原文：

Database indexes are data structures that improve query performance at the cost of additional storage and slower writes. A B-tree index maintains sorted data, enabling logarithmic-time lookups instead of full table scans. Without an index, a query must examine every row — an O(n) operation. With an appropriate index, the same query becomes O(log n). However, over-indexing is counterproductive because each insert and update must also modify the indexes. The key is creating indexes on columns frequently used in WHERE clauses and JOIN conditions.

翻译：

数据库索引是提高查询性能的数据结构，代价是额外存储和更慢的写入。B树索引维护排序数据，实现对数时间查找，而不是全表扫描。没有索引时，查询必须检查每一行——这是O(n)操作。有了适当的索引，同样的查询变为O(log n)。然而，过度索引会产生反效果，因为每次插入和更新也必须修改索引。关键是在WHERE子句和JOIN条件中频繁使用的列上创建索引。

词汇注释：

• index /ˈɪndeks/：索引
• query /ˈkwɪri/：查询
• B-tree /biː triː/：B树
• logarithmic /ˌlɔːɡəˈrɪðmɪk/：对数的
• full table scan /fʊl ˈteɪbl skæn/：全表扫描
• counterproductive /ˌkaʊntərprəˈdʌktɪv/：适得其反的
• WHERE clause /wer klɔːz/：WHERE子句
• JOIN condition /dʒɔɪn kənˈdɪʃn/：JOIN条件