typedef struct PageHeaderData
{
   XLogRecPtr    pd_lsn;      // xact log record for last change
   uint16        pd_tli;      // xact log reference information
   uint16        pd_flags;    // flag bits (e.g. free, full, ...
   LocationIndex pd_lower;    // offset to start of free space
   LocationIndex pd_upper;    // offset to end of free space
   LocationIndex pd_special;  // offset to start of special space
   uint16        pd_pagesize_version;
   TransactionId pd_prune_xid;// is pruning useful in data page?
   ItemIdData    pd_linp[1];  // beginning of line pointer array
} PageHeaderData;

typedef PageHeaderData *PageHeader;