Query Optimisation (cont)
Why do we not generate optimal query execution plans?
Finding an optimal query plan ...
- requires exhaustive search of a space of possible plans
- for each possible plan, need to estimate cost
(not cheap)
Even for relatively small query, search space is very large.
Compromise:
- do limited search of query plan space
(guided by heuristics)
- quickly choose a reasonably efficient execution plan
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