Postgresql source code learning (33) - transaction log ⑨ - see the overall process of log writing from the insert record

一、 整体流程

       Earlier we looked at the specific functions of each step separately,Here is another simple passinsertstatement tracking,Let's take a look at the overall process.目前只看WAL相关的部分,因为insertThere are a lot of things involved,Some have not yet been learned.

最简单的insertThe statement corresponding to the function is heap_insert,其中跟WAL相关的代码：

XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHeapInsert);

        xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
        xlhdr.t_infomask = heaptup->t_data->t_infomask;
        xlhdr.t_hoff = heaptup->t_data->t_hoff;

        /*
         * note we mark xlhdr as belonging to buffer; if XLogInsert decides to
         * write the whole page to the xlog, we don't need to store
         * xl_heap_header in the xlog.
         */
        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD | bufflags);
        XLogRegisterBufData(0, (char *) &xlhdr, SizeOfHeapHeader);
        /* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
        XLogRegisterBufData(0,
                            (char *) heaptup->t_data + SizeofHeapTupleHeader,
                            heaptup->t_len - SizeofHeapTupleHeader);

        /* filtering by origin on a row level is much more efficient */
        XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);

        recptr = XLogInsert(RM_HEAP_ID, info);

        PageSetLSN(page, recptr);

二、 代码调试

1. 调试方法

会话1

postgres=# create table t_insert(a int);
CREATE TABLE

postgres=# select pg_backend_pid();

 pg_backend_pid
----------------
           8632

会话2

vscode跟踪8632进程

会话1

postgres=# insert into t_insert values(1);

Add a breakpoint to WAL相关部分,点击继续

2. 调试过程

• 首先是XLogBeginInsert,做代码安全检查工作,另外是设置begininsert_called标志,防止递归调用日志生成函数.

• XLogRegisterData,将WAL日志数据注册到rdatas数组.

       参数的xlrec为xl_heap_insert结构体（日志记录中的main data部分）,So this step is building and registeringmain data数据到WAL记录中.

• 构建xl_heap_header（block data）

• XLogRegisterBuffer：Registration data page,将被修改的buffer页面信息注册到registerd_buffers数组.由于XLOG是page-oriented log,So you need to register the data page before registering the tuple.

• XLogRegisterBufData：注册xl_heap_header（block data）,这里的xlhdrThe parameters are constructed earlierxl_heap_header

• Register the actual tuple data

heaptup->t_data + SizeofHeapTupleHeaderis the actual tuple

• 组装WALRecord and writewal buffer

        XLogInsertThe log assembly function is called in the functionXLogRecordAssemble 以及 XLogInsertRecord完成上述工作,The last one returned is what was writtenEndPos.

• 设置PageLSN

       Set the page headerPageLSN为recptr（即EndPos）.Every page will have itPageLSN,页面中所有LSN<=Page LSN的WALIndicates that the corresponding operations have been placed.So on crash recovery,None of these operations need to be performedredo.

参考

https://blog.csdn.net/obvious__/article/details/119242908?spm=1001.2014.3001.5502

postgresql源码学习（23）—— 事务日志④-日志组装_Hehuyi_In的博客-CSDN博客_postgresql事务日志

https://blog.csdn.net/Hehuyi_In/article/details/125447500  postgresql源码学习（22）—— 事务日志③-Log registration_Hehuyi_In的博客-CSDN博客