PostgreSQL WAL解析：構建WAL記錄準備

以heap_insert為例，簡述WAL的插入過程。
在構建WAL日誌記錄的過程中主要涉及2個資料變數：static XLogRecData rdatas陣列和static registered_buffer registered_buffers陣列。這兩個陣列分別用來儲存WAL資料和管理rdatas連結串列。
主要涉及3個重要的函式：XLogRegisterData、XLogRegisterBuffer和XLogRegisterBufData。這三個函式的作用分別是將WAL記錄的特殊結構體資料註冊到WAL，比如heap_insert中的xl_heap_insert結構體；將涉及到的buf註冊到wal記錄，比如heap_insert中page頁賦予regbuf->page；將元組內容註冊到WAL記錄，比如insert語句的元組資料等。
下面首先介紹相關資料結構。
1、資料結構
HeapTupleData
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;
xl_heap_header
`/*

• 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; xl_heap_insert
  / 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; XLogRecData/
• 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; registered_buffer
  /
• 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;`
  2、heap_insert涉及WAL的流程
  

第一步中，得到如下結果，mainrdata_last儲存rdata[0]，儲存的是xl_heap_insert結構：

第二步，得到如下結果，取registered_buffer[0]，其rdata_head->next指向rdata[1]，儲存tuple記錄的頭資訊：

接著進入第三步，取rdata[2]，將其放到rdata[1]->next中，即加入registered_buffers[0]的rdata_head連結串列中，儲存TUPLE值：