看了一下 optimizer ，其他几个都懂，引擎依据 cost 选择了 temp 表

唯独这个

···
"considered_execution_plans": [
{
"plan_prefix": [
] /* plan_prefix */,
"table": "`t_deal`",
"best_access_path": {
"considered_access_paths": [
{
"access_type": "ref",
"index": "t_deal_entrust_IDX",
"usable": false,
"chosen": false
},
{
"rows_to_scan": 166,
"access_type": "scan",
"resulting_rows": 166,
"cost": 17.6,
"chosen": true
}
] /* considered_access_paths */
} /* best_access_path */,
"condition_filtering_pct": 100,
"rows_for_plan": 166,
"cost_for_plan": 17.6,
"sort_cost": 166,
"new_cost_for_plan": 183.6,
"chosen": true
}
···

为啥 useable 是 false ？ cost_for_plan 和 new_cost_for_plan 分别表示啥？