标签：#t

Read Committed: snapshot mỗi statement, và vì sao hai SELECT trong cùng transaction có thể trả khác nhau READ COMMITTED là isolation level mặc định của PostgreSQL, và là level mà phần lớn workload OLTP đang chạy mà không biết. Khác với mô hình "transaction lấy một snapshot rồi giữ nguyên" mà nhiều dev tưởng tượng từ MVCC, ở Read Committed mỗi statement lấy một snapshot mới tại thời điểm statement bắt đầu , không phải tại thời điểm BEGIN . Hậu quả thực tế: hai SELECT liên tiếp trong cùng một transaction có thể trả về dữ liệu khác nhau nếu giữa hai lần đó có transaction khác commit. Đây là non-repeatable read — đúng spec của Read Committed, không phải bug — và là nguồn của một class lỗi rất hay gặp: code đọc một giá trị, ra quyết định, rồi cập nhật dựa trên giá trị đã đọc, trong khi giá trị thực tế đã thay đổi. Cơ chế hoạt động Một transaction ở Read Committed không có transaction-level snapshot . Khi mỗi statement (mỗi SELECT , UPDATE , DELETE , INSERT ... SELECT ...) bắt đầu thực thi, backend lấy một snapshot mới gồm xmin , xmax và xip list — chính cái snapshot quyết định row version nào "visible" theo MVCC. Statement chỉ thấy: row có xmin đã commit trước thời điểm statement bắt đầu , và xmax chưa tồn tại hoặc thuộc một transaction chưa commit / đã abort. Ngay sau khi statement kết thúc, snapshot đó bị bỏ. Statement kế tiếp lấy snapshot mới — nếu trong khoảng giữa có transaction khác commit, statement này sẽ thấy dữ liệu mới đó. -- T1 BEGIN ; -- KHÔNG lấy snapshot ở đây SELECT balance FROM accounts WHERE id = 1 ; -- snapshot S1 -> trả 1000 -- ... T2 chạy: UPDATE accounts SET balance=500 WHERE id=1; COMMIT; SELECT balance FROM accounts WHERE id = 1 ; -- snapshot S2 -> trả 500 COMMIT ; Với UPDATE / DELETE / SELECT ... FOR UPDATE / FOR NO KEY UPDATE / FOR SHARE , Read Committed làm thêm một bước đặc biệt mà SELECT thường không làm: nếu target row bị một transaction khác đang lock (chưa commit), statement đợi transaction đó kết thúc. Khi unblock: nếu transaction kia ROL

Scale your creativity on editable canvas with agent memory Discussion | Link

There's a lot of info packed in the top area of an Android phone's screen

X is rolling out a new video editor and recorder for iOS with multilingual captions, green-screen effects, and other editing tools.

We recently got the shape() function and corner-shape property. What else could we possibly need as far as making shapes in CSS? Let me tell you: the border-shape property! Get Ready For the Powerful CSS border-shape Property! originally handwritten and published with love on CSS-Tricks . You should really get the newsletter as well.

Discord says a bug affecting its safety system caused it to mistakenly ban more than 8,000 accounts since May. The platform's statement follows a wave of reports from users over the past week, who say they've been banned for posting images containing grids, such as chessboards, game textures, and even Minecraft inventories. Stanislav Vishnevskiy, Discord […]

Chemistry Ventures, the VC firm launched by Bessemer, Index Ventures, and Andreessen Horowitz alums, is raising $500M for its second fund.

AI law startup Norm has raised a $120 million Series C round led by Khosla Ventures, valuing the startup at $1.2 billion.

You might have a great bed and a good sleepy time routine, but if you’re still struggling to fall (or stay) asleep, Anker’s Soundcore Sleep A20 are worth considering. They’re designed to help muffle distracting sounds, and they’ve hit a new all-time low price of $99.99 ($80 off) at Best Buy and Target. In his […]

Bidbus, which lets dealerships bid on used cars, has raised $15 million in a Series A round that was led by early-stage mobility fund Ibex Investors.

iRobot just announced its first-ever non-robotic floor cleaner. The $399 Roomba Electro Plus is a 5-in-1 hard-floor cleaner that combines vacuuming, mopping, and disinfecting, but you have to operate it yourself. The company also announced updates to its line of Roomba robot vacuums, launching five new models with higher suction power, smaller footprints, and lower […]

Microsoft is addressing a Windows 11 bug that caused a folder to take up several gigabytes of storage space. As spotted earlier by Windows Latest, Microsoft included the patch in its optional June 2026 update (KB5095093), which "improves disk space usage for the CapabilityAccessManager.db-wal file." The CapabilityAccessManager.db-wal file is installed on Windows 11 PCs by […]

The self-learning knowledge base that improves itself Discussion | Link

The data can help predict what a future pandemic virus might look like.

The U.S. Army has fixed two of its websites that were hacked to display messages calling President Trump a "pedophile" and a "thief."

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