How would you determine whether a SQL index should be added for a given SQL query?
💡 Model Answer
To decide if an index is needed, first analyze the query’s execution plan (EXPLAIN). Look for full table scans on large tables. If the query filters on a column with low cardinality or joins on a column, an index can reduce I/O. Consider the selectivity: columns with high uniqueness benefit more. Also check if the index can be a covering index (includes all columns used). Evaluate the write workload; indexes slow inserts/updates. Finally, test the query with and without the index to confirm performance gains.
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