Why Column Order in a Database Index Determines Whether It Gets Used

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A database index can exist yet still fail to speed up queries if its columns are ordered incorrectly. Multi-column indexes only work when queries reference columns starting from the leftmost position in the index definition, regardless of the order conditions appear in the SQL. Placing the filtered column first in the index can dramatically cut query time — one test on a million-row SQLite table dropped from 26.4 ms to 0.02 ms after reordering. Building a covering index, which includes every column a query needs, eliminates a second lookup to the base table and speeds reads further. However, each index adds overhead to every write operation, so indexes should be deliberately designed around actual query patterns rather than added indiscriminately.

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ProgrammingDEV Community ·

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ProgrammingDEV Community ·

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