1 / 5
You run EXPLAIN on a slow query. What does it show?
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EXPLAIN reveals the planner's chosen execution strategy without (usually) running the query.
2 / 5
The plan shows a 'full table scan'. Why is that often a concern?
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A full table scan examines all rows; on big tables a suitable index usually performs far better.
3 / 5
A developer says: 'The query isn't using the index — it's not sargable.' What does 'sargable' mean?
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Sargable (Search ARGument ABLE) predicates let the optimiser use an index; wrapping a column in a function often prevents it.
4 / 5
EXPLAIN reports a high 'estimated cost'. What is cost?
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Cost is the planner's abstract estimate of resource usage, used to compare alternative plans.
5 / 5
Which sentence correctly uses 'nested loop join'?
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A nested loop join iterates the inner relation for each outer row, efficient when one side is small/indexed.