mysql執行計劃
在企業的應用場景中，為了知道優化SQL語句的執行，需要查看SQL語句的具體執行過程，以加快SQL語句的執行效率。
可以使用explain+SQL語句來模擬優化器執行SQL查詢語句，從而知道mysql是如何處理sql語句的。
官網地址： https://dev.mysql.com/doc/refman/5.5/en/explain-output.html
1、執行計劃中包含的信息
| Column | Meaning |
|---|---|
| id | The SELECT identifier |
| select_type | The SELECT type |
| table | The table for the output row |
| partitions | The matching partitions |
| type | The join type |
| possible_keys | The possible indexes to choose |
| key | The index actually chosen |
| key_len | The length of the chosen key |
| ref | The columns compared to the index |
| rows | Estimate of rows to be examined |
| filtered | Percentage of rows filtered by table condition |
| extra | Additional information |
id
select查詢的序列號，包含一組數字，表示查詢中執行select子句或者操作表的順序
id號分為三種情況：
1、如果id相同，那么執行順序從上到下
2、如果id不同，如果是子查詢，id的序號會遞增，id值越大優先級越高，越先被執行
3、id相同和不同的，同時存在：相同的可以認為是一組，從上往下順序執行，在所有組中，id值越大，優先級越高，越先執行
select_type
主要用來分辨查詢的類型，是普通查詢還是聯合查詢還是子查詢
select_type Value |
Meaning |
|---|---|
| SIMPLE | Simple SELECT (not using UNION or subqueries) |
| PRIMARY | Outermost SELECT |
| UNION | Second or later SELECT statement in a UNION |
| DEPENDENT UNION | Second or later SELECT statement in a UNION, dependent on outer query |
| UNION RESULT | Result of a UNION. |
| SUBQUERY | First SELECT in subquery |
| DEPENDENT SUBQUERY | First SELECT in subquery, dependent on outer query |
| DERIVED | Derived table |
| UNCACHEABLE SUBQUERY | A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query |
| UNCACHEABLE UNION | The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY) |
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n91" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">--sample:簡單的查詢，不包含子查詢和union
explain select * from emp;
--primary:查詢中若包含任何復雜的子查詢，最外層查詢則被標記為Primary
explain select staname,ename supname from (select ename staname,mgr from emp) t join emp on t.mgr=emp.empno ;
--union:若第二個select出現在union之后，則被標記為union
explain select * from emp where deptno = 10 union select * from emp where sal >2000;
--dependent union:跟union類似，此處的depentent表示union或union all聯合而成的結果會受外部表影響
explain select * from emp e where e.empno in ( select empno from emp where deptno = 10 union select empno from emp where sal >2000)
--union result:從union表獲取結果的select
explain select * from emp where deptno = 10 union select * from emp where sal >2000;
--subquery:在select或者where列表中包含子查詢
explain select * from emp where sal > (select avg(sal) from emp) ;
--dependent subquery:subquery的子查詢要受到外部表查詢的影響
explain select * from emp e where e.deptno in (select distinct deptno from dept);
--DERIVED: from子句中出現的子查詢，也叫做派生類，
explain select staname,ename supname from (select ename staname,mgr from emp) t join emp on t.mgr=emp.empno ;
--UNCACHEABLE SUBQUERY：表示使用子查詢的結果不能被緩存
explain select * from emp where empno = (select empno from emp where deptno=@@sort_buffer_size);
--uncacheable union:表示union的查詢結果不能被緩存：sql語句未驗證</pre>
table
對應行正在訪問哪一個表，表名或者別名，可能是臨時表或者union合并結果集 1、如果是具體的表名，則表明從實際的物理表中獲取數據，當然也可以是表的別名
2、表名是derivedN的形式，表示使用了id為N的查詢產生的衍生表
3、當有union result的時候，表名是union n1,n2等的形式，n1,n2表示參與union的id
type
type顯示的是訪問類型，訪問類型表示我是以何種方式去訪問我們的數據，最容易想的是全表掃描，直接暴力的遍歷一張表去尋找需要的數據，效率非常低下，訪問的類型有很多，效率從最好到最壞依次是：
system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
一般情況下，得保證查詢至少達到range級別，最好能達到ref
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n100" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">--all:全表掃描，一般情況下出現這樣的sql語句而且數據量比較大的話那么就需要進行優化。
explain select * from emp;
--index：全索引掃描這個比all的效率要好，主要有兩種情況，一種是當前的查詢時覆蓋索引，即我們需要的數據在索引中就可以索取，或者是使用了索引進行排序，這樣就避免數據的重排序
explain select empno from emp;
--range：表示利用索引查詢的時候限制了范圍，在指定范圍內進行查詢，這樣避免了index的全索引掃描，適用的操作符： =, <>, >, >=, <, <=, IS NULL, BETWEEN, LIKE, or IN()
explain select * from emp where empno between 7000 and 7500;
--index_subquery：利用索引來關聯子查詢，不再掃描全表
explain select * from emp where emp.job in (select job from t_job);
--unique_subquery:該連接類型類似與index_subquery,使用的是唯一索引
explain select * from emp e where e.deptno in (select distinct deptno from dept);
--index_merge：在查詢過程中需要多個索引組合使用，沒有模擬出來
--ref_or_null：對于某個字段即需要關聯條件，也需要null值的情況下，查詢優化器會選擇這種訪問方式
explain select * from emp e where e.mgr is null or e.mgr=7369;
--ref：使用了非唯一性索引進行數據的查找
create index idx_3 on emp(deptno);
explain select * from emp e,dept d where e.deptno =d.deptno;
--eq_ref ：使用唯一性索引進行數據查找
explain select * from emp,emp2 where emp.empno = emp2.empno;
--const：這個表至多有一個匹配行，
explain select * from emp where empno = 7369;
--system：表只有一行記錄（等于系統表），這是const類型的特例，平時不會出現</pre>
possible_keys
顯示可能應用在這張表中的索引，一個或多個，查詢涉及到的字段上若存在索引，則該索引將被列出，但不一定被查詢實際使用
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n103" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;</pre>
key
實際使用的索引，如果為null，則沒有使用索引，查詢中若使用了覆蓋索引，則該索引和查詢的select字段重疊。
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n106" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;</pre>
key_len
表示索引中使用的字節數，可以通過key_len計算查詢中使用的索引長度，在不損失精度的情況下長度越短越好。
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n109" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;</pre>
ref
顯示索引的哪一列被使用了，如果可能的話，是一個常數
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n112" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;</pre>
rows
根據表的統計信息及索引使用情況，大致估算出找出所需記錄需要讀取的行數，此參數很重要，直接反應的sql找了多少數據，在完成目的的情況下越少越好
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n115" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">explain select * from emp;</pre>
extra
包含額外的信息。
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="sql" cid="n118" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">--using filesort:說明mysql無法利用索引進行排序，只能利用排序算法進行排序，會消耗額外的位置
explain select * from emp order by sal;
--using temporary:建立臨時表來保存中間結果，查詢完成之后把臨時表刪除
explain select ename,count(*) from emp where deptno = 10 group by ename;
--using index:這個表示當前的查詢時覆蓋索引的，直接從索引中讀取數據，而不用訪問數據表。如果同時出現using where 表名索引被用來執行索引鍵值的查找，如果沒有，表面索引被用來讀取數據，而不是真的查找
explain select deptno,count(*) from emp group by deptno limit 10;
--using where:使用where進行條件過濾
explain select * from t_user where id = 1;
--using join buffer:使用連接緩存，情況沒有模擬出來
--impossible where：where語句的結果總是false
explain select * from emp where empno = 7469;</pre>
