Query Optimisation (cont)
Query optimisation is a critical step in query evaluation.
The query optimiser
- takes relational algebra expression from SQL compiler
- produces sequence of RelOps to evaluate the expression
- query execution plan provides an efficient evaluation
"Optimisation" is a misnomer since query optimisers
- aim to find a good plan (maybe not optimal)
- within a reasonable amount of time
Observed Time = Planning time + Execution time
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