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日志記錄的過程中主要涉及2個數據變量：static XLogRecData *rdatas數組和static registered_buffer *registered_buffers數組。這兩個數組分別用來保存WAL數據和管理rdatas鏈表。

主要涉及3個重要的函數：XLogRegisterData、XLogRegisterBuffer和XLogRegisterBufData。這三個函數的作用分別是將WAL記錄的特殊結構體數據注冊到WAL，比如heap_insert中的xl_heap_insert結構體；將涉及到的buf注冊到wal記錄，比如heap_insert中page頁賦予regbuf->page；將元組內容注冊到WAL記錄，比如insert語句的元組數據等。

下面首先介紹相關數據結構。

1、數據結構

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保存rdata[0]，存儲的是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]，存儲tuple記錄的頭信息：

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鏈表中，存儲TUPLE值：

以上是構建WAL記錄的準備階段，下一節介紹WAL的構建及其通用結構。