PortalRunUtility函数执行utility语句，如果是TransactionStmt、LockStmt、VariableSetStmt、VariableShowStmt、ConstraintsSetStmt、FetchStmt、ListenStmt、NotifyStmt、UnlistenStmt、CheckPointStmt则不需要快照。

static void PortalRunUtility(Portal portal, PlannedStmt *pstmt, bool isTopLevel, bool setHoldSnapshot, DestReceiver *dest, char *completionTag) {
  Node     *utilityStmt = pstmt->utilityStmt;
  Snapshot  snapshot;

  /*
   * Set snapshot if utility stmt needs one.  Most reliable way to do this
   * seems to be to enumerate those that do not need one; this is a short
   * list.  Transaction control, LOCK, and SET must *not* set a snapshot
   * since they need to be executable at the start of a transaction-snapshot
   * mode transaction without freezing a snapshot.  By extension we allow
   * SHOW not to set a snapshot.  The other stmts listed are just efficiency
   * hacks.  Beware of listing anything that can modify the database --- if,
   * say, it has to update an index with expressions that invoke
   * user-defined functions, then it had better have a snapshot.
   */
  if (!(IsA(utilityStmt, TransactionStmt) ||
      IsA(utilityStmt, LockStmt) ||
      IsA(utilityStmt, VariableSetStmt) ||
      IsA(utilityStmt, VariableShowStmt) ||
      IsA(utilityStmt, ConstraintsSetStmt) ||
  /* efficiency hacks from here down */
      IsA(utilityStmt, FetchStmt) ||
      IsA(utilityStmt, ListenStmt) ||
      IsA(utilityStmt, NotifyStmt) ||
      IsA(utilityStmt, UnlistenStmt) ||
      IsA(utilityStmt, CheckPointStmt))) {
    snapshot = GetTransactionSnapshot();
    /* If told to, register the snapshot we're using and save in portal */
    if (setHoldSnapshot) { // 注册快照，并将快照放到portal中
      snapshot = RegisterSnapshot(snapshot);
      portal->holdSnapshot = snapshot;
    }
    PushActiveSnapshot(snapshot);
    /* PushActiveSnapshot might have copied the snapshot */
    snapshot = GetActiveSnapshot();
  } else
    snapshot = NULL;

  ProcessUtility(pstmt, portal->sourceText, isTopLevel ? PROCESS_UTILITY_TOPLEVEL : PROCESS_UTILITY_QUERY,portal->portalParams, portal->queryEnv, dest, completionTag);

  /* Some utility statements may change context on us */  // 切换到portal的内存上下文
  MemoryContextSwitchTo(portal->portalContext);

  /* Some utility commands may pop the ActiveSnapshot stack from under us, so be careful to only pop the stack if our snapshot is still at the top. */
  if (snapshot != NULL && ActiveSnapshotSet() && snapshot == GetActiveSnapshot())
    PopActiveSnapshot();  // 如果有快照就需要Pop快照
}

PostgreSQL数据库查询执行——PortalRunUtility_数据库