PostgreSQL启动恢复期间，恢复到的时间线的确定

1、启动恢复时，确定恢复到的时间线recoveryTargetTLI

 1）归档恢复点比checkpoint中记录的时间线大，那么选择归档恢复点作为目标时间线

 2）否则，checkpoint记录中的时间线作为目标时间线

StartupXLOG->  if (ControlFile->minRecoveryPointTLI >   ControlFile->checkPointCopy.ThisTimeLineID)   recoveryTargetTLI = ControlFile->minRecoveryPointTLI;  else   recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;  ...

2、接着从recovery.conf文件中读取

 1）若设置了recovery_target_timeline值，并且设为latest，那么history列表最大的时间线即为目标时间线

 2）否则是recovery.conf文件中设置的时间线值

 3）若没有设置recovery_target_timeline值，则目标时间线为第一步中的值

StartupXLOG->readRecoveryCommandFile()->  for (item = head; item; item = item->next){   if (strcmp(item->name, "restore_command") == 0){    ...   }else if ...   else if(strcmp(item->name, "recovery_target_timeline") == 0){    rtliGiven = true;    if (strcmp(item->value, "latest") == 0)     rtli = 0;    else     rtli = (TimeLineID) strtoul(item->value, NULL, 0);   }else if...  }  if (rtliGiven){   if (rtli){    recoveryTargetTLI = rtli;    recoveryTargetIsLatest = false;   }else{    /* We start the "latest" search from pg_control's timeline */    recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);    recoveryTargetIsLatest = true;   }  }