CS848: Topics in Databases: Foundations of Query Optimization Topics Covered Databases QL Query containment More on QL.

CS848: Topics in Databases: Foundations of Query Optimization

Topics Covered

Databases

QL

Query containment

More on QL


A simple case of finding a query plan

Subsystem3

Subsystem2

Subsystem1

SQL

Global Schema

A single table: T

Open, Scan

SQL Server

A “local as view (LAV) integration schema”: T ´ Q2. User submits Q1. Query optimizer must determine if a scan of T suffices. True iff Q1 is equivalent to Q2.


In the beginning …

Infinite countable sets of each of the following kinds of symbols:

C = {C1, C2, … } (primitive concepts)

A = {A1, A2, …} [ {B1, B2, …} (attributes)

R = {R1, R2, … } (roles)

Conventions: Attributes (resp. primitive concepts and roles) correspond to words in lower case or to positive integers (resp. words in upper case and words in mixed case).


For a particular database I

h, (¢)Ii

where is a countable possibly infinite domain, and where for each symbol

(C)I µ

(A)I : !

(R)I µ ( £ )


Partial Databases (Aboxes)

e

e : {C1, … , Cn}

e2e1

A

e2e1

R

e 2

e 2 (Ci)I

(A)I(e1) = e2

(e1, e2) 2 (R)I

e2e1 e1 e2

e2e1 e1 e2


Relational Databases

“John”“Mary”

3333

name ageEMP

e1 : {EMP}

e2 : {EMP}

33

“Mary”

“John”name

name

age


Relational Databases (cont’d)

{e1, e2} µ (EMP)I

(name)I(e1) = “John”

(age)I(e1) = (age)I(e2) = 33

{e1, e2, “John”, 33, “Mary”} µ

e1 e2

“John” ? 33

e1 : {EMP}

e2 : {EMP}

33

“Mary”

“John”name

name

age


Dialects of QL

(expressiveness)

(semantics)Conjunctive QL

withbag semantics†

Positive QL First order QLConjunctive QL

First order QLwith

bag semantics

Positive QLwith

bag semantics‡

†[Khizder et al., 1999], ‡[Lui et al., 2002]


Conjunctive QL

Q ::= D as A (quantification)

| A1 = A2.R (unnest)

| A1.Pf1 = A2.Pf2 (selection)

| elim A1, … , An Q (projection)

| true (null tuple)

| from Q1, Q2 (natural join)

| ( Q )

D ::= THING | C (basic description)

Pf ::= id | A.Pf (path function)


Well Formed Queries: (Q)

(D as A)´ {A}

(A1 = A2.R) ´ {A1, A2}

(A1.Pf1 = A2.Pf2) ´ {A1, A2}

(elim A1, … , An Q) ´ {A1, … , An}

(true) ´ ;

(from Q1, Q2) ´ (Q1) [ (Q2)

Require {A1, … , An} µ (Q) for projection operators.


Tuples and Bags

A (duplicate) tuple t with attribute bindings for attributes {A1, … , An} over a database I = h, (¢)Ii has the general form

hA1 : e1, … , An : en , cnt : ii,

where {e1, … , en} µ , “cnt” is a distinct attribute not used in queries, and i a positive integer.

A set of duplicate tuples that contain the same attribute bindings is called a bag.


Operations on Tuples

(t) ´ set of attributes occurring in t, excluding cnt.

t@cnt ´ integer i such that “cnt : i” occurs in t

t@A ´ element e 2 such that “A : e” occurs in t; defined only when A 2 (t)

t[{A1, … , An}] ´ {A1 : t@A1, … , An : t@An}; defined only when {A1, … , An} µ (t)

[t] ´ t[(t)]


SemanticsThe meaning of a query Q, denoted «Q¬, is a function that maps databases to bags. The behavior of this function on a particular database I = h, (¢)Ii is defined as follows.

«THING as A¬(I) ´ {hA : e, cnt : 1i : e 2 }

«C as A¬(I) ´ {hA : e, cnt : 1i : e 2 (C)I}

«A1 = A2.R¬(I) ´ {hA1 : e1, A2 : e2, cnt : 1i : (e2, e1) 2 RI}

«A1.Pf1 = A2.Pf2¬(I)´ {hA1 : e1, A2 : e2, cnt : 1i : (Pf1)I(e1) = (Pf2)I(e2)}

where

(id)I ´ {(e, e) : e 2 }

(A.Pf )I ´ {(e1, e2) : (Pf )I((A)I(e1)) = e2}


Semantics (cont’d)

«elim A1, … , An Q ¬(I) ´ ; , if not well formed; otherwise {hA1 : t@A1, … , An : t@An , cnt : 1i : t 2 «Q¬(I)}

«true¬(I) ´ {hcnt : 1i}

«from Q1, Q2¬(I) ´ {t : (t) = (Q1) [ (Q2) Æ 9 t1 2 «Q1¬(I), t2 2 «Q2¬(I) : t@cnt = t1@cnt £ t2@cnt Æ t[(t1)] = [t1] Æ t[(t2)] = [t2]}


Syntactic Sugar

A1. .An.id ´ A1. .An

select distinct A1, … , An Q ´ elim A1, … , An Q

select * Q ´ Q

Q1 where Q2 ´ from Q1, Q2

Q1 and Q2 ´ from Q1, Q2

from ´ true

from Q1, Q2, … , Qn ´ from (from Q1, Q2, …) Qn


Examples

The names of employees who have the same age as another employee with a given name.

select distinct :p, namefrom EMP as e, ( select distinct :p, e1

from EMP as e1, EMP as e2where e1.age = e2.age and e2.name = :p )

where e.name = nameand e.id = e1.id


Method Calls (more syntactic sugar)

A1.Pf1.C(A2.Pf2, … , An-1.Pfn-1) = An.Pfn

select distinct A1, … , An

´ from C as A where A.1 = A1.Pf1 and … and A.n = An.Pfn

A1.Pf1.C(A2.Pf2, … , An-1.Pfn-1) as An

´ A1.Pf1.C(A2.Pf2, … , An-1.Pfn-1) = An.id


Examples (cont’d)

select distinct namefrom EMP as e, ( select distinct e1

from EMP as e1, EMP as e2where e2.age.+(e2.age) = e1.age )

where e.name = nameand e.id = e1.id

The names of employees who have an age double that of another employee.


Conjunctive Datalog (more syntactic sugar)

C(A1, … , An)

select distinct A1, … , An

´ from C as A where A.1 = A1.id and … and A.n = An.id

(A1, … , Am) :- Q1, … , Qn.

´ select distinct A1, … , Am from Q1, … , Qn


Positive QL

Q ::= empty A1, … , An (empty set)

| Q1 union Q2 (union)

(empty A1, …, An) ´ {A1, …, An}

(Q1 union all Q2) ´ (Q1)

Require (Q1) = (Q2) in union operations.


Semantics

«empty A1, … , An¬(I) ´ ;

«Q1 union Q2¬(I)

´ {t : t@cnt = 1 Æ (t) = (Q1) Æ (t) = (Q2) Æ (

( 9 t1 2 «Q1¬(I) : [t] = [t1] Æ :9 t2 2 «Q2¬(I) : [t] = [t2] ) Ç ( 9 t2 2 «Q2¬(I) : [t] = [t2] Æ :9 t1 2 «Q1¬(I) : [t] = [t1] ) Ç ( 9 t1 2 «Q1¬(I), t2 2 «Q2¬(I) : [t] = [t1] Æ [t] = [t2] ) )}


First Order QL

Q ::= Q1 minus Q2 (difference)

(Q1 minus Q2) ´ (Q1)

Require (Q1) = (Q2) in difference operations.


Semantics

«Q1 minus Q2¬(I)

´ {t : t@cnt = 1 Æ (t) = (Q1) Æ (t) = (Q2) Æ ( 9t1 2 «Q1¬(I) : [t] = [t1] ) Æ ( :9t2 2 «Q2¬(I) : [t] = [t2] )}


QL with Duplicates

Q ::= select A1, … , An Q (duplicate preserving projection)

| Q1 union all Q2 (bag union)

| Q1 minus all Q2 (bag difference)


Well Formed Queries (cont’d)

(select A1, … , An Q) ´ {A1, … , An}

(Q1 union all Q2 ) ´ (Q1)

(Q1 minus all Q2) ´ (Q1)

Require (Q1) = (Q2) in bag union and bag difference operations, and that {A1, … , An} µ (Q) in (duplicate preserving) projection operations.


Semantics

«select A1, … , An Q ¬(I)

´ ; , if not well formed and representable†; otherwise

{hA1 : t1@A1, … , An : t1@An , cnt : ni : t1 2

«Q¬(I)

Æ n = (t2@cnt) } t2 2 «Q¬(I) :

t2[{A1, … , An}] = t1 [{A1, … , An}]

†The selection operation is representable on database I iff, for everyt1 2 «Q¬(I),

|{t2 2 «Q¬(I) : t2[{A1, … , An}] = t1 [{A1, … , An}]}|

is finite.


Example

A duplicate preserving projection operation that is not representable in any database with an infinite domain.

select e1 from THING as e1, THING as e2

Observation: All well-formed duplicate preserving projection operations on databases with finite domains are representable.


Semantics (cont’d) «Q1 union all Q2¬(I)

´ ; , if not well formed; otherwise

{t 2 «Q1¬(I) : :9 t2 2 «Q2¬(I) : [t] = [t2]}

[ {t 2 «Q2¬(I) : :9 t1 2 «Q1¬(I) : [t] = [t1]}

[ {t : 9 t1 2 «Q1¬(I), t2 2 «Q2¬(I) : [t] = [t1]} Æ [t] =

[t2]

Æ t@cnt = t1@cnt + t2@cnt} «Q1 minus all Q2¬(I)

´ ; , if not well formed; otherwise

{t 2 «Q1¬(I) : :9 t2 2 «Q2¬(I) : [t] = [t2]}

[ {t : 9 t1 2 «Q1¬(I), t2 2 «Q2¬(I) : [t] = [t1]} Æ [t] = [t2]

Æ t@cnt = t1@cnt t2@cnt

Æ t1@cnt t2@cnt }


Summary

at, =, elim,true, from,

select


empty, union


empty, union,minus

at, =, elim,true, from


select,empty, union all,

minus all


select,empty, union all

(conjunctive)

(bag semantics)

(set semantics)

(positive) (first order)


Query Contexts

An expression Q[] in the language QL enriched by an additional terminal symbol [] is called a query context.

For a query Q1 2 QL, the expression Q1[Q2] denotes the syntactical substitution of Q2 for [].

Q2 is compatible with Q1 if Q1[Q2] 2 QL. For example, Q2 is compatible with Q1 in the following.

Q2: EMP as e where e.name = :p

Q1: select distinct :p, d from DEPT as d, [] where d = e.dept


The Query Equivalence Problem

Q1 is equivalent to Q2 for database I, written I ² (Q1 ´ Q2 ),if «Q1¬(I) = «Q2¬(I).

A query equivalence dependency E has the form (Q1 ´ Q2).E = (Q1 ´ Q2) is an axiom if, for any database I, I ² (Q1 ´ Q2 ).

A query equivalence problem for a given set of query equivalence dependencies is to determine if a given member of the set is an axiom.


Some Axioms

Question: Is it true that any E with the following form is an axiom?

(elim A1, … , Am Q1)[elim B1, … , Bn Q2] ´ elim A1, … , Am Q1 [Q2]

Answer: No. However, any such E is an axiom if any attribute in

(Q2) – {B1, … , Bn})

does not occur in query context

(elim A1, … , Am Q1 []).


Excluding variable reuse in QL

Q has an occurrence of variable reuse if there is a query context Q1[] and a query of the form

elim A1, … , An Q2

or of the form

select A1, … , An Q2

such that Q = Q1[Q2] and there exists A in ((Q2) – {A1, … , An}) thatalso occurs in Q1[].

Observation: For any Q1, there exists an equivalent class of query Q2 that has no occurrence of variable reuse.


The Query Containment Problem

Q1 is contained in Q2 for database I, written I ² (Q1 v Q2), if, for any tuple t1 in «Q1¬(I), there exists t2 in «Q2¬(I) such that [t1] = [t2] and t1@cnt t2@cnt.

A query containment dependency C has the form (Q1 v Q2). C = (Q1 v Q2) is an axiom if, for any database I, I ² (Q1 v Q2).

A query containment problem for a given set of query containment dependencies is to determine if a given member of the set is an axiom.


Equivalence and Containment

Observation: Equivalence reduces to containment.

Q1 ´ Q2 iff Q1 v Q2 and Q2 v Q1

Observation: Containment reduces to equivalence in first order QL.

Q1 v Q2 iff (Q1 minus all Q2) ´ empty (Q1)


Some Complexity Results

Theorem: The query equivalence and containment problems for conjunctive QL is NP-complete.†

†Chandra, A. K. and P. M. Merlin. Optimal implementation of conjunctive queries in relational databases. Proc. Ninth Annual ACM Symposium on the Theory of Computing, pp. 77–90, 1977.


A Decision Procedure

Theorem: The following procedure decides if C = (Q1 v Q2) is anaxiom for conjunctive QL.†

1. Freeze the body of Q1 by creating a partial database consisting of individuals that include its variables.

2. If the tuple

hA1 : A1 , … , An : An , cnt : 1i occurs in «Q2¬(I), where (Q1) = {A1, … , An}, then return true;

otherwise return false.‡

†Derived from [Ullman, 1999].‡Use forced semantics for selection operations.


Obtaining a Partial Database from Q

A1.A2. .Am = B1.B2. .Bn

A1

A2 Am…

B1

B2 Bn…

THING as A A

C as A A : {C}

A1 = A2.R

A2

RA1


Deriving Partial Databases (cont’d)

w : L u : L1

A

v : L2

A

w : L u : L1

A

v : L2

A

u : L1 v : L3

A

x : L4

Aw : L2

u : L1 v : L3

A

x : L4

Aw : L2


Deriving Partial Databases (cont’d)

n1 : L1 n2 : L2 n1 : L1 [ L2 n2 : L1 [ L2

n1 : L1 n2 : L2 n3 : L3

n1 : L1 n2 : L2 n3 : L3


Evaluating Selections on Partial Databases

Note that selection conditions can navigate missing attribute values. In such cases, assume a forced semantics. In particular, two nodes n1 and n2 satisfy a selection condition iff the condition has the form

n1.Pf1.Pf = n2.Pf2.Pf

where (Pf1)I(n1) and (Pf2)I(n2) are defined and lead to nodes connected by an equality arc.


Some Complexity Results (cont’d)

Theorem: The query equivalence problem for conjunctive QL with bag semantics is NP-complete.

Observation: The complexity of the query containment problem for conjunctive QL with bag semantics remains open at this time.

Example:† In conjunctive QL with bag semantics, the query containment dependency Q1 v Q2 is an axiom, where Q1 and Q2 have the respective definitions

select x, z select x, z from P as x, R as z from P as x, R as z where x = u.Q and z = v.Q where y = u.Q and y = v.Q

† [Chaudhuri and Vardi, 1993]


The Query Membership Problem

A database schema, denoted T, consists of a finite set {C1, … , Cn} of query containment dependencies.

C is an axiom relative to database schema T = {C1, … , Cn}, writtenT ² C, if, for any database I, I ² C if I ² Ci for each i.

A query membership problem for a given set of query containment dependencies is to determine if a given member of the set is an axiom relative to a given database schema also consisting of members of the set.


More Complexity Results

Theorem: The query membership problem for conjunctive QL is undecidable.

Theorem: The query membership problem for first order QL is equivalent to the query containment problem for first order QL.

Proof: Assignment.


More on QL

Defining database schema

Expressing access plans


Modeling Generalization Taxonomies

Consider a simple object-oriented schema language consisting of sentencesof the following form.†

class C {A1 : ref C1, … , Am : ref Cm} [isa C1 , … , Cn];

Assignment: Encode a fixed collection of such sentences as a databaseschema in conjunctive QL. Your encoding should be as compact as possibleand should enable the following questions to be expressed as querycontainment dependencies over your schema.

1. Is C a defined class?

2. Is attribute A defined on class C?

3. Can an object reside in both class C1 and class C2?†Assume that any object in a database was created with respect to a singleclass.


Modeling Pipelined Query Access Plans

(syntax) (defn of (¢))

(parameter) Q ::= (PARAM as A) {A}(index scan) | (from C as A, A.1 = B1, … , A.n = Bn) {A}(nested loops) | (from Q1, Q2) (Q1) [ (Q2)(noop) | (select A1, … , An Q) (Q) Å {A1, … , An}(record field access) | (A1 = A2.B) {A1}(comparison) | (A1 = A2) ;(catenation) | (Q1 union all Q2) (Q1) Å (Q2)(cut) | (elim A1, … , An Q) ;

| …

Require:1. ((Q2) – (Q2)) µ (Q1) for nested loops, and2. (Q) = (Q) for top-level queries.


Alternative Semantics

Require richer models theories for

1. sort operations, and

2. named cuts.

CS848: Topics in Databases: Foundations of Query Optimization Topics Covered Databases QL Query containment More on QL.

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