事務是否在SELECT上啟動？
[英]Does a transaction start even on SELECT?
I read in the docs :
我在文檔中讀到：
...since transactions start when a cursor execute a query, but end when COMMIT or ROLLBACK is executed by the Connection object.
...因為事務在游標執行查詢時開始，但在Connection對象執行COMMIT或ROLLBACK時結束。
import MySQLdb
db = MySQLdb.connect(user="root", db="test")
c = db.cursor()
c.execute("SELECT * FROM books")
print c.fetchall()
I suspect that MySQLdb starts a transaction even on queries that do not modify data (like SELECT), because it is difficult to know if a query only reads data and doesn't write it.
我懷疑MySQLdb甚至在不修改數據的查詢（如SELECT）上啟動事務，因為很難知道查詢是否只讀取數據而不寫入數據。
- Is it true?
- 這是真的嗎？
- If so, this means i should do
cursor.commit()after every query, to be sure that no table is locked? - 如果是這樣，這意味着我應該在每次查詢后執行cursor.commit（），以確保沒有表被鎖定？
- Other issues i am not aware of?
- 我不知道的其他問題？
Thank you
謝謝
2 个解决方案
#1
3
Yes, a SELECT statement is like other so the transaction starts.
是的，SELECT語句與其他語句一樣，因此事務開始。
If you want to avoid this, you could do something like that:
如果你想避免這種情況，你可以這樣做：
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
SELECT * FROM books ;
COMMIT ;
In detail:
詳細：
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
says that the following instruction could read the rows that have been modified but that haven't received a COMMIT yet. That kind of transaction dind't get exclusive locks.
表示以下指令可以讀取已修改但尚未收到COMMIT的行。這種交易不會得到獨家鎖。
The second part SELECT * FROM books ; is obviously a SQL statement and the third part COMMIT ; ends the transaction and make it "permanent". In that case no writes are done, so the COMMIT is used only to end the transaction and
第二部分SELECT * FROM books;顯然是一個SQL語句和第三部分COMMIT;結束交易並使其“永久”。在這種情況下，沒有寫入，因此COMMIT僅用於結束事務和
#2
2
it is true,
but it as well automatically commits after each query, because mysql clients start withautocommit=1by default這是真的，但它也會在每次查詢后自動提交，因為mysql客戶端默認以autocommit = 1開頭
you should not, since
SELECTdoes not hold any locks after the statement is executed. In practice, explicit commits might even cause a significant slow down.你不應該，因為SELECT在執行語句后沒有持有任何鎖。在實踐中，顯式提交甚至可能導致顯着減速。
Just might be useful: Why connection in Python's DB-API does not have "begin" operation?
可能有用：為什么Python的DB-API中的連接沒有“開始”操作？
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