PAGE(FILES)

NAME
       page  structure - Postgres database file default page for-
       mat

DESCRIPTION
       This section provides an overview of the page format  used
       by Postgres classes.  User-defined access methods need not
       use this page format.

       In the following explanation, a byte is assumed to contain
       8  bits.   In addition, the term item refers to data which
       is stored in Postgres classes.

       The first 8 bytes of each page consists of a  page  header
       (PageHeaderData).   Within  the  header,  the  first three
       2-byte integer fields, lower, upper, and  special,  repre-
       sent  byte  offsets  to the start of unallocated space, to
       the end of unallocated space, and to the start of "special
       space."   Special space is a region at the end of the page
       which is allocated at page initialization time  and  which
       contains  information  specific  to an access method.  The
       last 2 bytes of the page header, opaque, encode  the  page
       size  and information on the internal fragmentation of the
       page.  Page size is stored in each page because frames  in
       the  buffer  pool may be subdivided into equal sized pages
       on a frame by frame basis within a  class.   The  internal
       fragmentation  information  is  used to aid in determining
       when page reorganization should occur.

       Following the page header are  item  identifiers  (ItemId-
       Data).   New item identifiers are allocated from the first
       four bytes of unallocated space.  Because an item  identi-
       fier  is  never  moved until it is freed, its index may be
       used to indicate the location of an item on  a  page.   In
       fact,  every  pointer  to an item (ItemPointer) created by
       Postgres consists of a frame number and  an  index  of  an
       item  identifier.  An item identifier contains a byte-off-
       set to the start of an item, its length in  bytes,  and  a
       set of attribute bits which affect its interpretation.

       The items, themselves, are stored in space allocated back-
       wards from the end of  unallocated  space.   Usually,  the
       items  are  not interpreted.  However when the item is too
       long to be placed on a single page or  when  fragmentation
       of the item is desired, the item is divided and each piece
       is handled as distinct items in the following manner.  The
       first through the next to last piece are placed in an item
       continuation   structure   (ItemContinuationData).    This
       structure  contains  itemPointerData  which  points to the
       next piece and the piece itself.  The last piece  is  han-
       dled normally.

FILES

       .../data/...
            Location of shared (global) database files.

       .../data/base/...
            Location of local database files.

BUGS
       The  page  format may change in the future to provide more
       efficient access to large objects.

       This section contains insufficient detail  to  be  of  any
       assistance in writing a new access method.