace_name, sum(bytes) bytes from dba_free_space group by tablespace_name) f WHERE
d.tablespace_name = a.tablespace_name(+) AND d.tablespace_name = f.tablespace_name(+) AND NOT
(d.extent_management like 'LOCAL' AND d.contents like 'TEMPORARY')
UNION ALL
SELECT d.status
"Status", d.tablespace_name "Name",
TO_CHAR(NVL(a.bytes / 1024 / 1024 /1024, 0),'99,999,990.90') "Size (GB)",
TO_CHAR(NVL(t.bytes,0)/1024/1024 /1024,'99999999.99') "Used (GB)",
TO_CHAR(NVL((a.bytes -NVL(t.bytes, 0)) / 1024 / 1024 /1024, 0),'99,999,990.90') "Free (GB)",
TO_CHAR(NVL(t.bytes / a.bytes * 100, 0), '990.00') "(Used) %"
FROM sys.dba_tablespaces d,
(select tablespace_name, sum(bytes) bytes from dba_temp_files group by tablespace_name) a,
(select tablespace_name, sum(bytes_cached) bytes from v$temp_extent_pool group by tablespace_name) t
WHERE d.tablespace_name = a.tablespace_name(+) AND d.tablespace_name = t.tablespace_name(+) AND
d.extent_management like 'LOCAL' AND d.contents like 'TEMPORARY'; 
Tablespace Threshold Values
Your aim as a DBA is to always have good space inside tablespaces so that database users do not encounter an error. This is achieved by maintaining a great tablespace threshold percentages (%) or good free space.
As a thumb rule, make sure all your database tablespaces have at least 20% free space (or 80% utilized space).
But when your tablespace size shoots into TB, even 20% free space will be equal to 200 GB! In such big tablespaces, you can have 10% or sometimes 5% free space.
Further Read
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