Query Processing and Optimization
Query Processing and Optimization
Query Processing and Optimization
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Unnesting Nested Queries<br />
• Uncorrelated sub-queries with aggregates.<br />
• Most systems would compute the average only once.<br />
SELECT ssn<br />
FROM emp<br />
WHERE salary > (SELECT AVG(salary) FROM emp)<br />
• Uncorrelated sub-queries without aggregates.<br />
SELECT ssn<br />
FROM emp<br />
WHERE dept IN (SELECT dept FROM techdept)<br />
• Some systems may not use emp's index on dept, so a<br />
transformation is desirable.<br />
SELECT ssn<br />
FROM emp, techdept<br />
WHERE emp.dept = techdept.dept<br />
When is this<br />
acceptable?<br />
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