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;
  }
 }