Explain format=json difference in cost

| {

  "query_block": {

    "select_id": 1,

    "cost": 0.00524312,

    "nested_loop": [

      {

        "table": {

          "table_name": "t1",

          "access_type": "range",

          "possible_keys": ["a"],

          "key": "a",

          "key_length": "6",

          "used_key_parts": ["a"],

          "loops": 1,

          "rows": 2,

          "cost": 0.00524312,

          "filtered": 100,

          "index_condition": "t1.a between '2010-01-01 00:00:00' and '2010-12-31 23:59:59'",

          "attached_condition": "t1.c < (subquery#2)"

        }

      }

    ],

    "subqueries": [

      {

        "query_block": {

          "select_id": 2,

          "cost": 0.00273041,

          "nested_loop": [

            {

              "table": {

                "table_name": "t1",

                "access_type": "range",

                "possible_keys": ["a"],

                "key": "a",

                "key_length": "6",

                "used_key_parts": ["a"],

                "loops": 1,

                "rows": 2,

                "cost": 0.00273041,

                "filtered": 100,

                "attached_condition": "t1.a between '2010-01-01 00:00:00' and '2010-12-31 23:59:59'",

                "using_index": true

              }

            }

          ]

        }

      }

    ]

  }

} |

| {

  "query_block": {

    "select_id": 1,

    "cost": 0.00273041,

    "nested_loop": [

      {

        "table": {

          "table_name": "t1",

          "access_type": "range",

          "possible_keys": ["a"],

          "key": "a",

          "key_length": "6",

          "used_key_parts": ["a"],

          "loops": 1,

          "rows": 2,

          "cost": 0.00273041,

          "filtered": 100,

          "index_condition": "t1.a between '2010-01-01 00:00:00' and '2010-12-31 23:59:59'",

          "attached_condition": "t1.c < (subquery#2)"

        }

      }

    ],

    "subqueries": [

      {

        "query_block": {

          "select_id": 2,

          "cost": 0.00273041,

          "nested_loop": [

            {

              "table": {

                "table_name": "t1",

                "access_type": "range",

                "possible_keys": ["a"],

                "key": "a",

                "key_length": "6",

                "used_key_parts": ["a"],

                "loops": 1,

                "rows": 2,

                "cost": 0.00273041,

                "filtered": 100,

                "attached_condition": "t1.a between '2010-01-01 00:00:00' and '2010-12-31 23:59:59'",

                "using_index": true

              }

            }

          ]

        }

      }

    ]

  }

} |

set optimizer_record_context=ON;

create table t1 (pk int primary key, a datetime, c int, key(a));

insert into t1 (pk,a,c) values (1,'2009-11-29 13:43:32', 2);

insert into t1 (pk,a,c) values (2,'2009-11-29 03:23:32', 2);

insert into t1 (pk,a,c) values (3,'2009-10-16 05:56:32', 2);

insert into t1 (pk,a,c) values (4,'2010-11-29 13:43:32', 2);

insert into t1 (pk,a,c) values (5,'2010-10-16 05:56:32', 2);

insert into t1 (pk,a,c) values (6,'2011-11-29 13:43:32', 2);

insert into t1 (pk,a,c) values (7,'2012-10-16 05:56:32', 2);

explain format=json update t1 set c = 0

  where year(a) = 2010 and c < (select count(*) from t1 where year(a) = 2010);

SELECT context INTO DUMPFILE 'context1.txt' FROM INFORMATION_SCHEMA.OPTIMIZER_CONTEXT;

source context1.txt