[prev] 2 [next]

Query Processing So Far

Steps in processing an SQL statement
  • parse, map to relation algebra (RA) expression
  • transform to more efficient RA expression
  • instantiate RA operators to DBMS operations
  • execute DBMS operations (aka query plan)
Cost-based optimisation:
  • generate possible query plans  (via heuristics)
  • estimate cost of each plan  (sum costs of operations)
  • choose the lowest-cost plan  (... and choose quickly)