v$type_size 对理解 block structure 很有用，简单记录一下

_sys@DAVID> select * from V$TYPE_SIZE ;


COMPONEN TYPE     DESCRIPTION                       TYPE_SIZE
-------- -------- -------------------------------- ----------
S        EWORD    EITHER WORD                               4
S        EB1      EITHER BYTE 1                             1
S        EB2      EITHER BYTE 2                             2
S        EB4      EITHER BYTE 4                             4
S        UWORD    UNSIGNED WORD                             4
S        UB1      UNSIGNED BYTE 1                           1
S        UB2      UNSIGNED BYTE 2                           2
S        UB4      UNSIGNED BYTE 4                           4
S        SWORD    SIGNED WORD                               4
S        SB1      SIGNED BYTE 1                             1
S        SB2      SIGNED BYTE 2                             2
S        SB4      SIGNED BYTE 4                             4
S        BOOLEAN  BOOLEAN                                   4
S        FLOAT    FLOAT                                     4
S        DOUBLE   DOUBLE                                    8
S        SIZE_T   SIZE_T                                    8
S        DSIZE_T  DSIZE_T                                   4
S        PTR_T    PTR_T                                     8
K        KDBA     DATABASE BLOCK ADDRESS                    4
K        KTNO     TABLE NUMBER IN CLUSTER                   1
K        KSCN     SYSTEM COMMIT NUMBER                      8
K        KXID     TRANSACTION ID                            8
K        KUBA     UNDO ADDRESS                              8
KCB      KCBH     BLOCK COMMON HEADER                      20
KTB      KTBIT    TRANSACTION VARIABLE HEADER              24
KTB      KTBBH    TRANSACTION FIXED HEADER                 48
KTB      KTBBH_BS TRANSACTION BLOCK BITMAP SEGMENT          8
KDB      KDBH     DATA HEADER                              14
KDB      KDBT     TABLE DIRECTORY ENTRY                     4
KTE      KTECT    EXTENT CONTROL                           44
KTE      KTECH    EXTENT CONTROL                           72
KTE      KTETB    EXTENT TABLE                              8
KTS      KTSHC    SEGMENT HEADER                            8
KTS      KTSFS    SEGMENT FREE SPACE LIST                  20
KTS      KTSPHW   PAGE TABLE SEGMENT HWM                   60
KTS      KTSPHC   PAGE TABLE SEGMENT HEADER               112
KTS      KTSPFHC  LEVEL 1 BITMAP BLOCK HEADER             184
KTS      KTSPSHC  LEVEL 2 BITMAP BLOCK HEADER              96
KTS      KTSPTHC  LEVEL 3 BITMAP BLOCK HEADER              88
KTU      KTUBH    UNDO HEADER                              16
KTU      KTUXE    UNDO TRANSACTION ENTRY                   40
KTU      KTUXC    UNDO TRANSACTION CONTROL                104
KDX      KDXCO    INDEX HEADER                             16
KDX      KDXLE    INDEX LEAF HEADER                        32
KDX      KDXBR    INDEX BRANCH HEADER                      24


45 rows selected.