What's the real cost of adding an index, beyond disk space?
Every index must be updated on every INSERT, UPDATE, or DELETE that touches its columns, so more indexes mean slower writes. A table with ten indexes pays that update cost ten times per write. Indexes are a read/write trade-off, not a free performance upgrade — index the columns your queries actually filter or sort on, not every column that might someday be useful.
Answered in
Database Indexing Explained: Which Index Type Actually Fits Your QueryB-tree, hash, composite, and covering indexes each solve a different query shape. Pick the wrong one and you pay index overhead without the speedup.
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