对mapping 取值，放在memory 还是storage 中省gas

// SPDX-License-Identifier: MIT

pragma solidity 0.8.20;

contract StorageUnOptimized {

struct User {

uint256 id;

string name;

uint256 lastSee;

}

constructor() {

users[0] = User(0, "tom Doe", block.timestamp);

}

mapping(uint256 => User) public users;

function reSecondsAgo(uint256 _id) public view returns (uint256) {

//User memory _user = users[_id]; //gas 7928

User storage _user = users[_id]; //gas 2984

uint256 lastSeen = block.timestamp - _user.lastSee;

return lastSeen;

}

}

大家考虑下，如果对mapping 做循环处理呢？