How much space does a blob use?

Question

How much space does a BLOB use?
Does it use allocated anyspace if its empty?

Answer 1

It will size appropriately:
TEST.SQL>CREATE TABLE TBLOB
  2  (
  3  A BLOB
  4  ) TABLESPACE _SANITIZED_;
TABLE CREATED.
TEST.SQL>SELECT INITIAL_EXTENT FROM DBA_TABLESPACES WHERE TABLESPACE_NAME='_SANITIZED_';
INITIAL_EXTENT
       5242880
TEST.SQL>SELECT * FROM DBA_EXTENTS WHERE SEGMENT_NAME='TBLOB';
OWNER                          SEGMENT_NAME                                                                      PARTITION_NAME                 SEGMENT_TYPE
TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  0         16    1201289    5242880        640           16
TEST.SQL>SELECT * FROM DBA_OBJECTS WHERE CREATED > SYSDATE-1;
OWNER                          OBJECT_NAME
SUBOBJECT_NAME                  OBJECT_ID DATA_OBJECT_ID OBJECT_TYPE        CREATED  LAST_DDL TIMESTAMP           STATUS  T G S
YJAM                           SYS_LOB0000920875C00001$$
                                   920876         920876 LOB                14:41:36 14:41:36 2005-11-16:14:41:36 VALID   N Y N
YJAM                           TBLOB
                                   920875         920875 TABLE              14:41:36 14:41:36 2005-11-16:14:41:36 VALID   N N N
TEST.SQL>SELECT * FROM DBA_EXTENTS WHERE SEGMENT_NAME='SYS_LOB0000920875C00001$$';
OWNER                          SEGMENT_NAME                                                                      PARTITION_NAME                 SEGMENT_TYPE
TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
YJAM                           SYS_LOB0000920875C00001$$                                                                                        LOBSEGMENT
_SANITIZED_                                  0         16    1202569    5242880        640           16
TEST.SQL>DECLARE
  2    VALINS VARCHAR2(4000);
  3  BEGIN
  4    VALINS:='1';
  5    FOR I IN 1..100000
  6    LOOP
  7      INSERT INTO TBLOB VALUES (RPAD(VALINS,4000,'0'));
  8    END LOOP;
  9  END;
10  /
PL/SQL procedure successfully completed.
TEST.SQL>COMMIT;
Commit complete.
TEST.SQL>SELECT * FROM DBA_EXTENTS WHERE SEGMENT_NAME IN ('SYS_LOB0000920875C00001$$','TBLOB');
OWNER                          SEGMENT_NAME                                                                      PARTITION_NAME                 SEGMENT_TYPE
TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  0         16    1201289    5242880        640           16
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  1         17    1169929    5242880        640           17
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  2         18    1163529    5242880        640           18
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  3         19    1176969    5242880        640           19
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  4          5     379529    5242880        640            5
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                  5         14     375689    5242880        640           14
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                 51         18    1172489    5242880        640           18
YJAM                           TBLOB                                                                                                            TABLE
_SANITIZED_                                 52         19    1191689    5242880        640           19
YJAM                           SYS_LOB0000920875C00001$$                                                                                        LOBSEGMENT
_SANITIZED_                                  0         16    1202569    5242880        640           16
54 rows selected.
TEST.SQL>ANALYZE TABLE TBLOB COMPUTE STATISTICS;
Table analyzed.
TEST.SQL>SELECT * FROM DBA_TABLES WHERE TABLE_NAME='TBLOB';
OWNER                          TABLE_NAME                     TABLESPACE_NAME                CLUSTER_NAME                   IOT_NAME                         PCT_FREE   PCT_USED
INI_TRANS  MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS PCT_INCREASE  FREELISTS FREELIST_GROUPS LOG B   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE  CHAIN_CNT
AVG_ROW_LEN AVG_SPACE_FREELIST_BLOCKS NUM_FREELIST_BLOCKS DEGREE     INSTANCES  CACHE TABLE_LO SAMPLE_SIZE LAST_ANA PAR IOT_TYPE     T S NES BUFFER_ ROW_MOVE GLO USE
DURATION        SKIP_COR MON CLUSTER_OWNER                  DEPENDEN COMPRESS
YJAM                           TBLOB                          _SANITIZED_                                                                                               10
         1        255        5242880     5242880           1  2147483645            0                            YES N     100000      33547          373       1977          0
       2042                         0                   0          1          1     N ENABLED       100000 14:51:22 NO               N N NO  DEFAULT DISABLED NO  NO
                DISABLED NO                                 DISABLED DISABLED
TEST.SQL>SELECT BYTES/1024 FROM DBA_SEGMENTS WHERE SEGMENT_NAME='TBLOB';
BYTES/1024
    271360   
TEST.SQL>TRUNCATE TABLE TBLOB;
Table truncated.
TEST.SQL>BEGIN
  2    FOR i IN 1..100000
  3    LOOP
  4      INSERT INTO TBLOB VALUES ('1');
  5    END LOOP;
  6  END;
  7  /
PL/SQL procedure successfully completed.
TEST.SQL>COMMIT;
Commit complete.
TEST.SQL>ANALYZE TABLE TBLOB COMPUTE STATISTICS;
Table analyzed.
TEST.SQL>SELECT BYTES/1024 FROM DBA_SEGMENTS WHERE SEGMENT_NAME='TBLOB';
BYTES/1024
      5120Note: there is a deported blob segment for the pointers I forgot in my previous post.
HTH,
Yoann.
Message was edited by:
Yoann Mainguy
Hmm, forgot that for a full comparison of the avg linesize:
TEST.SQL>SELECT * FROM DBA_TABLES WHERE TABLE_NAME='TBLOB';
OWNER                          TABLE_NAME                     TABLESPACE_NAME                CLUSTER_NAME                   IOT_NAME                         PCT_FREE   PCT_USED
INI_TRANS  MAX_TRANS INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS MAX_EXTENTS PCT_INCREASE  FREELISTS FREELIST_GROUPS LOG B   NUM_ROWS     BLOCKS EMPTY_BLOCKS  AVG_SPACE  CHAIN_CNT
AVG_ROW_LEN AVG_SPACE_FREELIST_BLOCKS NUM_FREELIST_BLOCKS DEGREE     INSTANCES  CACHE TABLE_LO SAMPLE_SIZE LAST_ANA PAR IOT_TYPE     T S NES BUFFER_ ROW_MOVE GLO USE
DURATION        SKIP_COR MON CLUSTER_OWNER                  DEPENDEN COMPRESS
YJAM                           TBLOB                          _SANITIZED_                                                                   10
         1        255        5242880     5242880           1  2147483645            0                            YES N     100000        628           12       1223          0
         41                         0                   0          1          1     N ENABLED       100000 14:56:56 NO               N N NO  DEFAULT DISABLED NO  NO
                DISABLED NO                                 DISABLED DISABLED