XML Publishing

1
XML Publishing

• XML publishing overview
• SilkRoute (ATT)
• XPERANTO (IBM Research)
• Clio (IBM Research and U. Toronto)
• Schema-directed XML Publishing
• ATG (Bell Labs and U. Edinburgh)
• PRATA implementation of ATG
• Extensions of ATG

2
XML -- its an exchange format
Data base
Data base
HTML
Data base!

• Most data is stored in pre-existing databases
• Need to provide XML wrappers to export data

3
XML publishing

• An XML view definition language specifying
  desired mapping
• Efficient implementation of the view language

Q XML view
4
From relations to XML Views
ltActorgt ltLnamegtViterellilt/Lnamegt
ltFnamegtJoelt/Fnamegt ltMovie year1999gt Analyze
This lt/Moviegt ltMovie year2001gt See Spot
Run lt/Moviegt lt/Actorgt ltActorgt
ltLnamegtWinterlt/Lnamegt ltFnamegtAlexlt/Fnamegt
ltMovie year1988gt Bill and Teds Excellent
Adventure lt/Moviegt
Actor(aid, lname, fname) lt001, Viterelli,
Joegt, Movie(mid, title, year) lt011,
Analyze This, 1999gt, lt032, See Spot Run,
2001gt Appearance(mid, aid) lt001, 011gt,
lt001, 032gt
5
Commercial systems -- canonical publishing

• Canonical publishing the universal-relation
  approach
• Embedding single SQL query in XSL stylesheet
• Result canonical XML representation of
  relations
• Systems
• Oracle 10g XML SQL facilities SQL/XML, XMLGen
• IBM DB2 XML Extender SQL/XML, DAD
• Microsoft SQL Server 2005 FOR-XML, XSD
• incapable of expressing practical XML publishing
  default fixed XML document template

6
Canonical publishing
A.lname A.fname M.year M.title Viterelli
Joe 1999 See Spot Run Viterelli Joe
2001 Analyze This
IBM DB2 SQL Statement giving big relation
ltSQL_stmtgt SELECT A.lname,A.fname, M.year,
M.title FROM Movie M, Actor A, Appearance
Ap WHERE M.midAp.mid AND M.aidA.Aid ORDER BY
aid lt/SQL_stmtgt
ltActorgt ltLnamegtViterellilt/Lnamegt
ltFnamegtJoelt/Fnamegt ltMovie year1999gt Analyze
This lt/Moviegt ltMovie year2001gt See Spot
Run lt/Moviegt lt/Actorgt ltActorgt
ltLnamegtWinterlt/Lnamegt ltFnamegtAlexlt/Fnamegt
Formatting Template annotated with columns of
the universal relation
ltelement_node Actorgt ltelement_node Lnamegt
lttext_nodegt ltColumn nameA.lname/gt
lt/text_nodegt
7
Middleware Approach
View Definition
Query-cost Estimates Source Capabilities
Query Generator
Request
SQL Queries
Tagger
8
XPERANTO

• Commercial system IBM DB2 XML extender,
  SQL/XML
• Middleware (vendor-independent) XPERANTO
• Extending SQL with XML constructors
• select XML-aggregation
• from R1, . . ., Rn
• where conditions
• XML constructors (XML-aggregation) functions
• Input tables and XML trees (forest)
• Output XML tree

9
XML publishing with XPERANTO (SQL/XML)

• Relational schema
• Extended SQL
• select XMLAGG( ACT(lname, fname,
• select XMLAGG ( MOV(title, year)
• from Appearance Ap, Movie M
• where Ap.aid A.aid and Ap.mid
  M.mid
• group order by A.lname, A. fname
  ))
• from Actor A

ltActorgt ltLnamegtViterellilt/Lnamegt
ltFnamegtJoelt/Fnamegt ltMovie year1999gt . . .
lt/Moviegt ltMovie year2001gt . . .
lt/Moviegt lt/Actorgt ltActorgt ltLnamegtWinterlt/Lnamegt
ltFnamegtAlexlt/Fnamegt
10
XML constructors

• Actor constructor
• create function ACT(lname str, fname str,
  mlist XML)
• ltActorgt
• ltLnamegt lname lt/Lnamegt
• ltFnamegt fname lt/Fnamegt
• mlist
• lt/Actorgt
• Movie constructor (mlist)
• create function Mov(title str, year int)
• ltMovie yearyeargt title lt/Moviegt
• Verbose and cumbersome
• small document tedious
• large documents unthinkable

11
SilkRoute

• Annotated template embedding SQL in a fixed XML
  tree
• Middleware SilkRoute
• Commercial SQL Server 2005 XSD, IBM DB2 DAD
• Advantages
• More modular comparing to the universal
  relation approach
• Limited schema-driven conforming to a fixed doc
  template

12
Clio

• Schema mapping based on inter-schema constraints
• Given two schemas S1 and S2, we want to translate
  instances of S1 to instances of S2.
  Constraint-driven approach
• Start with a set of referential integrity
  constraints (foreign keys) from S1 to S2 (schema
  matching)
• Derive schema mapping from S1 to S2, by reasoning
  about the given integrity constraints (chasing
  technique)
• Pros and cons
• Generic system, and can be used for XML
  publishing
• Semi-automated
• Chasing may not terminates for XML DTD/Schema
• Recursive target schema (S2)? unclear

13
Getting real data exchange on the Web

• All members of a community (industry) agree on a
  DTD and then exchange data w.r.t. it
  e-commerce, health-care, ...
• XML Publishing
• mapping relational data to XML
• conforming to the predefined DTD

Web
DTD
XML
XML
Q XML view
DB1
DB2
14
Data exchange insurance company and hospital

• Daily report
• Relational database R at the hospital
• Patient (SSN, name, tname, policy, date)
• inTreatment (tname, cost)
• outTreatment (tname, referral)
• Procedure (tname1, tname2)
• treatment
• in hospital composition hierarchy in Procedure
• outside of the hospital referral

hospital
insurance company
XML view
R
XML
15
Example insurance company and hospital

• DTD D predefined by the insurance company
• report ? patient
• patient ? SSN, pname,
  treatment, policy
• treatment ? tname, (inTreatment
  outTreatment)
• inTreatment ? treatment
• outTreatment ? referral
• How to define a mapping ? such that for any
  instance DB of R,
• ? (DB) is an XML document containing all the
  patients and their treatments (hierarchy,
  referral) from DB, and
• ? (DB) conforms to D?

16
Challenge recursive type

• XML data unbounded depth -- cannot be decided
  statically
• treatment ? tname, (inTreatment
  outTreatment)
• inTreatment ? treatment ---
  recursive

SSN
123
17
Challenge non-determinism

• The choice of a production (element type
  definition)
• treatment ? tname, (inTreatment outTreatment)
• -- depends on the underlining relational data

report
18
Existing systems

• fixed XML tree template or ignoring
  DTD-conformance
• middleware SilkRoute (ATT), XPERANTO (IBM),
• systems SQL Server 2005, IBM DB2 XML extender,
• incapable of coping with a predefined DTD (e.g.
  recursion)
• type checking define a view and then check its
  conformance
• undecidable in general, co-NEXPTIME for extremely
  restricted view definitions
• no guidance on how to define XML views that
  typecheck
• one gets an XML view that typechecks only after
  repeated failures and with luck

19
XML Publishing

• XML publishing overview
• SilkRoute (ATT)
• XPERANTO (IBM Research)
• Clio (IBM Research and U. Toronto)
• Schema-directed XML Publishing
• ATG (Bell Labs and U. Edinburgh)
• PRATA implementation of ATG
• Extensions of ATG

20
Attribute Translation Grammar (ATG)

• DTD normalized element type definitions e ? ?
• ? PCDATA ? e1, , en e1
  en e
• Attributes e associated with each element type
  e
• e tuple-valued, to pass data value as well as
  control
• Rules associated with each e ? ? for e in ?,
  e Q(e)
• SQL query Q extracts data from DB
• parent attribute e as a constant parameter in Q
  

21
Semantics conceptual evaluation

• Top-down
• report ? patient
• patient ? select SSN, name, tname,
  policy
• from Patient --- SQL
  query
• recall Patient (SSN, name, tname, policy)
• Data-driven a patient element for each tuple in
  Patient relation

patient
patient
patient
patient
22
Inherited attributes

• Inherited child is computed using parent
• patient ? SSN, name, treatment, policy
• SSN patient.SSN, name
  patient.name
• treatment patient.tname policy
  patient.policy
• recall patient (SSN, name, tname, policy)
• SSN ? PCDATA
• PCDATA SSN

report
...
patient
patient
patient
patient
patient
treatment
name
policy
SSN
SSN
123
Joe
LU23
PCDATA
23
Coping with non-determinism

• treatment ? tname, (inTreatment
  outTreatment)
• tname treatment
• (inTreatment, outTreatment)
• case Qc(treatment).tag
  --- conditional query
• 1 (treatment, null)
• else (null, treatment)
• Qc select 1 as tag from inTreatment where
  tname treatment
• conditional query the choice of production
• parent as constant parameter in SQL query

...
treatment
treatment
tname
tname
inTreatment
outTreatment
24
Coping with recursion

• inTreatment ? treatment
• treatment ? select tname2
• from Procedure
• where inTreatment tname1
• recall Procedure (tname1, tname2)
• parent as constant parameter in SQL query Q
• inTreatment is further expanded as long as Q(DB)
  is nonempty

treatment
treatment
25
DTD-directed publishing with ATGs

• DTD-directed the XML tree is constructed
  strictly following the productions of a DTD ---
  DTD conformance
• Data-driven the choice of productions and
  expansion of the XML tree (recursion) depends on
  relational data

report
...
patient
patient
patient
patient
treatment
name
SSN
inTreatment
...
tname
Joe
123
treatment
treatment
...
...
26
ATGs vs. existing systems

• DTD-conformance
• ATGs provide guidance for how to define
  DTD-directed publishing
• Other systems based on a fixed tree template
• Expressive power strictly more expressive than
  others
• ATGs capable of expressing XML views supported
  by other systems
• Other systems cannot handle recursion/nondetermin
  ism

27
ATGs vs. Attribute Grammars (AGs)

• AGs
• Definition w.r.t. a CFG
• Evaluation parse a string with the CFG, then
  evaluate attributes given the parse tree
• ATGs combining DTD and database operations
• Definition w.r.t. an ECFG (DTD) and SQL queries
• Evaluation given DB, extract relevant data from
  DB with SQL queries to build an XML tree of the
  DTD
• It does not make sense to parse a database
  w.r.t. a DTD
• ATGs are not a mild variation of AGs

28
XML Publishing

• XML publishing overview
• SilkRoute (ATT)
• XPERANTO (IBM Research)
• Clio (IBM Research and U. Toronto)
• Schema-directed XML Publishing
• ATG (Bell Labs and U. Edinburgh)
• PRATA implementation of ATG
• Possible extensions of ATG

29
PRATA middleware based on ATGs
query plan generation, evaluation
relations
ATG graph
XML
ATG
tagging
parsing
cost estimate query
statistics query results

• R

• ATG graph representing the ATG
• relations representing root-leaf paths ---gt
• tagging one pass

30
Evaluation of ATGs

• Conceptual evaluation defining semantics, but
  not efficient
• Techniques proved useful
• partitioning reduce db visits, make use of DBMS
  optimizer (handling annotated templates
  complete tree)
• Relations representing XML trees root-leaf paths
• Separate tagging

31
ATG graph

• edge labels SQL queries
• cyclic introduced by recursion -- not in
  other systems
• iterative unfolding partial ATG tree to a
  depth d

report
Q1

patient
Q5
Q2
policy
Q3
Q4
Q8
name
SSN
treatment
Q7
Qc
tname
outTreatment
inTreatment
Q9
Q6
referral
procedure

32
Partitioning partial ATG trees

• query composition (outer join)
• challenge how to partition into clusters?
• finer ? fewer nulls, smaller queries sent to
  DBMS
• coarser ? leverage DBMS optimizer, less DB visits
• Finding an optimal strategy NP-hard

report
composed Q
Q1

patient
Q2
Q5
Q3
SSN
policy
Q4
name
composed Q
treatment
Qc
Q7
outTreatment
tname
inTreatment
Q9
Q6
procedure
referral
...

Q8
treatment
33
Materialization of intermediate results

• Partitioning root-leaf relation R ?
  intermediate R1, R2, R3, R4
• repeated computation each Ri carrying a key from
  the root
• materialization storing keys in temporary
  tables
• cons overhead with temporary tables
• pros reducing recomputation in descendant
  queries
• What keys to materialize? Not supported by other
  systems

output relation R
R1
sort merge
key
R2
R3
sort merge
key
R4
34
Query plan generation

• New challenge find best partitioning and
  materialization
• mutually dependent partitioning and
  materialization must be taken together
• expensive exponentially many choices NP-hard
• Efficient cost-based heuristic
• DBMS statistics arity/cardinality of a table,
  cost to evaluate a query/create temporary table
• cost estimate simple formulae for computing the
  benefit of materialization/partitioning --
  effective in practice
• optimal strategy -- dynamic programming
  decides partitioning w.r.t. the benefit of
  materialization

35
Putting these together

• ATGs
• ensure DTD-conformance automatically
• support recursion/non-determinism naturally
• simple no need for a new language knowing DTD
  SQL is all that one needs for writing an ATG
• PRATA middleware based on ATGs
• novel technique of combining partitioning and
  materialization
• a practical solution for DTD-directed publishing
• ATG the first systematic way for DTD-directed
  publishing

36
XML Publishing

• XML publishing overview
• SilkRoute (ATT)
• XPERANTO (IBM Research)
• Clio (IBM Research and U. Toronto)
• Schema-directed XML Publishing
• ATG (Bell Labs and U. Edinburgh)
• PRATA implementation of ATG
• Extensions of ATG

37
Extension Capturing integrity constraints

• XML schema
• Type (DTD)
• Integrity constraints keys, foreign keys
• Schema-directed XML publishing automatically
  guarantee that the target document both conforms
  to the type and satisfies the constraints -- in
  a single framework
• Challenge consistency analyses undecidable

DTD
XML ATG
DB
38
Data integration in XML

• multiple, heterogeneous data sources
  multi-source queries, query decomposition, object
  fusion,
• distributed sources scheduling of query
  execution
• schema-conformance,

query
answer
schema
(D, ?)
XML
query translation
integration
Integration middleware
updates
DB4
DB1
DB2
DB3
39
Example hospital and insurance company

• patient (SSN, name, policy)
• visitInfo (SSN, trId, date)

billing (trId, price)
cover (policy, trId)
daily XML report
treatment (trId, name) procedure (trId1, trId2)

• Given a date, for each patient of that day,
  report
• SSN, name (DB1)
• treatments (hierarchy) covered by insurance
  (DB1, DB3, DB4)
• cost of all and only those treatments received
  (DB2)

40
Predefined schema (D, ?) DTD

• report ? patient
• patient ? SSN, name,
  treatments, bill
• treatments ? treatment
• treatment ? trId, tname,
  treatments
• bill ? item
• item ? trId, price

report
date
patient
patient
patient
treatments
bill
name
SSN
item
treatment
item
treatment
. . .
price
trId
trId
tname
treatments
41
Predefined schema (D, ?) XML constraints ?

• constraints relative to each patient,
• key each treatment is charged only once
• patient ( item.trId ? item )
• foreign key every treatment has a billing record
  
• patient ( treatment.trId ? item.trId )

report
date
patient
patient
patient
name
treatments
SSN
bill
treatment
item
item
treatment
. . .
price
trId
treatments
42
More on XML integrity constraints

• absolute on the entire document
• key country.name ? country
• relative on a subdocument rooted at a country
• key country (province.name ?
  province)
• foreign key country (capital.inProvince ?
  province.name)
• (value inclusion)

43
Challenge context dependency

• bill subtree all and only the trIds in the
  treatments subtree
• controlled derivation the bill subtree cannot be
  started before the treatments subtree is
  completed.
• information passing downward, upward, sideways

date
report
patient
date
SSN
treatments
name
SSN
bill
trIdS
trId
trId
unbounded
44
Challenges

• DTD-conformance recursive, nondeterministic
• integrity constraints validation during
  document generation
• multi-source queries a single one involves
  several databases
• context-dependency not strictly top-down or
  bottom-up
• Previous work
• XML publishing single data source, no
  constraints, top-down.
• XML integration little support for XML
  schema-conformance.
• XML query languages type checking provide no
  guidance for how to ensure schema-conformance
  optimization hard.

Schema-directed XML integration is nontrivial!
45
Middleware for schema-directed integration

view definition language
optimization techniques

• A lightweight language Attribute Integration
  Grammar (AIG)
• Cost-based optimization in light of
  context-dependency

DTD constraints
semantic attributes

semantic rules
46
Attribute Integration Grammar (AIG)

• DTD element type definitions e ? ?
• ? PCDATA ? e1, , en
  e1 en e
• Attributes associated with each element type e
• Inh(e) inherited from parent/siblings
  (top-down/sideways)
• Syn(e) synthesized from children (bottom-up)
• Syn(e), Inh(e) tuple or set/bag-valued
• Rules associated with each production e ? ?, for
  e in ?
• Inh(child) Inh(e) Q(Inh(parent),
  Syn(sibling) )
• Syn(parent) Syn(e) U (Syn(children))
  -- union
• Q multi-source SQL query with parameters
• Dependency e2 must be evaluated before e1 if
• Inh(e1) Q( Syn(e2) ) -- acyclic graph
  (DAG)

47
AIG semantics conceptual evaluation

• following the dependency ordering starting from
  the root
• report ? patient
• Inh(patient) ? Q1 (Inh(report))
• select Inh(report) as date, p.SSN,
  p.name, p.policy
• from DB1 patient p, DB1
  visitInfo v
• where p.SSN v.SSN and v.date
  Inh(report)
• Recall DB1 patient (SSN, name, policy),
  visitInfo (SSN, trId, date)
• Parameter in a query Inh(report) as a constant
• Data driven the number of patients depends on Q1

report
Inh
patient
patient
patient
48
Multi-source query

• patient ? SSN, name, treatments, bill
• Inh(SSN) Inh(patient).SSN, . . . ,
• Inh(treatments) Q2(v) -- v
  Inh(patient)
• select t.trId, t.tname
• from DB1 visitInfo i, DB3 cover
  c, DB4 treatment t
• where i.SSN v.SSN and i.date
  v.date and t.trId i.trId
• and c.trId i.trId and c.policy
  v.policy
• a single query uses DB1, DB3 and DB4
• tuple- and set-valued attributes (Inh(SSN),
  Inh(treatments))

• Recall DB1 patient(SSN, name, policy),
  visitInfo(SSN, trId, date)
• DB3 cover (policy, trId)
• DB4 treatment (trId, name),
  procedure (trId1, trId2)

49
Initial top-down pass context-dependent

• patient ? SSN, name, treatments, bill
• Inh(SSN) Inh(patient).SSN, Inh(name)
  Inh(patient).name,
• Inh(treatments) Q2(Inh(patient))
• Inh(bill) Syn(treatments) lt--
  halt
• DTD-directed generate children following the
  production
• Inh(bill) defined with sibling --
  Syn(treatments), dependency ordering evaluate
  bill after treatments

lt- - halt
50
Initial top-down pass recursion

• treatments ? treatment
• Inh(treatment) ? Inh(treatments) -- set of
  (trId, tname)
• Data driven treatments expansion depends on
  Inh(treatments)
• empty expansion terminates, Syn(treatments) is
  empty
• nonempty expands.

51
Leaf step

• treatment ? trId, tname, treatments
• Inh(trId) Inh(treatment).trId, . . .,
• trId ? PCDATA

Syn(trId) Inh(trId)
treatments
Inh (trId, tname)
treatment
treatment
treatments
tname
trId
Inh
52
Bottom-up step synthesize attributes

• treatment ? trId, tname, treatments
• treatments ? treatment
• Syn(treatments) U Syn(treatment)
• Processing of an element e Inh(e) ? subtree(e) ?
  Syn(e)

Syn(treatment) Syn(trId) ? Syn(treatments)
53
Sideways step controlled derivation

• patient ? SSN, name, treatments, bill
• Inh(bill) Syn(treatments)
• bill ? item
• Inh(item) ? Q(Inh(bill) )
• select trId, price
• from DB2 billing
• where trId in Inh(bill) -- set
  membership test
• DTD-directed each step of construction follows a
  production

Recall DB2 billing (trId, price)
54
Constraint compilation

• Captured with rules on synthesized attributes of
  patient
• trIdB bag-valued, collecting trIds under item
• trIdS1 set-valued, collecting trIds under
  treatment
• trIdS2 set-valued, collecting trIds under item
• key patient ( item.trId ? item )
• unique (Syn(patient).trIdB) -- no
  duplicates in the bag
• foreign key patient ( treatment.trId ?
  item.trId )
• subset ( Syn(patient).trIdS1,
  Syn(patient).trIdS2)
• compilation semantic rules and attributes for
  constraints are automatically generated and
  evaluated

55
Advantages of AIG

• DTD-directed view definition automatically
  ensures conformance to DTD -- recursive,
  nondeterministic
• Constraint compilation automatically captures
  integrity constraints in a uniform framework
• performance avoid post-materialization checking
• optimization jointly with query evaluation
• exception handling actions when constraints are
  violated
• Controlled derivation supports context-dependent
  generation.
• Information passing top-down, bottom-up,
  sideways
• Multi-source queries optimizer-based
  decomposition
• One sweep each node is visited at most twice
  evaluates its inherited attribute, subtree, then
  its synthesized attribute

56
Middleware evaluation of AIGs
optimizer
AIG
XML
merging
query plan execution
pre-processing
tagging
scheduling
data
query
DB3
cost statistics
DB2

• pre-processing
• constraint compilation
• multi-source query decomposition ? single-source
  queries
• optimizer query plan generation using cost
  statistics
• execution SQL queries ? data sources, results
  ? mediator
• tagging relational tables (paths from root) ?
  XML view via merge-sorting

• DB1

57
Optimization

• Goal reduce response time
• costs query execution, data transfer, storage
  (caching)
• constraints query dependency graph (DAG)
• nodes queries computing inherited/synthesized
  attributes
• edges dependency relation (producer-consumer
  relation)
• recursion iterative unfolding by a certain depth
• Optimization techniques
• query merging
• query scheduling

58
Query scheduling

• Goal reduce the total response time by
  increasing parallelism
• Ordering execution of queries on the same site
• e.g., ltQ4, Q5, Q3gt vs. ltQ3, Q5, Q4gt on DB2
• Constraints
• costs execution, communication, caching
  overheads
• dependency relation
• Finding an optimal schedule NP-hard

59
Query merging

• Goal reduce DB visits, leverage DB optimizer
• Composition of queries on the same site
  outer-join/union
• Tradeoffs
• large result tables with null
  communication/caching cost
• impact on scheduling -- changing query dependency
  graph

Finding an optimal strategy for merging and
scheduling NP-hard
60
Cost-based heuristic scheduling

• cost estimate given a fixed schedule, estimate
  the completion time of a query Q, comp_time(Q)
• statistics eval_cost(Q), size(Q), trans_cost(S,
  S, size)
• dependency Q cant start before comp_time(Q) if
  Q -gt Q
• scheduling given a fixed query dependency graph,
  a heuristic based on dynamic programming to
• find the most costly trailing path of each
  query
• sort queries to favor critical paths

61
Cost-based heuristic merging

• greedy algorithm repeat until no further
  improvement
• merge each pair of queries on the same source
• modify the query dependency graph accordingly
• invoke scheduling w.r.t. the modified dependency
• estimate the cost
• pick the pair with the biggest improvement to
  merge

Interaction between query scheduling and merging
62
AIG Summary

• AIG a novel specification language
• ensures DTD-conformance (recursion/nondeterminism)
  
• captures integrity constraints in a uniform
  framework
• supports complex transformations controlled
  derivation (context-dependency), multi-source
  queries, . . .
• Optimization techniques nontrivial optimization
  problems
• constraint compilation, multi-source query
  decomposition
• query scheduling w.r.t. query dependency graph
• query merging and its interaction with scheduling

63
More on Commercial System MS SQL Server 2005

• Annotated schema (XSD) fixed tree templates
• nonrecursive schema
• associate elements and attributes with table and
  column names
• Given a relational database, XSD populates an XML
  elements/attributes with corresponding
  tuples/columns
• FOR-XML
• An extension of SQL with an FOR-XML construct
• Nested FOR-XML to construct XML documents
• Summary
• incapable of supporting schema-directed
  publishing
• cant define recursive XML views (bounded
  recursion depth)

64
Commercial System IBM DB2 XML Extender

• User-defined mapping through DAD (Document Access
  Definition) a fixed XML tree template
  (nonrecusive)
• SQL mapping a single SQL query, constructing XML
  trees of depth bounded by the arity of the tuples
  returned and group-by
• RDB node mapping a fixed tree template with
  nodes annotated with conjunctive queries
• SQL/XML an extension of SQL with XML
  constructors (XMLAGG, XMLELEMENT, etc) as
  discussed earlier
• Summary
• incapable of supporting schema-directed
  publishing
• cant define recursive XML views

65
Commercial System Oracle 10g XML DB

• SQL/XML
• DBMS_XMLGEN, a PL/SQL package
• Supports recursive XML view definition (via
  linear recursion of SQL99)
• does not support schema-directed XML publishing

66
Summary and review

• Why publish XML data? What is the major
  difficulty?
• One can publish relational data via an XQuery
  view. How to do it? What are the pros and cons of
  this approach?
• What is schema-directed publishing? Why do we
  need it?
• Why does ATG automatically ensure
  DTD-conformance?
• How to ensure both DTD conformance and constraint
  satisfaction?
• Homework
• Consider projects related to ATG
• How to answer queries over XML views? Read papers
  on query composition (SilkRoute, XPERANTO).