PostgreSQLWAL解析：构建WAL记录准备
以heap_insert为例，简述WAL的插入过程。
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在构建WAL日志记录的过程中主要涉及2个数据变量：static XLogRecData *rdatas数组和static registered_buffer *registered_buffers数组。这两个数组分别用来保存WAL数据和管理rdatas链表。
主要涉及3个重要的函数：XLogRegisterData、XLogRegisterBuffer和XLogRegisterBufData。这三个函数的作用分别是将WAL记录的特殊结构体数据注册到WAL，比如heap_insert中的xl_heap_insert结构体；将涉及到的buf注册到wal记录，比如heap_insert中page页赋予regbuf->page；将元组内容注册到WAL记录，比如insert语句的元组数据等。
下面首先介绍相关数据结构。
1、数据结构
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，得到如下结果，mainrdata_last保存rdata[0]，存储的是xl_heap_insert结构：
第二步，得到如下结果，取registered_buffer[0]，其rdata_head->next指向rdata[1]，存储tuple记录的头信息：
接着进入第三步，取rdata[2]，将其放到rdata[1]->next中，即加入registered_buffers[0]的rdata_head链表中，存储TUPLE值：
以上是构建WAL记录的准备阶段，下一节介绍WAL的构建及其通用结构。
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