文章出處：http://inter12.iteye.com/blog/1430144
MYSQL的全表掃描，主鍵索引(聚集索引、第一索引)，非主鍵索引(非聚集索引、第二索引)，覆蓋索引四種不同查詢的分析
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1.前置條件：
本次是基于小數據量，且數據塊在一個頁中的最理想情況進行分析，可能無具體的實際意義，但是可以借鑒到各種復雜條件下，因為原理是相同的,知小見大，見微知著！
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打開語句分析并確認是否已經打開
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Java代碼 ?
mysql>?set?profiling=1;?????Query?OK,?0?rows?affected?(0.00?sec)????mysql>?select?@@profiling;??+-------------+??|?@@profiling?|??+-------------+??|???????????1?|??+-------------+??1?row?in?set?(0.01?sec)?? ?
2.數據準備：
2.1全表掃描數據
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Java代碼 ?
create?table?person4all(id?int?not?null??auto_increment,?name?varchar(30)?not?null,?gender?varchar(10)?not?null?,primary?key(id));??insert?into?person4all(name,gender)?values("zhaoming","male");??insert?into?person4all(name,gender)?values("wenwen","female");?? ?
2.2根據主鍵查看數據
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Java代碼 ?
create?table?person4pri(id?int?not?null??auto_increment,?name?varchar(30)?not?null,?gender?varchar(10)?not?null?,primary?key(id));??insert?into?person4pri(name,gender)?values("zhaoming","male");??insert?into?person4pri(name,gender)?values("wenwen","female");?? ?
2.3根據非聚集索引查數據
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Java代碼 ?
create?table?person4index(id?int?not?null??auto_increment,?name?varchar(30)?not?null,?gender?varchar(10)?not?null?,primary?key(id)?,?index(gender));??insert?into?person4index(name,gender)?values("zhaoming","male");??insert?into?person4index(name,gender)?values("wenwen","female");?? ?
2.4根據覆蓋索引查數據
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Java代碼 ?
create?table?person4cindex(id?int?not?null??auto_increment,?name?varchar(30)?not?null,?gender?varchar(10)?not?null?,primary?key(id)?,?index(name,gender));??insert?into?person4cindex(name,gender)?values("zhaoming","male");??insert?into?person4cindex(name,gender)?values("wenwen","female");?? ?
主要從以下幾個方面分析：查詢消耗的時間，走的執行計劃等方面。
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3.開工測試：
第一步：全表掃描
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Java代碼 ?
mysql>?select?*?from?person4all?;??+----+----------+--------+??|?id?|?name?????|?gender?|??+----+----------+--------+??|??1?|?zhaoming?|?male???|??|??2?|?wenwen???|?female?|??+----+----------+--------+??2?rows?in?set?(0.00?sec)?? ?
查看其執行計劃：
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Java代碼 ?
mysql>?explain?select?*?from?person4all;??+----+-------------+------------+------+---------------+------+---------+------+------+-------+??|?id?|?select_type?|?table??????|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?Extra?|??+----+-------------+------------+------+---------------+------+---------+------+------+-------+??|??1?|?SIMPLE??????|?person4all?|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|???????|??+----+-------------+------------+------+---------------+------+---------+------+------+-------+??1?row?in?set?(0.01?sec)?? ?
我們可以很清晰的看到走的是全表掃描，而沒有走索引！
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查詢消耗的時間：
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Java代碼 ?
mysql>?show?profiles;??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+??|?Query_ID?|?Duration???|?Query?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????|??|???????54?|?0.00177300?|?select?*?from?person4all??????????????????????????????????????????????????????????????????????????????????????????????????????????|??|???????55?|?0.00069200?|?explain?select?*?from?person4all??????????????????????????????????????????????????????????????????????????????????????????????????|??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+?? ?
全表掃描總共話了0.0017730秒
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各個階段消耗的時間是：
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Java代碼 ?
mysql>?show?profile?for?query?54;??+--------------------------------+----------+??|?Status?????????????????????????|?Duration?|??+--------------------------------+----------+??|?starting???????????????????????|?0.000065?|??|?checking?query?cache?for?query?|?0.000073?|??|?Opening?tables?????????????????|?0.000037?|??|?System?lock????????????????????|?0.000024?|??|?Table?lock?????????????????????|?0.000053?|??|?init???????????????????????????|?0.000044?|??|?optimizing?????????????????????|?0.000022?|??|?statistics?????????????????????|?0.000032?|??|?preparing??????????????????????|?0.000030?|??|?executing??????????????????????|?0.000020?|??|?Sending?data???????????????????|?0.001074?|??|?end????????????????????????????|?0.000091?|??|?query?end??????????????????????|?0.000020?|??|?freeing?items??????????????????|?0.000103?|??|?storing?result?in?query?cache??|?0.000046?|??|?logging?slow?query?????????????|?0.000019?|??|?cleaning?up????????????????????|?0.000020?|??+--------------------------------+----------+??17?rows?in?set?(0.00?sec)?? ?
第一次不走緩存的話，需要檢查是否存在緩存中，打開表，初始化等操作，最大的開銷在于返回數據。
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第二步：根據主鍵查詢數據。
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Java代碼 ?
mysql>?select?name?,gender?from?person4pri?where?id?in?(1,2);??+----------+--------+??|?name?????|?gender?|??+----------+--------+??|?zhaoming?|?male???|??|?wenwen???|?female?|??+----------+--------+??2?rows?in?set?(0.01?sec)?? ?
查看其執行計劃:
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Java代碼 ?
mysql>?explain?select?name?,gender?from?person4pri?where?id?in?(1,2);??+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+??|?id?|?select_type?|?table??????|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?Extra???????|??+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+??|??1?|?SIMPLE??????|?person4pri?|?range?|?PRIMARY???????|?PRIMARY?|?4???????|?NULL?|????2?|?Using?where?|??+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+??1?row?in?set?(0.00?sec)?? ?
從執行計劃中我們可以看出，走的是范圍索引。
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再看其執行消耗的時間：
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Java代碼 ?
mysql>?show?profiles;??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+??|?Query_ID?|?Duration???|?Query?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????|??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+??|???????63?|?0.00135700?|?select?name?,gender?from?person4pri?where?id?in?(1,2)?????????????????????????????????????????????????????????????????????????????|??|???????64?|?0.00079200?|?explain?select?name?,gender?from?person4pri?where?id?in?(1,2)?????????????????????????????????????????????????????????????????????|??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------+??15?rows?in?set?(0.01?sec)?? ?
這次查詢消耗時間為0.00079200。
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查看各個階段消耗的時間：
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Java代碼 ?
mysql>?show?profile?for?query?63;??+--------------------------------+----------+??|?Status?????????????????????????|?Duration?|??+--------------------------------+----------+??|?starting???????????????????????|?0.000067?|??|?checking?query?cache?for?query?|?0.000146?|??|?Opening?tables?????????????????|?0.000342?|??|?System?lock????????????????????|?0.000027?|??|?Table?lock?????????????????????|?0.000115?|??|?init???????????????????????????|?0.000056?|??|?optimizing?????????????????????|?0.000032?|??|?statistics?????????????????????|?0.000069?|??|?preparing??????????????????????|?0.000039?|??|?executing??????????????????????|?0.000022?|??|?Sending?data???????????????????|?0.000100?|??|?end????????????????????????????|?0.000075?|??|?query?end??????????????????????|?0.000022?|??|?freeing?items??????????????????|?0.000158?|??|?storing?result?in?query?cache??|?0.000045?|??|?logging?slow?query?????????????|?0.000019?|??|?cleaning?up????????????????????|?0.000023?|??+--------------------------------+----------+??17?rows?in?set?(0.00?sec)?? ?
看出最大的消耗也是在Sending data，第一次也是需要一些初始化操作。
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第三步：根據非聚集索引查詢
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Java代碼 ?
mysql>?select?name?,gender?from?person4index?where?gender?in?("male","female");??+----------+--------+??|?name?????|?gender?|??+----------+--------+??|?wenwen???|?female?|??|?zhaoming?|?male???|??+----------+--------+??2?rows?in?set?(0.00?sec)?? 查看器執行計劃：
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Java代碼 ?
mysql>?explain?select?name?,gender?from?person4index?where?gender?in?("male","female");??+----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+??|?id?|?select_type?|?table????????|?type??|?possible_keys?|?key????|?key_len?|?ref??|?rows?|?Extra???????|??+----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+??|??1?|?SIMPLE??????|?person4index?|?range?|?gender????????|?gender?|?12??????|?NULL?|????2?|?Using?where?|??+----+-------------+--------------+-------+---------------+--------+---------+------+------+-------------+??1?row?in?set?(0.00?sec)?? ?
可以看出，走的也是范圍索引。同主鍵查詢，那么就看其消耗時間了
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Java代碼 ?
mysql>?show?profiles;??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+??|?Query_ID?|?Duration???|?Query???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????|??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+??|???????68?|?0.00106600?|?select?name?,gender?from?person4index?where?gender?in?("male","female")?????????????????????????????????????????????????????????????????????????????|??|???????69?|?0.00092500?|?explain?select?name?,gender?from?person4index?where?gender?in?("male","female")?????????????????????????????????????????????????????????????????????|??+----------+------------+-----------------------------------------------------------------------------------------------------------------------------------------------------+??15?rows?in?set?(0.00?sec)?? ?
這個非主鍵索引消耗的時間為：0.00106600，可以看出略大于組件索引消耗的時間。
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看其具體消耗的階段：
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Java代碼 ?
mysql>?show?profile?for?query?68?;??+--------------------------------+----------+??|?Status?????????????????????????|?Duration?|??+--------------------------------+----------+??|?starting???????????????????????|?0.000059?|??|?checking?query?cache?for?query?|?0.000111?|??|?Opening?tables?????????????????|?0.000085?|??|?System?lock????????????????????|?0.000023?|??|?Table?lock?????????????????????|?0.000067?|??|?init???????????????????????????|?0.000183?|??|?optimizing?????????????????????|?0.000031?|??|?statistics?????????????????????|?0.000139?|??|?preparing??????????????????????|?0.000035?|??|?executing??????????????????????|?0.000020?|??|?Sending?data???????????????????|?0.000148?|??|?end????????????????????????????|?0.000024?|??|?query?end??????????????????????|?0.000019?|??|?freeing?items??????????????????|?0.000043?|??|?storing?result?in?query?cache??|?0.000042?|??|?logging?slow?query?????????????|?0.000017?|??|?cleaning?up????????????????????|?0.000020?|??+--------------------------------+----------+??17?rows?in?set?(0.00?sec)?? ?
看幾個關鍵詞的點；init,statistics,Sending data 這幾個關鍵點上的消耗向比較主鍵的查詢要大很多，特別是Sending data。因為若是走的非聚集索引，那么就需要回表進行再進行一次查詢，多消耗一次IO。
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第四部：根據覆蓋索引查詢數據
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Java代碼 ?
mysql>?select?gender?,name?from?person4cindex?where?gender?in?("male","female");??+--------+----------+??|?gender?|?name?????|??+--------+----------+??|?female?|?wenwen???|??|?male???|?zhaoming?|??+--------+----------+??2?rows?in?set?(0.01?sec)?? ?
這里需要注意的是，我的字段查詢順序變了，是gender,name而不在是前面的name,gender，這樣是為了走覆蓋索引。具體看效果吧
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還是先看執行計劃：
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Java代碼 ?
mysql>?explain?select?gender?,name?from?person4cindex?where?gender?in?("male","female");??+----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+??|?id?|?select_type?|?table?????????|?type??|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?Extra????????????????????|??+----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+??|??1?|?SIMPLE??????|?person4cindex?|?index?|?NULL??????????|?name?|?44??????|?NULL?|????2?|?Using?where;?Using?index?|??+----+-------------+---------------+-------+---------------+------+---------+------+------+--------------------------+??1?row?in?set?(0.00?sec)?? ?
最后欄Extra中表示走的就是覆蓋索引。
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看消耗的時間吧：
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Java代碼 ?
mysql>?show?profiles;??+----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+??|?Query_ID?|?Duration???|?Query????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????|??+----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+??|???????83?|?0.00115400?|?select?gender?,name?from?person4cindex?where?gender?in?("male","female")?????????????????????????????????????????????????????????????????????????????????????????|??|???????84?|?0.00074000?|?explain?select?gender?,name?from?person4cindex?where?gender?in?("male","female")?????????????????????????????????????????????????????????????????????????????????|??+----------+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+?? ?
我們看到消耗的時間是0.00115400，看這個數字好像挺高的，那么都花在什么地方了呢？
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看下具體的消耗情況：
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Sql代碼 ?
mysql>?show?profile?for?query?83?;??+--------------------------------+----------+??|?Status?????????????????????????|?Duration?|??+--------------------------------+----------+??|?starting???????????????????????|?0.000083?|??|?checking?query?cache?for?query?|?0.000113?|??|?Opening?tables?????????????????|?0.000039?|??|?System?lock????????????????????|?0.000026?|??|?Table?lock?????????????????????|?0.000075?|??|?init???????????????????????????|?0.000128?|??|?optimizing?????????????????????|?0.000193?|??|?statistics?????????????????????|?0.000056?|??|?preparing??????????????????????|?0.000038?|??|?executing??????????????????????|?0.000021?|??|?Sending?data???????????????????|?0.000121?|??|?end????????????????????????????|?0.000042?|??|?query?end??????????????????????|?0.000021?|??|?freeing?items??????????????????|?0.000112?|??|?storing?result?in?query?cache??|?0.000043?|??|?logging?slow?query?????????????|?0.000021?|??|?cleaning?up????????????????????|?0.000022?|??+--------------------------------+----------+??17?rows?in?set?(0.00?sec)?? ?
很驚奇吧，在初始化和優化上消耗了這么多時間，取數據基恩差不多。
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總結：
? ? 有了上面這些數據，那么我們整理下吧。未存在緩存下的數據。
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看這個表，全表掃描最慢，我們可以理解，同時主鍵查詢比覆蓋所有掃描慢也還能接受，但是為什么主鍵掃描會比非主鍵掃描慢？而且非主鍵查詢需要消耗的1次查詢的io+一次回表的查詢IO，理論上是要比主鍵掃描慢，而出來的數據缺不是如此。那么就仔細看下是個查詢方式在各個主要階段消耗的時間吧。
查詢是否存在緩存，打開表及鎖表這些操作時間是差不多，我們不會計入。具體還是看init，optimizing等環節消耗的時間。
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1.從這個表中，我們看到非主鍵索引和覆蓋索引在準備時間上需要開銷很多的時間，預估這兩種查詢方式都需要進行回表操作，所以花在準備上更多時間。
2.第二項optimizing上，可以清晰知道，覆蓋索引話在優化上大量的時間，這樣在二級索引上就無需回表。
3. Sendingdata，全表掃描慢就慢在這一項上，因為是加載所有的數據頁，所以花費在這塊上時間較大，其他三者都差不多。
4. 非主鍵查詢話在freeingitems上時間最少，那么可以看出它在讀取數據塊的時候最少。
5.相比較主鍵查詢和非主鍵查詢，非主鍵查詢在Init，statistics都遠高于主鍵查詢，只是在freeingitems開銷時間比主鍵查詢少。因為這里測試數據比較少，但是我們可以預見在大數據量的查詢上，不走緩存的話，那么主鍵查詢的速度是要快于非主鍵查詢的，本次數據不過是太小體現不出差距而已。
6.在大多數情況下，全表掃描還是要慢于索引掃描的。
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tips:
過程中的輔助命令：
1.清楚緩存
reset query cache ;
flush tables;
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2.查看表的索引：
show index from tablename;
總結
以上是生活随笔為你收集整理的MYSQL的全表扫描，主键索引(聚集索引、第一索引)，非主键索引(非聚集索引、第二索引)，覆盖索引四种不同查询的分析...的全部內容，希望文章能夠幫你解決所遇到的問題。
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