Почему так по-разному работает COUNT(*) в MySQL?

MariaDB [AB_FORUMS]> explain select count(*) from posts;
±-----±------------±------±------±--------------±-----------±--------±-----±--------±------------+
| id   | select_type | table | type  | possible_keys | key        | key_len | ref  | rows    | Extra       |
±-----±------------±------±------±--------------±-----------±--------±-----±--------±------------+
|    1 | SIMPLE      | posts | index | NULL          | is_deleted | 1       | NULL | 3316686 | Using index |
±-----±------------±------±------±--------------±-----------±--------±-----±--------±------------+
1 row in set (0.03 sec)

MariaDB [AB_FORUMS]> explain select count(*) from posts_cache;
±-----±------------±------------±------±--------------±--------±--------±-----±--------±------------+
| id   | select_type | table       | type  | possible_keys | key     | key_len | ref  | rows    | Extra       |
±-----±------------±------------±------±--------------±--------±--------±-----±--------±------------+
|    1 | SIMPLE      | posts_cache | index | NULL          | PRIMARY | 4       | NULL | 2377671 | Using index |
±-----±------------±------------±------±--------------±--------±--------±-----±--------±------------+
1 row in set (0.00 sec)

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show create table posts\G
*************************** 1. row ***************************
Table: posts
Create Table: CREATE TABLE posts (
id int(10) unsigned NOT NULL AUTO_INCREMENT,
poster varchar(32) NOT NULL DEFAULT ‘’,
poster_id int(10) unsigned DEFAULT ‘1’,
avatar_id int(11) DEFAULT NULL,
poster_ip varchar(255) CHARACTER SET latin1 COLLATE latin1_general_ci DEFAULT NULL,
poster_ua varchar(255) NOT NULL,
poster_email varchar(50) DEFAULT NULL,
source longtext NOT NULL,
source_html longtext NOT NULL,
hide_smilies tinyint(1) NOT NULL DEFAULT ‘0’,
posted int(10) unsigned NOT NULL DEFAULT ‘0’,
edited int(10) unsigned DEFAULT NULL,
edited_by varchar(40) DEFAULT NULL,
topic_id int(10) unsigned NOT NULL DEFAULT ‘0’,
page smallint(5) unsigned DEFAULT NULL,
add_type varchar(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL DEFAULT ‘’,
original_id int(11) DEFAULT NULL,
answer_to_post_id int(11) NOT NULL DEFAULT ‘0’,
answer_to_user_id int(11) NOT NULL,
have_answers int(11) DEFAULT NULL,
answers_count int(11) NOT NULL COMMENT ‘Сколько всего ответов на данное сообщение в дереве ответов. Для первых сообщений топиков считаются все ответы темы.’,
have_attach int(11) DEFAULT NULL COMMENT ‘NULL/0, если аттача нет, -1, если аттачи в таблице, №, если аттач один.’,
have_cross tinyint(1) unsigned DEFAULT NULL COMMENT ‘0/1’,
order int(11) NOT NULL,
is_deleted tinyint(1) unsigned NOT NULL,
is_hidden enum(‘0’,‘1’) CHARACTER SET latin1 COLLATE latin1_general_ci DEFAULT NULL,
is_moderatorial tinyint(1) unsigned NOT NULL,
is_incorrect int(11) DEFAULT NULL,
is_moderated int(11) DEFAULT NULL,
is_spam int(11) DEFAULT NULL,
last_moderator_id int(11) NOT NULL,
title varchar(255) DEFAULT NULL,
score int(11) DEFAULT NULL,
markup_class_name varchar(64) CHARACTER SET latin1 COLLATE latin1_general_ci DEFAULT NULL,
field4 int(11) DEFAULT NULL,
field_int int(11) DEFAULT NULL,
field_str varchar(255) DEFAULT NULL,
year smallint(5) unsigned NOT NULL,
original_topic_id int(10) unsigned DEFAULT NULL,
infonesy_uuid varchar(255) DEFAULT NULL,
infonesy_data text,
PRIMARY KEY (id),
KEY posts_topic_id_idx (topic_id),
KEY posts_multi_idx (poster_id,topic_id),
KEY poster (poster),
KEY poster_id (poster_id),
KEY posted (posted),
KEY original_id (original_id),
KEY poster_ip (poster_ip),
KEY answer_to (answer_to_post_id),
KEY index_for_joins (topic_id,id),
KEY for_posted_sort_wo_filesort (topic_id,posted),
KEY order (order),
KEY page (page),
KEY topic_id (topic_id,page),
KEY topic_id_2 (topic_id,page,id),
KEY poster_id_2 (poster_id,posted),
KEY have_answers (have_answers),
KEY is_deleted (is_deleted),
KEY anwer_to_user_id (answer_to_user_id),
KEY answer_uid__posted (answer_to_user_id,posted),
KEY joined_to_topic_id (last_moderator_id),
KEY is_spam (is_spam),
KEY answers_count (answers_count),
KEY is_moderatorial (is_moderatorial),
KEY edited_idx (edited),
KEY by_tid_ordered (topic_id,is_deleted,order,posted),
KEY poster_id__is_deleted__posted (poster_id,is_deleted,posted),
KEY original_topic_id (original_topic_id),
KEY year (year),
CONSTRAINT posts_ibfk_1 FOREIGN KEY (poster_id) REFERENCES users (id) ON DELETE SET NULL ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4207770 DEFAULT CHARSET=utf8mb4

show create table posts_cache\G
*************************** 1. row ***************************
Table: posts_cache
Create Table: CREATE TABLE posts_cache (
id int(10) unsigned NOT NULL COMMENT ‘post_id’,
body text COMMENT ‘post body, html’,
body_ts timestamp NULL DEFAULT NULL COMMENT ‘body timestamp’,
is_body_temporary tinyint(1) unsigned DEFAULT NULL,
PRIMARY KEY (id),
CONSTRAINT posts_cache_ibfk_1 FOREIGN KEY (id) REFERENCES posts (id) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)

select count([em]) from posts;
select count([/em]) from posts_cache;

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CONSTRAINT posts_cache_ibfk_1 FOREIGN KEY (id) REFERENCES posts (id) ON DELETE CASCADE ON UPDATE CASCADE

count([em])

select count(X) from posts_calculated_fields;
.----------.
| count(X) |
.----------.
|  3808257 |
.----------.
1 row in set (2.47 sec)

show create table posts_calculated_fields\G
Table: posts_calculated_fields
Create Table: CREATE TABLE posts_calculated_fields (
post_id int(10) unsigned NOT NULL,
answers_total int(10) unsigned DEFAULT NULL,
answers_other_topics int(10) unsigned DEFAULT NULL,
best10_ts timestamp NULL DEFAULT NULL,
root_post_id int(10) unsigned DEFAULT NULL,
PRIMARY KEY (post_id),
KEY answers_total (answers_total),
KEY answers_other_topics (answers_other_topics),
KEY best10_ts (best10_ts),
KEY root_post_id (root_post_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

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innodb_buffer_pool_size
innodb_additional_mem_pool_size
innodb_log_buffer_size
innodb_file_per_table

| innodb_buffer_pool_size                   | 8589934592             |
| innodb_additional_mem_pool_size           | 33554432               |
| innodb_log_buffer_size                    | 8388608                |
| innodb_file_per_table                     | ON                     |

select count(*) from posts_calculated_fields;
+----------+
| count(*) |
+----------+
|  3808511 |
+----------+
1 row in set (25.38 sec)

MariaDB [AB_FORUMS]> explain select count(*) from posts_calculated_fields;
+------+-------------+-------------------------+-------+---------------+---------------+---------+------+---------+-------------+
| id   | select_type | table                   | type  | possible_keys | key           | key_len | ref  | rows    | Extra       |
+------+-------------+-------------------------+-------+---------------+---------------+---------+------+---------+-------------+
|    1 | SIMPLE      | posts_calculated_fields | index | NULL          | answers_total | 5       | NULL | 3664599 | Using index |
+------+-------------+-------------------------+-------+---------------+---------------+---------+------+---------+-------------+