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Sự khác nhau giữa các mức cách ly giao dịch trong PostgreSQL là gì?

Mức cách ly giao dịch là một khái niệm quan trọng trong quản lý cơ sở dữ liệu, ảnh hưởng đến cách mà các giao dịch tương tác với nhau. Trong PostgreSQL, có bốn mức cách ly giao dịch tiêu chuẩn được định nghĩa theo tiêu chuẩn SQL, mỗi mức có những đặc điểm riêng. Bài viết này sẽ khám phá sự khác nhau giữa các mức cách ly giao dịch trong PostgreSQL.

Các mức cách ly giao dịch

PostgreSQL hỗ trợ bốn mức cách ly giao dịch chính:

  1. Read Uncommitted
  2. Read Committed
  3. Repeatable Read
  4. Serializable

Mỗi mức cách ly giao dịch có cách xử lý khác nhau đối với các vấn đề như không đồng nhất, ghi lại các thay đổi của giao dịch và khả năng nhìn thấy dữ liệu.

1. Read Uncommitted

  • Định nghĩa: Cho phép một giao dịch đọc dữ liệu chưa được cam kết từ các giao dịch khác. Điều này có nghĩa là dữ liệu có thể bị thay đổi sau khi được đọc.
  • Vấn đề: Cách ly này có thể dẫn đến hiện tượng "bẩn" (dirty reads), nơi một giao dịch đọc dữ liệu có thể bị thay đổi hoặc hoàn tác bởi giao dịch khác.
  • Sử dụng: Không được hỗ trợ trong PostgreSQL, mặc dù có thể mô phỏng bằng cách sử dụng các mức thấp hơn.

2. Read Committed

  • Định nghĩa: Là mức cách ly mặc định trong PostgreSQL. Tại đây, một giao dịch chỉ có thể đọc các thay đổi đã được cam kết bởi các giao dịch khác.
  • Vấn đề: Không xảy ra hiện tượng "bẩn" nhưng có thể gặp phải hiện tượng "non-repeatable reads", nơi dữ liệu đọc có thể thay đổi giữa các lần đọc trong cùng một giao dịch.
  • Sử dụng: Thích hợp cho nhiều ứng dụng, đặc biệt là những nơi cần độ tin cậy cao mà không cần phải lo lắng về dữ liệu chưa cam kết.

3. Repeatable Read

  • Định nghĩa: Một giao dịch có thể đọc cùng một tập dữ liệu nhiều lần mà không bị thay đổi giữa các lần đọc. Điều này đảm bảo rằng dữ liệu đọc sẽ không thay đổi cho đến khi giao dịch hoàn tất.
  • Vấn đề: Tránh được hiện tượng "non-repeatable reads", nhưng có thể xảy ra "phantom reads", nơi các bản ghi mới có thể được thêm vào giữa các lần đọc trong cùng một giao dịch.
  • Sử dụng: Phù hợp với các ứng dụng yêu cầu tính nhất quán cao hơn và tránh việc dữ liệu thay đổi.

4. Serializable

  • Định nghĩa: Là mức cách ly cao nhất, đảm bảo rằng các giao dịch sẽ hoạt động như thể chúng được thực hiện một cách tuần tự, tức là không có giao dịch nào ảnh hưởng đến giao dịch khác.
  • Vấn đề: Tránh cả hai hiện tượng "non-repeatable reads" và "phantom reads". Tuy nhiên, có thể dẫn đến lỗi và phải chờ đợi nếu có nhiều giao dịch cố gắng thay đổi cùng một tập dữ liệu.
  • Sử dụng: Phù hợp cho những ứng dụng yêu cầu tính nhất quán tuyệt đối, như các hệ thống tài chính.

Tóm tắt các mức cách ly giao dịch

Mức cách lyDirty ReadsNon-repeatable ReadsPhantom ReadsSử dụng
Read UncommittedCóCóCóKhông hỗ trợ trong PostgreSQL
Read CommittedKhôngCóCóMặc định trong PostgreSQL
Repeatable ReadKhôngKhôngCóTính nhất quán cao hơn
SerializableKhôngKhôngKhôngTính nhất quán tuyệt đối

Kết luận

Sự lựa chọn mức cách ly giao dịch trong PostgreSQL phụ thuộc vào yêu cầu cụ thể của ứng dụng và cách mà nó tương tác với dữ liệu. Mỗi mức cách ly có ưu và nhược điểm riêng, và việc hiểu rõ chúng sẽ giúp bạn tối ưu hóa hiệu suất cũng như độ tin cậy của ứng dụng của mình. Hãy cân nhắc kỹ lưỡng khi thiết lập mức cách ly giao dịch để đảm bảo rằng nó phù hợp với nhu cầu của hệ thống của bạn.

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File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

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Severity: Notice

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Filename: v1/question.php

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Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">Sự khác biệt giữa Serializable và Repeatable Read trong PostgreSQL là gì?

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Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 99
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

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Severity: Notice

Message: Undefined index: link

Filename: v1/question.php

Line Number: 102

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">Memcached là gì? Cách sử dụng Memcached

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Line Number: 99

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 99
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

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Severity: Notice

Message: Undefined index: link

Filename: v1/question.php

Line Number: 102

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">Cách phát hiện và khắc phục deadlock trong PostgreSQL?

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Severity: Notice

Message: Undefined variable: username

Filename: v1/question.php

Line Number: 99

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 99
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

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Severity: Notice

Message: Undefined index: link

Filename: v1/question.php

Line Number: 102

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">Tại sao Redis được gọi là database còn Memcached thì không

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Severity: Notice

Message: Undefined variable: username

Filename: v1/question.php

Line Number: 99

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 99
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

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Severity: Notice

Message: Undefined index: link

Filename: v1/question.php

Line Number: 102

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">Lệnh để khóa bảng trong PostgreSQL là gì?

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Severity: Notice

Message: Undefined variable: username

Filename: v1/question.php

Line Number: 99

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 99
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

A PHP Error was encountered

Severity: Notice

Message: Undefined index: link

Filename: v1/question.php

Line Number: 102

Backtrace:

File: /var/www/vnengineer.net/public_html/application/views/v1/question.php
Line: 102
Function: _error_handler

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 57
Function: view

File: /var/www/vnengineer.net/public_html/application/controllers/Entity.php
Line: 65
Function: index

File: /var/www/vnengineer.net/public_html/index.php
Line: 240
Function: require_once

">bảng mysql khoảng 3 triệu dữ liệu kiểm tra và thêm mới nhiều thi thoảng bị crash là sao?