Sedna XML Database: Query Parser & Optimizing Rewriter

Sedna Query Parser & Optimizing Rewriter Dmitry Lizorkin [email_address] Ph.D., software developer Sedna team

Goals
Wide range of queries/statements support: XQuery queries, XML update statements, data definition language statements
High performance for both query evaluation and updates execution
Query optimization strategies designed in correspondence with Sedna internal data representation design

Query processing steps query execution plan (QEP): to Sedna executor operation tree Physical Plan Generator Optimizing Rewriter Static Analyzer Parser XML update statement Data Definition Language statement XQuery query

Query parser
Input: 3 types of queries/statements
XQuery queries
XML update statements
Data Definition Language statements (e.g. create document statement)
Output: uniform operation tree for all above 3 query/statement types

Static analyzer
Query static analysis phase
Static context is initialized with XQuery Functions and Operators and augmented with query prolog declarations
Query operation tree is expanded with imported XQuery modules
All namespace prefixes, function names and variable names are resolved
XQuery static errors are reported, if any

Optimizing rewriter step Physical Plan Generator Optimizing Rewriter query execution plan Static Analyzer Parser operation tree XML update statement Data Definition Language statement XQuery query

Optimizing Rewriter
Optimization based on query rewriting
Removing unnecessary ordering operations
Combining abbreviated descendant-or-self path step with a next path step
Removing unnecessary node copies for constructed content
Analyzing nested for-clauses

Ordering operations: challenges
Many XQuery expressions have the semantics for their resulting sequences to be ordered in document order with duplicate nodes removed
“ Distinct-Document Order” semantics is expressed via explicit DDO operations in Sedna query operation tree
DDO operations decrease query execution performance
Require the whole argument sequence to be evaluated before a first result item could be produced
Break execution pipeline

Ordering operations: optimization
Idea: removing unnecessary ordering operations
Analysis: for each operation in the query operation tree, the following properties for the resulting sequence are recursively found out
whether in DDO
whether consists of no more than a single item
whether consists of nodes on a common level of an XML tree
Result: a DDO operation is removed if
either its argument is known to be in DDO, or
DDO is not required for the resulting sequence

Descendant-or-self: challenges
The “//” abbreviated path step (expanded into descendant-or-self::node() ) is frequently used in practical XQuery queries
Expensive to evaluate
Bad selectivity: generally, selects almost all nodes in an XML document
Does not allow to use benefits of Sedna descriptive schema-driven storage strategy

Descendant-or-self: optimization idea
Idea: combining the // step with a next path step
E.g., //para transformed to /descendant::para
Better intermediate selectivity
Benefits of Sedna schema-driven storage
Technical issue: context position/size
“ The path expression //para[1] does not mean the same as the path expression /descendant::para[1] ” (XQuery Spec., Subsect. 3.2.4)

Descendant-or-self: solution
For the // path step, its next step predicate expressions are analyzed (if any)
If predicate expressions results do not depend on context position and size (neither explicitly, nor implicitly),
than the // step can be combined with its next step while keeping the original query semantics

Removing unnecessary node copies
An XQuery constructor semantics implies constructor content being new nodes, “ even if they are copies of existing nodes ”
Problem: making a deep copy of an XML subtree is expensive to evaluate
Idea: avoiding node copies that do not affect query result semantics
Algorithm: a constructed node needs not be copied if
it is a part of the query result sequence, or
it becomes a direct child of another constructed node

Analyzing nested for-clauses
FLWOR-expressions generally contain multiple iteration variables in for-clauses
for $u in doc('users')//user_tuple,
$i in doc('items')//item_tuple
where ... return ...
Binding sequences with nested loop semantics
An expression associated with an inner iteration variable is analyzed
If the associated expression does not depend on outer iteration variables, it is marked as “lazy”
Lazy associated expression value can be evaluated just once, with the query semantics preserved

Physical plan generation step Physical Plan Generator Optimizing Rewriter query execution plan Static Analyzer Parser operation tree XML update statement Data Definition Language statement XQuery query

Query physical plan generation
Query execution plan (QEP) for the query operation tree is constructed
Structural location path fragments are extracted
A path that starts from an XML document node and contains only descending axes and no predicates
Mapped to Sedna descriptive schema access operations
For & order-by clauses are mapped to tuple stream generation & reordering operations respectively

Implementation details
Query parser is implemented with ANTLR parser generator (www.antlr.org)
ANTLR native representation for an operation tree is an S-expression
S-expression is a native data representation for Scheme programming language
Scheme provides extensive native features for S-expressions rewriting purposes
Optimizing rewriter is implemented in Scheme
The Scheme-to-C compiler produces high-performance code

Summary
Complete query static analysis phase and rewriting-based optimization processing (i.e. from query textual representation to physical plan)

Thank you for your attention!

Future work: Cost-based optimization
Problem analysis
Join operations are primary candidates to benefit from cost-based optimization implemented
In XML, the problem of join evaluation is not as vital as for relational data
One-to-many entity relationships can often be modeled via XML elements nesting mechanism
For many-to-many relationships, Sedna explicit indexes can be used

Cost-based optimization: plans
Join operations extraction in a query operation tree
Selectivity estimation mechanism for XQuery expressions
A storage for selectivity statistics is required (XML?)
Cost-based physical plan selection
is a complicated task in general; however, a lot of related work exists
can be relatively easily implemented for simple cases
Hints can be used for complex cases

Sedna XML Database: Query Parser & Optimizing Rewriter

by Ivan Shcheklein on Apr 07, 2010

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Sedna XML Database: Query Parser & Optimizing Rewriter — Presentation Transcript