Managing XML and Semistructured Data

1
Managing XML and Semistructured Data

• Lecture 9 Query Languages -
• StruQL and XSL

Prof. Dan Suciu
Spring 2001
2
In this lecture

• Website management with Strudel
• Background on skolem functions
• Skolem functions in StruQL
• Structural recursion
• XSL
• Resources
• Catching the boat with Strudel VLDBJ 2001
• UnQL A Query Language and Algebra for
  Semistructured Data Based on Structural Recursion
  Buneman, Fernandez, Suciu.VLDBJ 2000
• Data on the Web Abiteboul, Buneman, Suciu
  sections 5.2, 6.4, 6.5

3
Strudel and StruQL

• Strudel a Website management tool
• Idea separate the following three tasks
• Management of data
• use some database
• Management of the sites structure
• use StruQL
• Management of the sites presentation
• use HTML templates (this was before XML...)

4
Example Bibliography Data
Input data
Bib

• Bib paper author Jones,
• author Smith,
• title The Comma,
• year 1994 ,
• paper author Jones,
• title The Dot,
• year 1998 ,
• paper author Mark,
• ....
• . . .

paper
paper
paper
author
year
author
title
Jones
Smith
The Comma
.....
5
Simple Website Definition in StruQL
WHERE Root -gt Bib.paper.author -gt A CREATE
Root(), HomePage(A) LINK Root() -gt
person -gt HomePage(A),
HomePage(A) -gt name -gt A
HomePage(A) -gt home -gt Root()
StruQL query
Result
home
home
home
name
name
name
Smith
Jones
Mark
Root(), HomePage(A) Skolem Functions (more
later)
6
Complex Website Definition in StruQL
WHERE Root -gt Bib -gt X, X -gt paper -gt P,
P -gt author -gt A, P -gt title -gt T,
P -gt year -gt Y CREATE Root(), HomePage(A),
YearPage(A,Y), PubPage(P) LINK Root() -gt
person -gt HomePage(A), HomePage(A)
-gt yearentry -gt YearPage(A,Y),
YearPage(A,Y) -gt publication -gt PubPage(P),
PubPage(P) -gt author -gt HomePage(A),
PubPage(P) -gt title -gt T
7
Example A Complex Web Site
The Comma
The Dot
8
Skolem Functions

• Maier, 1986
• in OO systems
• Kifer et al, 1989
• F-logic
• Hull and Yoshikawa, 1990
• deductive db (ILOG)
• Papakonstantinou et al., 1996
• semistructured db (MSL)

9
Skolem Functions in Logic

• Origins First Order Logic
• The Satisfiability problemgiven a formula ?,
  does it have a model ?

10
Skolem Functions in Logic

• Example does ? have a model ?
• Skolem functions replace ? with functions, drop
  ?
• Fact ? has a model iff ? has a model

11
Skolem Functions in Databases

• Recall Datalog
• Means

Answer(title, author) - Paper(author, title,
year)
12
Skolem Functions in Databases

• Now consider
• I want to create a new object x. What meaning ?

Answer(author, x) - Paper(author, title, year)
13
Skolem Functions in Databases

• Better use Skolem functions directly in Datalog
• Choices

Answer(author, NewObj(author)) - Paper(author,
title, year)
Answer(author, NewObj(author,title)) -
Paper(author, title, year)
Answer(author, NewObj(title,year)) -
Paper(author, title, year)
Answer(author, NewObj()) - Paper(author, title,
year)
14
Skolem Functions in StruQL

• StruQLs semantics
• Input graph (Node, Edge)
• Output graph(Node, Edge)
• Example

WHERE Root -gt Bib.paper.author -gt A CREATE
Root(), HomePage(A) LINK Root() -gt
person -gt HomePage(A),
HomePage(A) -gt name -gt A
HomePage(A) -gt home -gt Root()
Node(Root()) - Node(HomePage(A)) -
Edge(Root,Bib,X), Edge(X,paper,Y),Edge(Y
,author,A)Edge(Root,person,HomePage(A)) -
Edge(Root,Bib,X), Edge(X,paper,Y),Edge(Y,author,A)
Edge(HomePage(A),person, A) -
Edge(Root,Bib,X), Edge(X,paper,Y),Edge(Y,author,A)
Edge(HomePage(A),home,Root()) -
Edge(Root,Bib,X), Edge(X,paper,Y),Edge(Y,author,A)
15
A Different ParadigmStructural Recursion

• Data as sets with a union operator
• a3, abone, c5, b4
• a3 U abone,c5 U b4

16
Structural Recursion

• Example retrieve all integers in the data

f(T1 U T2) f(T1) U f(T2) f(L T)
f(T) f() f(V)
if isInt(V) then result V
else
17
Structural Recursion

• What does this do ?

f(T1 U T2) f(T1) U f(T2) f(L T)
if La then bf(T) else Lf(T) f()
f(V) V
Returns the same tree with a-edges replaced by
b-edges
18
Structural Recursion

• What does this do ?

f(T1 U T2) f(T1) U f(T2) f(L T)
LLf(T) f()
f(V) V
Input tree with n nodes Output tree with 2n
nodes (every edge is doubled)
19
Structural Recursion

• Example increase all engine prices by 10

20
Structural Recursion

• Retrieve all subtrees reachable by (a.b).a

a
b
a
21
Structural Recursion General Form
f1(T1 U T2) f1(T1) U f1(T2) f1(L T)
E1(L, f1(T),...,fk(T), T) f1()
f1(V)
. . . .
fk(T1 U T2) fk(T1) U fk(T2) fk(L T)
Ek(L, f1(T),...,fk(T), T) fk()
fk(V)
Each of E1, ..., Ek consists only of _ _, U,
if_then_else_
22
Evaluating Structural Recursion

• Recursive Evaluation
• Compute the functions recursively, starting with
  f1 at the root
• Termination is guaranteed.
• How efficiently can we evaluate this ?

23
Structural Recursion

• Consider this

f(T1 U T2) f(T1) U f(T2) f(L T)
Lf(T), Lf(T) f()
f(V) V
24
Naive Recursive Evaluation
a
a
a
b
b
b
b
b
c
c
c
c
c
c
c
c
c
d
Input tree n nodes Output tree 2n1 1 nodes
25
Efficient Recursive Evaluation
Recursive Evaluation with function
memorization. PTIME complexity.
f(T1 U T2) f(T1) U f(T2) f(L T)
Lf(T), Lf(T) f()
f(V) V
Alternatively apply the function in parallel to
each input edge ? Bulk Evaluation
26
Bulk Evaluation
Sometimes f doesnt return anything ? use ? edges
f(T1 U T2) f(T1) U f(T2) f(L T)
if Lc then T else f(T) f()
f(V) V
27
Epsilon Edges

• Meaning of ? edges

a
b
a
b
?

d
c
c
d
c
d
28
Epsilon Edges

• Note union becomes easy to draw with ? edges
• Example

?
?
T1
T2
U

T1
T2
?
?
a
b
U
a
b
c
d
e

c
d
e

e
a
c
d
b
29
Bulk Evaluation

• Idea apply E1, ..., Ek independently on each
  edge, then connect with ? edges ? PTIME

30
Bulk Evaluation
Recall (a.b).a
a
b
b
a
a
a
a
a
b
d
a
b
b
a
a
c
b
d
a
a
b
d
d
c
b
b
c
c
31
Structural Recursion

• Can evaluate in two ways
• Recursively memorize functions results
• Bulk apply all functions on all edges, in
  parallel, connect, eliminate what is useless
• Complexity PTIME
• More precisely NLOGSPACE
• Works on graphs with cycles too !

32
XSL

• XSLT 1.0 (a recommendation)
• http//www.w3.org/TR/xslt.html
• XSLT 1.1 (a working draft)
• http//www.w3.org/TR/xslt11/
• In commercial products (e.g. IE5.0)

33
XSL

• Purpose stylesheet specification language
• stylesheet XML -gt HTML
• in general XML -gt XML
• Uses XPath

34
XSL Program

• XSL program template-rule ... template-rule
• template-rule match pattern template

Example Retrieve all book titles
ltxsltemplate match /gt
ltxslapply-templates/gt lt/xsltemplategt ltxsltemp
late match /bib//titlegt ltresultgt
ltxslvalue-of select . /gt lt/resultgt lt/xsltem
plategt
35
Simple XSL Program

• Copy the input

ltxsltemplate match /gt
ltxslapply-templates/gt lt/xsltemplategt ltxsltemp
late match text()gt ltxslvalue-of
select./gtlt/xsltemplategt ltxsltemplate match
gt ltxslelement namename(.)gt
ltxslapply-templates/gt
lt/xslelementgt lt/xsltemplategt
36
Flow Control in XSL
ltxsltemplate match /gt ltxslapply-template
s/gt lt/xsltemplategt ltxsltemplate matchagt
ltAgtltxslapply-templates/gtlt/Agt lt/xsltemplategt ltxs
ltemplate matchbgt ltBgtltxslapply-templates/gtlt/
Bgt lt/xsltemplategt ltxsltemplate matchcgt
ltCgtltxslvalue-of/gtlt/Cgt lt/xsltemplategt
37

• ltagt ltegt ltbgt ltcgt 1 lt/cgt
• ltcgt 2 lt/cgt
• lt/bgt
• ltagt ltcgt 3 lt/cgt
• lt/agt
• lt/egt
• ltcgt 4 lt/cgt
• lt/agt

• ltAgt ltBgt ltCgt 1 lt/Cgt
• ltCgt 2 lt/Cgt
• lt/Bgt
• ltAgt ltCgt 3 lt/Cgt
• lt/Agt
• ltCgt 4 lt/Cgt
• lt/Agt

38
XSL is Structural Recursion

• Equivalent to

f(T1 U T2) f(T1) U f(T2) f(L T) if L
c then C t else L b then
B f(t) else L a then A
f(t) else f(t) f()
f(V) V
? ltxsltemplate matchcgt
? ltxsltemplate matchbgt
? ltxsltemplate matchagt
? ltxsltemplate match /gt
XSL query single function XSL query with modes
multiple function (next)
39
Modes in XSLT
Compute the path (a.b)
f(T1 U T2) f(T1) U f(T2) f(a T)
resultT U g(T) f() f(V)
V g(T1 U T2) g(T1) U g(T2) g(b T)
f(T) g() g(V)
V
ltxsltemplate match /gt
ltxslapply-templates modef/gt
lt/xsltemplategt ltxsltemplate match
modef/gt ltxsltemplate matcha modefgt
ltresultgt ltxslcopy-of match./gt lt/resultgt
ltxslapply-templates modeg/gt lt/xsltemplategt lt
xsltemplate match modeggt ltxsltemplate
matchb modeggt ltxslapply-templates
modef/gt lt/xsltemplategt
ltxslcopy-of ... gt copies the input to the output
ignoring modes, this computes (ab)
40
Modes in XSLT

• Mode a name for a group of template rules
• No mode empty mode
• Same as having multiple recursive functions

41
Conflict Resolutionfor Template Rules

• If several template rules match, choose that with
  highest priority.
• Explicit priority ltxsltemplate matchabc
  priority1.41gt
• Computing implicit priority ad-hoc rules given
  by the W3C, based on match
• matchP1 P2 ... ? transform to a set of
  template rules.
• matchabc ? the priority is 0.
• match... some namespace name... ? the
  priority is -0.25.
• matchnode() ? the priority is -0.5.
• Otherwise, the priority is 0.5

It is an error if this leaves more than one
matching template rule.
42
Built-in Template Rules

• Keeps us goingltxsltemplate match /gt
  ltxslapply-templates/gtlt/xsltemplategtthere is
  one such rule for each mode
• Copies what we forgotltxsltemplate match
  text()_at_gtltxslvalue-of select./gtlt/xsltempl
  ategtthere is only one rule, for the empty mode
• Lowest priorities among all rules hence, can be
  easily overridden

43
XSL Template

• ltxsltemplate match expression mode name
  priority number name name gt
• Body
• lt/xsltemplategt
• Default mode priority (computed as
  explained earlier) name when no match, no mode
• Body
• XML constructors ltmyTaggt...lt/myTaggt ltbgt
  ... lt/bgt ...
• XSL instructions
• ltxslapply-templatesgt ( recursive call)
• ltxslvalue-ofgt ( copy the value)
• ltxslcopygt ( shallow copy)
• ltxslcopy-ofgt (deep copy)
• ltxslelementgt ( more flexible than XML
  constructors)
• ltxslattributegt ( add an attribute to the
  element)
• ltxslifgt ( conditional)
• ltxslfor-eachgt
• Instructions for variables

44
XSL Apply Templates

• ltxslapply-templates select expression mode
  name gt
• Body
• lt/xsl apply-templatesgt
• Default
• select (children)
• mode (empty mode)
• Body
• Sort instructions
• Paramemter instructions

45
XSL Variables

• Declaring a variable
• ltxslvariable name vname select valuegt
  value lt/xslvariablegt
• Value either in select, or in body
• Either in ltxsltemplategt ... lt/xsltemplategt or
  at top level
• Declaring a parameter
• ltxslparam select valuegt value lt/xslparamgt
• In ltxsltemplategt ... lt/xsltemplategt, at the
  beginning
• Passing a paramemter
• ltxslwith-param select valuegt value
  lt/xslparamgt
• In ltxslapply-templatesgt ... lt/xslapply-templates
  gt
• Using variables vname

46
XSL and Structural Recursion

• XSL
• mainly on trees
• may loop

• Structural Recursion
• arbitrary graphs
• always terminates

add the following rule
ltxsltemplate match egt
ltxslapply-patterns select//gt lt/xsltemplategt
stack overflow on IE 5.0