PostgreSQL WAL解析：构建WAL记录准备

typedef struct HeapTupleData
{
    uint32 t_len; /* length of *t_data */
    ItemPointerData t_self; /* SelfItemPointer */
    Oid t_tableOid; /* table the tuple came from */
    HeapTupleHeader t_data; /* -> tuple header and data */
} HeapTupleData;

/*
 * We don't store the whole fixed part (HeapTupleHeaderData) of an inserted
 * or updated tuple in WAL; we can save a few bytes by reconstructing the
 * fields that are available elsewhere in the WAL record, or perhaps just
 * plain needn't be reconstructed.  These are the fields we must store.
 * NOTE: t_hoff could be recomputed, but we may as well store it because
 * it will come for free due to alignment considerations.
 */
typedef struct xl_heap_header
{
    uint16 t_infomask2;
    uint16 t_infomask;
    uint8 t_hoff;
} xl_heap_header;

/* This is what we need to know about insert */
typedef struct xl_heap_insert
{
    OffsetNumber offnum; /* inserted tuple's offset */
    uint8 flags;
 
/* xl_heap_header & TUPLE DATA in backup block 0 */
} xl_heap_insert;

/*
 * The functions in xloginsert.c construct a chain of XLogRecData structs
 * to represent the final WAL record.
 */
typedef struct XLogRecData
{
    struct XLogRecData *next; /* next struct in chain, or NULL */
    char    *data; /* start of rmgr data to include */
    uint32 len; /* length of rmgr data to include */
} XLogRecData;

/*
 * For each block reference registered with XLogRegisterBuffer, we fill in
 * a registered_buffer struct.
 */
typedef struct
{
bool in_use; /* is this slot in use? */
uint8 flags; /* REGBUF_* flags */
RelFileNode rnode; /* identifies the relation and block */
ForkNumber forkno;
BlockNumber block;
Page page; /* page content */
uint32 rdata_len; /* total length of data in rdata chain */
XLogRecData *rdata_head; /* head of the chain of data registered with this block */
XLogRecData *rdata_tail; /* last entry in the chain, or &rdata_head if empty */
XLogRecData bkp_rdatas[2]; /* temporary rdatas used to hold references to
 * backup block data in XLogRecordAssemble() */
/* buffer to store a compressed version of backup block image */
char compressed_page[PGLZ_MAX_BLCKSZ];
} registered_buffer;