我有以下两个MySQL表：
CREATE TABLE `english_british` (
`word_id` int(11) NOT NULL,
`word` text,
`word_transcription` text,
`line_complete` text,
`pos` text,
`hunspell_rules` text,
`frequency_coca` int(8) NOT NULL DEFAULT '0',
`to_check` tinyint(4) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
ALTER TABLE `english_british`
ADD PRIMARY KEY (`word_id`);
ALTER TABLE `english_british`
MODIFY `word_id` int(11) NOT NULL AUTO_INCREMENT;
此表有112,711条记录，根据phpMyAdmin的重量为11.7 MiB。
第二张桌几乎相同：
CREATE TABLE `english_american` (
`word_id` int(11) NOT NULL,
`word` text,
`word_transcription_source` text,
`word_transcription` text,
`o_to_check` tinyint(4) NOT NULL DEFAULT '0',
`pos` text,
`frequency_coca` int(8) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
ALTER TABLE `english_american`
ADD PRIMARY KEY (`word_id`);
ALTER TABLE `english_american`
MODIFY `word_id` int(11) NOT NULL AUTO_INCREMENT;
此表有32,338条记录，重量为2.5 MiB。
现在我运行两个类似的查询：
SELECT word FROM english_british WHERE `word`='I\'m'
SELECT word FROM english_american WHERE `word`='I\'m'
第一个查询（对于较大的表）只需0.0446秒即可执行。第二个查询（对于较小的表）需要0.1197秒。确切的时间可能会有所改变，但仍然会慢2.5-3倍。
我知道你要告诉我应该为列“word”创建索引，首先我需要将其类型更改为varchar。我会这样做，但首先我只是想了解为什么在较小的表上运行相同的查询需要这么长时间？
编辑：两个查询只返回一行。
编辑2：两个查询的EXPLAIN EXTENDED完全相同（行数除外）。
编辑3：SHOW PROFILES的结果：
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
| starting | 0.000081 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | NULL | NULL | NULL |
| checking permissions | 0.000008 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 6043 |
| Opening tables | 0.000016 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 4519 |
| After opening tables | 0.000006 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 4757 |
| System lock | 0.000006 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 308 |
| Table lock | 0.000007 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 313 |
| init | 0.000019 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3427 |
| optimizing | 0.000015 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1092 |
| statistics | 0.000018 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1373 |
| preparing | 0.000017 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1398 |
| executing | 0.000005 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 2551 |
| Sending data | 0.121468 | 0.046875 | 0.078125 | NULL | NULL | 62672 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3223 |
| end | 0.000017 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3462 |
| query end | 0.000010 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 5679 |
| closing tables | 0.000004 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 930 |
| Unlocking tables | 0.000017 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 395 |
| freeing items | 0.000017 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 7331 |
| updating status | 0.000065 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 1936 |
| cleaning up | 0.000022 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 1955 |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
19 rows in set (0.00 sec)
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
| Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
| starting | 0.000059 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | NULL | NULL | NULL |
| checking permissions | 0.000007 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 6043 |
| Opening tables | 0.000020 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 4519 |
| After opening tables | 0.000006 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 4757 |
| System lock | 0.000007 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 308 |
| Table lock | 0.000008 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 313 |
| init | 0.000020 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3427 |
| optimizing | 0.000012 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1092 |
| statistics | 0.000013 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1373 |
| preparing | 0.000014 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 1398 |
| executing | 0.000004 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 2551 |
| Sending data | 0.049592 | 0.062500 | 0.000000 | NULL | NULL | 43 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3223 |
| end | 0.000017 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_select.cc | 3462 |
| query end | 0.000005 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 5679 |
| closing tables | 0.000004 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_base.cc | 930 |
| Unlocking tables | 0.000012 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | lock.cc | 395 |
| freeing items | 0.000006 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 7331 |
| updating status | 0.000065 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 1936 |
| cleaning up | 0.000025 | 0.000000 | 0.000000 | NULL | NULL | 0 | 0 | NULL | NULL | 0 | NULL | NULL | <unknown> | sql_parse.cc | 1955 |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------+---------------+-------------+
19 rows in set (0.00 sec)
编辑4： 以下查询
SELECT COUNT(DISTINCT word) from english_british
SELECT COUNT(DISTINCT word) from english_american
分别返回111996和32197.
编辑5： 这似乎与缓存的结果无关。即使query_cache_size设置为零，速度也是相同的。
答案 0 :(得分：0)
word列的群集索引并将其更改为varchar。这是因为存储text字段所需的空间不仅仅是存储varchar，因此需要更多磁盘 - ＆gt;记忆读。我不能告诉你一个关于为什么执行时间那么多的确切方法，但我可以建议你查看执行计划（EXPLAIN命令）来自己解决。
答案 1 :(得分：0)
尝试进行计数（*）并查看每个表返回的行数。也许返回的行数减慢了它。