Actual indicator of full tablespace in oracle - oracle

Question

What is the actual indicator of a full tablespace? The physical file size(allocated) or the used space ?
We have only one schema using DATA01 as the main tablespace in database.
Refer sql below:
Select Bytes/1024/1024/1024,Tablespace_Name,File_Name From Dba_Data_Files;
or
Select df.tablespace_name Tablespace,
totalusedspace Used_MB,
(df.totalspace - tu.totalusedspace) Free_MB,
Df.Totalspace Total_Mb,
round(100 * ( (df.totalspace - tu.totalusedspace)/ df.totalspace)) Pct_Free
from
(select tablespace_name,
round(sum(bytes) / 1048576) TotalSpace
from dba_data_files
Group By Tablespace_Name) Df,
(select round(sum(bytes)/(1024*1024)) totalusedspace, tablespace_name from dba_segments
group by tablespace_name) tu
where df.tablespace_name = tu.tablespace_name;
The first sql give following result:
SQL> Select Bytes/1024/1024/1024,Tablespace_Name,File_Name From Dba_Data_Files;
BYTES/1024/1024/1024 TABLESPACE_NAME
-------------------- ------------------------------
FILE_NAME
------------------------------------------------------------------------------------------------------------------------
.004882813 USERS
+DATA/xxx/datafile/users.289.863434089
1.43066406 UNDOTBS1
+DATA/xxx/datafile/undotbs1.288.863434089
5 SYSAUX
+DATA/xxx/datafile/sysaux.287.863434089
5 SYSTEM
+DATA/xxx/datafile/system.286.863434087
.537109375 UNDOTBS2
+DATA/xxx/datafile/undotbs2.294.863434493
31.9999847 DATA01
+DATA/xxx/datafile/data01a.dbf
31.9999847 DATA01
+DATA/xxx/datafile/data01b.dbf
31.9999847 DATA01
+DATA/xxx/datafile/data01c.dbf
.09765625 DATA02
+DATA/xxx/datafile/data02a.dbf
6.4831543 INDEX02
+DATA/xxx/datafile/index02a.dbf
31.9414063 DATA01
+DATA/xxx/datafile/data01d.dbf
The second sql give following result:
TABLESPACE USED_MB FREE_MB TOTAL_MB PCT_FREE
------------------------------ ---------- ---------- ---------- ----------
UNDOTBS1 27 1438 1465 98
SYSAUX 2211 2909 5120 57
SYSTEM 2579 2541 5120 50
DATA01 86363 44649 131012 34
UNDOTBS2 17 533 550 97
INDEX02 3103 3536 6639 53
As shown, the four datafiles data01a/data01b/data01c/data01d is having size nearly 32G, but why the result of second sql showing there are still 34% free space for tablespace DATA01?
Also, it is much more strange when DBA send the screenshot below from db console:
Datafile Name| Usage | Size | Used | Free |
data01d.dbf| 0% | 31.96 Gb | 47Mb| 31.9Gb
Questions:
As captioned, which statement should be the main reference for full table space? To check the physical files size or the usage?
Why it is showing 0% usage in screenshot for data01d.dbf but the actual physical file size is about 31.94G ?
What is causing the dbf file size expanded to size 31.96Gb ?
Thank you very much for any comments/advice, appreciated that.

Answer 1

A tablespace is simply an amount of space pre-allocated on the disk. It's the database that manages the space usage in the tablespace, so I would tend to go with what the database reports - ie. your 2nd query.
So, to answer your questions:
I'd go with your second query.
Probably because most of the space in that particular file hadn't been used yet, but I would expect that most of the space in the other 3 files has been used.
The apparent difference in size is probably due to rounding issues.

Answer 2

Expanded datafile sizes and actual database usage are two different things. A datafile could be expanded 100% (in your case to 32GB ) but not actually have any meaningful data in that file (but it still takes up all that space on your drive). For example the datafile expanded but later the data was deleted. The datafile itself would not shrink. It stays expanded.
You likely have set your database files to expand up to 32GB each before they move on to fill up the next available file. Check your settings in Oracle. There is a balancing act when it comes to datafile sizes. It might be better to chop up your datafiles to be in smaller chunks for performance, corruption protection, faster backups and restores. Check out:
https://docs.oracle.com/cd/B28359_01/server.111/b28310/dfiles003.htm#ADMIN11423