How to Decide upto what level you can decrement your datafile size. ( Shrink Datafile)

We can directly resize datafiles

TEST.SQL>SELECT FILE_NAME, BYTES FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_NAME BYTES

—————- ———-

/…/dbsGNX.dbf 419430400

TEST.SQL>ALTER DATABASE DATAFILE ‘/…/dbsGNX.dbf’ RESIZE 390M;

Database altered.

TEST.SQL>SELECT FILE_NAME, BYTES FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_NAME BYTES

————— ———-

/…/dbsGNX.dbf 408944640

But the minimum file size is the size of the extend the furthest in the datafile:

TEST.SQL>SELECT FILE_ID,FILE_NAME FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_ID FILE_NAME

———- —————–

1 /…/dbsGNX.dbf

TEST.SQL>SELECT MAX(BLOCK_ID) MBID FROM DBA_EXTENTS WHERE FILE_ID=1;

MBID

———-

25129

TEST.SQL>SELECT SEGMENT_NAME,OWNER,SEGMENT_TYPE FROM DBA_EXTENTS WHERE FILE_ID=1 AND BLOCK_ID=25129;

SEGMENT_NAME OWNER SEGMENT_TYPE

——————– —————————— ——————

I_OBJAUTH2 SYS INDEX

TEST.SQL>SHOW PARAMETER BLOCK_SIZE

NAME TYPE VALUE

———————————— ——————————– ——————————

db_block_size integer 8192

TEST.SQL>SELECT 8192*25129 FROM DUAL;

8192*25129

———-

205856768

about 200M.

We can directly resize datafiles

TEST.SQL>SELECT FILE_NAME, BYTES FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_NAME BYTES

—————– ———-

/…/dbsGNX.dbf 419430400

TEST.SQL>ALTER DATABASE DATAFILE ‘/…/dbsGNX.dbf’ RESIZE 390M;

Database altered.

TEST.SQL>SELECT FILE_NAME, BYTES FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_NAME BYTES

—————- ———-

/…/dbsGNX.dbf 408944640

But the minimum file size is the size of the extend the furthest in the datafile:

TEST.SQL>SELECT FILE_ID,FILE_NAME FROM DBA_DATA_FILES WHERE TABLESPACE_NAME=’SYSTEM’;

FILE_ID FILE_NAME

———- ————————-

1 /…/dbsGNX.dbf

TEST.SQL>SELECT MAX(BLOCK_ID) MBID FROM DBA_EXTENTS WHERE FILE_ID=1;

MBID

———-

25129

TEST.SQL>SELECT SEGMENT_NAME,OWNER,SEGMENT_TYPE FROM DBA_EXTENTS WHERE FILE_ID=1 AND BLOCK_ID=25129;

SEGMENT_NAME OWNER SEGMENT_TYPE

——————— —————————— ——————

I_OBJAUTH2 SYS INDEX

TEST.SQL>SHOW PARAMETER BLOCK_SIZE

NAME TYPE VALUE

———————————— ——————————– ——————————

db_block_size integer 8192

TEST.SQL>SELECT 8192*25129 FROM DUAL;

8192*25129

———-

205856768

About 200M.

Reference : http://forums.oracle.com/forums/message.jspa?messageID=1058350