Database Indexes Explained Simply: Why Some Queries Take 1ms and Others Take 10 Seconds

Short summary

Database indexes use B-Tree structures to enable rapid data retrieval, reducing query time from seconds to milliseconds by jumping to relevant rows instead of scanning entire tables. Indexes significantly improve read performance but slow writes; composite indexes require careful column ordering via the leftmost prefix rule. Understanding indexing is essential for scaling databases, and EXPLAIN ANALYZE helps verify indexes are actually being used.

• •Indexes are data structures that enable fast retrieval by narrowing search spaces hierarchically instead of scanning all rows
• •B-Tree indexes work via row elimination; composite indexes must follow the leftmost prefix rule for optimal query planning
• •Over-indexing hurts write performance and storage; use EXPLAIN ANALYZE to verify the query optimizer actually uses your indexes

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