问邮政和表格内部组织

EN

/*
 * A line pointer on a buffer page.  See buffer page definitions and comments
 * for an explanation of how line pointers are used.
 *
 * In some cases a line pointer is "in use" but does not have any associated
 * storage on the page.  By convention, lp_len == 0 in every line pointer
 * that does not have storage, independently of its lp_flags state.
 */
typedef struct ItemIdData
{
    unsigned    lp_off:15,      /* offset to tuple (from start of page) */
                lp_flags:2,     /* state of line pointer, see below */
                lp_len:15;      /* byte length of tuple */
} ItemIdData;

typedef ItemIdData *ItemId;

/*
 * A postgres disk page is an abstraction layered on top of a postgres
 * disk block (which is simply a unit of i/o, see block.h).
 *
 * specifically, while a disk block can be unformatted, a postgres
 * disk page is always a slotted page of the form:
 *
 * +----------------+---------------------------------+
 * | PageHeaderData | linp1 linp2 linp3 ...           |
 * +-----------+----+---------------------------------+
 * | ... linpN |                                      |
 * +-----------+--------------------------------------+
 * |           ^ pd_lower                             |
 * |                                                  |
 * |             v pd_upper                           |
 * +-------------+------------------------------------+
 * |             | tupleN ...                         |
 * +-------------+------------------+-----------------+
 * |       ... tuple3 tuple2 tuple1 | "special space" |
 * +--------------------------------+-----------------+
 *                                  ^ pd_special
 *
 * NOTES:
 *
 * linp1..N form an ItemId (line pointer) array.  ItemPointers point
 * to a physical block number and a logical offset (line pointer
 * number) within that block/page.  Note that OffsetNumbers
 * conventionally start at 1, not 0.
 *
 * tuple1..N are added "backwards" on the page.  Since an ItemPointer
 * offset is used to access an ItemId entry rather than an actual
 * byte-offset position, tuples can be physically shuffled on a page
 * whenever the need arises.  This indirection also keeps crash recovery
 * relatively simple, because the low-level details of page space
 * management can be controlled by standard buffer page code during
 * logging, and during recovery.