Category: Database
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MongoDB Change Stream: react to real-time data changes
Category: Database, JavaScript
MongoDB Change Stream is a high-level API that allows you to subscribe to real-time notifications whenever there is a change in your MongoDB collections, databases, or the entire cluster, in an event-driven fashion.
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MongoDB operations: Replica Set
A replica set is a group of servers that maintain the same data set, with one primary which takes client requests, and multiple secondaries that keep copies of the primary's data. If the primary crashes, secondaries can elect a new primary from amongst themselves.
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Import MySQL database but ignore specific tables
You could just exclude tables you do not need.
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MongoDB cookbook: Indexes
Category: Database
The order of fields in an index matters, you must consider Index Cardinality and Selectivity. Instead, the order of fields in a find() query doesn't affect whether it can use an index or not.
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MongoDB cookbook: Queries and Aggregations
Category: Database, JavaScript, Python
Frequently accessed items are cached in memory, so that MongoDB can provide optimal response time.
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MySQL system error codes
Print all OS error codes and MySQL error codes using the perror command.
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Create a custom migration in Django
ALTER TABLE app_repostarring CHANGE repo_description repo_description VARCHAR(191) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
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Locking and MVCC in MySQL InnoDB 鎖機制與多版本並發控制
Category: Database
最近在研究 MySQL 5.6 / 5.7 InnoDB storage engine 的 Transaction、Locking 和 Multi-Version Concurrency Control (MVCC) 機制，基本上只討論 Isolation level 是 REPEATABLE READ 或 READ COMMITTED 的情況，這一篇是過程中記錄下來的筆記。
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Django's get_or_create() may raise IntegrityError but subsequent get() raises DoesNotExist
Category: Database, Python, Web Development
get_or_create() 拋出 IntegrityError 但是接著 get() 一次卻會是 DoesNotExist，以上的錯誤常常會發生在 MySQL 的 isolation level 是 REPEATABLE READ 的情況下。MySQL 默認採用 REPEATABLE READ（而 PostgreSQL 預設是 READ COMMITTED）。