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這篇文章主要介紹“怎么理解mysql中read commited與repeatable read”,在日常操作中,相信很多人在怎么理解mysql中read commited與repeatable read問題上存在疑惑,小編查閱了各式資料,整理出簡單好用的操作方法,希望對大家解答”怎么理解mysql中read commited與repeatable read”的疑惑有所幫助!接下來,請跟著小編一起來學(xué)習(xí)吧!
兩個事物,A與B
1. 先看read committed級別的情況:
--A事物
mysql> set session transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
mysql> set autocommit = 0;
Query OK, 0 rows affected (0.01 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | aaa |
+------+------+
1 row in set (0.00 sec)
--B事物
mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set comm = 'bbb' where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
--A事物此時查詢表test,此時B事物未提交,因此讀到的仍然是舊的數(shù)據(jù)
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | aaa |
+------+------+
1 row in set (0.00 sec)
--B事物提交
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
--A事物再次查詢,數(shù)據(jù)已發(fā)生改變。
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | bbb |
+------+------+
1 row in set (0.00 sec)
可見,在 read committed級別,顧名思義,A事物在事物進(jìn)行過程中,讀取不到B事物未提交的數(shù)據(jù),但是可以讀取到B事物已經(jīng)提交的數(shù)據(jù)修改
2.再看repeatable read級別
--A事物:
mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)
mysql> set session transaction isolation level repeatable read;
Query OK, 0 rows affected (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | aaa |
+------+------+
1 row in set (0.00 sec)
--B事物:
mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set comm = 'bbb' where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
--A事物此時查詢表test,此時B事物未提交,因此讀到的仍然是舊的數(shù)據(jù)
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | aaa |
+------+------+
1 row in set (0.00 sec)
--B事物提交
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
--A事物再次查詢,發(fā)現(xiàn)數(shù)據(jù)未發(fā)生改變
mysql> select * from test;
+------+------+
| id | comm |
+------+------+
| 1 | aaa |
+------+------+
1 row in set (0.00 sec)
可見,在 repeatable read 級別,顧名思義,A事物在事物進(jìn)行過程中,是不能讀取到B事物未提交和已提交的數(shù)據(jù)修改。
對于repeatable read 級別,事物只能讀取到事物開始時的數(shù)據(jù),因此在事物進(jìn)行過程中讀取到的數(shù)據(jù)時一致的,而對于read commited級別,在事物進(jìn)行過程中,讀取到的數(shù)據(jù)可能是不一致的。
到此,關(guān)于“怎么理解mysql中read commited與repeatable read”的學(xué)習(xí)就結(jié)束了,希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學(xué)習(xí),快去試試吧!若想繼續(xù)學(xué)習(xí)更多相關(guān)知識,請繼續(xù)關(guān)注億速云網(wǎng)站,小編會繼續(xù)努力為大家?guī)砀鄬嵱玫奈恼拢?/p>
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