What is a composite index and why does column order matter?
A composite index covers multiple columns together, like (last_name, first_name). It can serve queries filtering on last_name alone, or on both columns together — but not on first_name alone, because the index is physically sorted by the first column first. Column order should match your most common filter patterns, leading with the column most queries filter on.
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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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