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+ CSE 4/562 - Database Systems
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Query Evaluation
+CSE 4/562 – Database Systems
+February 12, 2018
+Query Evaluation Styles
+ +-
+
- All-At-Once (Collections) +
- Bottom-up, one operator at a time. + +
- Volcano-Style (Iterators) +
- Operators "request" one tuple at a time from children. + +
- Push-Style (Buffers) +
- Operators continuously produce/consume tuples. +
Basic Mindset
+
+
+ r = get_table("R")
+
+ s = get_table("S")
+
+ temp1 = apply_join(r, s, "R.B = S.B")
+
+ temp2 = apply_select(temp1, "S.C = 10")
+
+ result = apply_projection(temp2, "R.A")
+ Basic Mindset
+
+ def build_tree(operator):
+
+ if """ operator is a base table """:
+ return get_table(...)
+
+ elif """ operator is a selection """:
+ return apply_select(operator.child, operator.condition)
+
+ elif """ handle remaining cases similarly """:
+ Select
+ ++ $$\sigma_{A \neq 3} R$$ +
+| A | B |
|---|---|
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |
Select
+ +
+ def apply_select(input, condition)
+ result = []
+
+ for row in input:
+ if condition(row):
+ result += [row]
+
+ return result;
+ (All-At-Once)
+Select
+ ++ $$\sigma_{A \neq 3} R$$ +
+| A | B | |
|---|---|---|
getNext() | for row in input: | |
| 1 | 2 | return row; |
getNext() | for row in input: | |
| 3 | 4 | X |
| 5 | 6 | return row; |
getNext() | for row in input: | |
None | return None; | |
Select
+ +Project
+ +Union
+ +Cross
+
+ def apply_cross(lhs, rhs):
+ result = []
+
+ for r in lhs:
+ for s in rhs:
+ result += [r + s]
+
+ return result
+ Cross
+ +What's the complexity of this cross-product algorithm?
+... in terms of compute
+... in terms of IOs
+Cross Product Problems
+-
+
- Need to scan the inner relation multiple times! +
- Load data intelligently to mitigate expensive IOs + +
- Every tuple needs to be paired with every other tuple! +
- Exploit join conditions to minimize pairs of tuples +
Preloading Data
+Nested-Loop Join
+
+ def apply_cross(lhs, rhs):
+ result = []
+
+ while r = lhs.next():
+ while s = rhs.next():
+ result += [r + s]
+ rhs.reset()
+
+ return result
+ Need to evaluate rhs iterator once per record in lhs
Preloading Data
+ +Naive Solution: Preload records from lhs
+ def apply_cross(lhs, rhs):
+ result = []
+ rhs_preloaded = []
+
+ while s = rhs.next():
+ rhs_preloaded += [s]
+
+ while r = lhs.next():
+ for s in rhs_preloaded:
+ result += [r + s]
+
+ return result
+ Any problems with this?
+Preloading Data
+ +Better Solution: Load both lhs and rhs records in blocks.
+ def apply_cross(lhs, rhs):
+ result = []
+
+ while r_block = lhs.take(100):
+ while s_block = rhs.take(100):
+ for r in r_block:
+ for s in s_block:
+ result += [r + s]
+ rhs.reset()
+
+ return result
+