Database management systems notes
Why the Index Is Ignored
Six reasons a perfectly good index goes unused, and the fix for each. Every section is a drawn page beside a clear written explanation, about 20 minutes in all.
- Chapter 22 of 34
- About 20 minutes
- 7 sections
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In this chapter
- First: is the database wrong?
- Cause 1: the column is wrapped in something
- Cause 2: an implicit type conversion
- Cause 3: statistics that no longer describe the data
- Cause 4: columns the planner assumes are independent
- Causes 5 and 6: OR conditions, and invisible parameters
- The checklist
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