Discussion
Which of the following improves a query's processing time?
Answer: D
Correct answer: Use compatible data types.
Comparing columns of different types forces the database to convert one of them, and a converted column generally cannot use its index - so a query that should have been an index lookup becomes a full scan. The other three options all add work: complexity, self-joins and subqueries each give the optimiser more to do rather than less.
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