MySQL函数sysdate()与now()的区别测试用例对比

mysql> SELECT NOW(), SLEEP(2), NOW();
+---------------------+----------+---------------------+
| NOW()               | SLEEP(2) | NOW()               |
+---------------------+----------+---------------------+
| 2023-12-14 15:13:09 |        0 | 2023-12-14 15:13:09 |
+---------------------+----------+---------------------+
1 row in set (2.00 sec)
mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE();
+---------------------+----------+---------------------+
| SYSDATE()           | SLEEP(2) | SYSDATE()           |
+---------------------+----------+---------------------+
| 2023-12-14 15:13:19 |        0 | 2023-12-14 15:13:21 |
+---------------------+----------+---------------------+
1 row in set (2.00 sec)

mysql> create table t1(
    ->   id int primary key auto_increment,
    ->   create_time datetime default current_timestamp,
    ->   uname varchar(20),
    ->   key idx_create_time(create_time)
    -> );
Query OK, 0 rows affected (0.02 sec)
mysql> insert into t1(id) values(null),(null),(null);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> insert into t1(id) values(null),(null),(null);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
mysql> select * from t1;
+----+---------------------+-------+
| id | create_time         | uname |
+----+---------------------+-------+
|  1 | 2023-12-14 15:34:30 | NULL  |
|  2 | 2023-12-14 15:34:30 | NULL  |
|  3 | 2023-12-14 15:34:30 | NULL  |
|  4 | 2023-12-14 15:34:37 | NULL  |
|  5 | 2023-12-14 15:34:37 | NULL  |
|  6 | 2023-12-14 15:34:37 | NULL  |
+----+---------------------+-------+
6 rows in set (0.00 sec)

mysql> explain select * from t1 where create_time<sysdate()\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 33.33
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from t1 where create_time<now()\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
   partitions: NULL
         type: range
possible_keys: idx_create_time
          key: idx_create_time
      key_len: 6
          ref: NULL
         rows: 6
     filtered: 100.00
        Extra: Using index condition
1 row in set, 1 warning (0.00 sec)

"rows_estimation": [                  
## 估算使用各个索引进行范围扫描的成本
    {
      "table": "`t1`",
      "range_analysis": {
        "table_scan": {
          "rows": 6,
          "cost": 2.95
        },
        "potential_range_indexes": [
          {
            "index": "PRIMARY",
            "usable": false,
            "cause": "not_applicable"
          },
          {
            "index": "idx_create_time",
            "usable": true,
            "key_parts": [
              "create_time",
              "id"
    ............................................
        "setup_range_conditions": [
        ],
        "group_index_range": {
          "chosen": false,
          "cause": "not_group_by_or_distinct"
        },
        "skip_scan_range": {
          "chosen": false,
          "cause": "disjuntive_predicate_present"
        }
    ............................................
  "considered_execution_plans": [       
  ## 对比各可行计划的代价，选择相对最优的执行计划
    {
      "plan_prefix": [
      ],
      "table": "`t1`",
      "best_access_path": {
        "considered_access_paths": [
          {
            "rows_to_scan": 6,
            "access_type": "scan",
            "resulting_rows": 6,
            "cost": 0.85,
            "chosen": true
          }
        ]
      },
      "condition_filtering_pct": 100,
      "rows_for_plan": 6,
      "cost_for_plan": 0.85,
      "chosen": true
    ............................................

"rows_estimation": [                  
## 估算使用各个索引进行范围扫描的成本
  ............................................
      "analyzing_range_alternatives": {
        "range_scan_alternatives": [
          {
            "index": "idx_create_time",
            "ranges": [
              "NULL < create_time < '2023-12-14 15:48:39'"
            ],
            "index_dives_for_eq_ranges": true,
            "rowid_ordered": false,
            "using_mrr": false,
            "index_only": false,
            "in_memory": 1,
            "rows": 6,
            "cost": 2.36,
            "chosen": true
          }
        ],
  ............................................
      },
      "chosen_range_access_summary": {
        "range_access_plan": {
          "type": "range_scan",
          "index": "idx_create_time",
          "rows": 6,
          "ranges": [
            "NULL < create_time < '2023-12-14 15:48:39'"
          ]
        },
        "rows_for_plan": 6,
        "cost_for_plan": 2.36,
        "chosen": true
 .............................................
 "considered_execution_plans": [      
 ## 对比各可行计划的代价，选择相对最优的执行计划                            
  {
    "plan_prefix": [
    ],
    "table": "`t1`",
    "best_access_path": {
      "considered_access_paths": [
        {
          "rows_to_scan": 6,
          "access_type": "range",
          "range_details": {
            "used_index": "idx_create_time"
          },
          "resulting_rows": 6,
          "cost": 2.96,
          "chosen": true
        }
      ]
    },
    "condition_filtering_pct": 100,
    "rows_for_plan": 6,
    "cost_for_plan": 2.96,
    "chosen": true
 .............................................