Command Language

Language

Teiid sends commands to your Translator in object form. These classes are all defined in the "org.teiid.language" package. These objects can be combined to represent any possible command that Teiid may send to the Translator. However, it is possible to notify Teiid that your Translator can only accept certain kinds of constructs via the capabilities defined on the "ExecutionFactory" class. Refer to Translator Capabilities for more information.

The language objects all extend from the LanguageObject interface. Language objects should be thought of as a tree where each node is a language object that has zero or more child language objects of types that are dependent on the current node.

All commands sent to your Translator are in the form of these language trees, where the root of the tree is a subclass of Command. Command has several sub-interfaces, namely:

  • QueryExpression

  • Insert

  • Update

  • Delete

  • BatchedUpdates

  • Call

Important components of these commands are expressions, criteria, and joins, which are examined in closer detail below. For more on the classes and interfaces described here, refer to the Teiid JavaDocs http://docs.jboss.org/teiid/7.6/apidocs.

Expressions

An expression represents a single value in context, although in some cases that value may change as the query is evaluated.  For example, a literal value, such as 5 represents an integer value.  An column reference such as "table.EmployeeName" represents a column in a data source and may take on many values while the command is being evaluated.

  • Expression – base expression interface

  • ColumnReference – represents an column in the data source

  • Literal – represents a literal scalar value.

  • Parameter – represents a parameter with multiple values. The command should be an instance of BatchedCommand, which provides all values via getParameterValues.

  • Function – represents a scalar function with parameters that are also Expressions

  • AggregateFunction – represents an aggregate function which can hold a single expression

  • WindowFunction – represents an window function which holds an AggregateFunction (which is also used to represent analytical functions) and a WindowSpecification

  • ScalarSubquery – represents a subquery that returns a single value

  • SearchedCase, SearchedWhenClause – represents a searched CASE expression.  The searched CASE expression evaluates the criteria in WHEN clauses till one evaluates to TRUE, then evaluates the associated THEN clause.

  • Array – represents an array of expressions, currently only used by the engine in multi-attribute dependent joins - see the supportsArrayType capability.

Condition

A criteria is a combination of expressions and operators that evaluates to true, false, or unknown.  Criteria are most commonly used in the WHERE or HAVING clauses.

  • Condition – the base criteria interface

  • Not – used to NOT another criteria

  • AndOr – used to combine other criteria via AND or OR

  • SubuqeryComparison – represents a comparison criteria with a subquery including a quantifier such as SOME or ALL

  • Comparison – represents a comparison criteria with =, >, <, etc.

  • BaseInCondition – base class for an IN criteria

  • In – represents an IN criteria that has a set of expressions for values

  • SubqueryIn – represents an IN criteria that uses a subquery to produce the value set

  • IsNull – represents an IS NULL criteria

  • Exists – represents an EXISTS criteria that determines whether a subquery will return any values

  • Like – represents a LIKE/SIMILAR TO/LIKE_REGEX criteria that compares string values

The FROM Clause

The FROM clause contains a list of TableReference’s.

  • NamedTable – represents a single Table

  • Join – has a left and right TableReference and information on the join between the items

  • DerivedTable – represents a table defined by an inline QueryExpression

A list of TableReference are used by default, in the pushdown query when no outer joins are used. If an outer join is used anywhere in the join tree, there will be a tree of Join s with a single root. This latter form is the ANSI preferred style. If you wish all pushdown queries containing joins to be in ANSI style have the capability "useAnsiJoin" return true. Refer to Command Form for more information.

QueryExpression Structure

QueryExpression is the base for both SELECT queries and set queries. It may optionally take an OrderBy (representing a SQL ORDER BY clause), a Limit (represent a SQL LIMIT clause), or a With (represents a SQL WITH clause).

Select Structure

Each QueryExpression can be a Select describing the expressions (typically elements) being selected and an TableReference specifying the table or tables being selected from, along with any join information.  The Select may optionally also supply an Condition (representing a SQL WHERE clause), a GroupBy (representing a SQL GROUP BY clause), an an Condition (representing a SQL HAVING clause).

SetQuery Structure

A QueryExpression can also be a SetQuery that represents on of the SQL set operations (UNION, INTERSECT, EXCEPT) on two QueryExpression. The all flag may be set to indicate UNION ALL (currently INTERSECT and EXCEPT ALL are not allowed in Teiid)

With Structure

A With clause contains named QueryExpressions held by WithItems that can be referenced as tables in the main QueryExpression.

Insert Structure

Each Insert will have a single NamedTable specifying the table being inserted into.  It will also has a list of ColumnReference specifying the columns of the NamedTable that are being inserted into. It also has InsertValueSource, which will be a list of Expressions (ExpressionValueSource) or a QueryExpression

Update Structure

Each Update will have a single NamedTable specifying the table being updated and list of SetClause entries that specify ColumnReference and Expression pairs for the update. The Update may optionally provide a criteria Condition specifying which rows should be updated.

Delete Structure

Each Delete will have a single NamedTable specifying the table being deleted from. It may also optionally have a criteria specifying which rows should be deleted.

Call Structure

Each Call has zero or more Argument objects. The Argument objects describe the input parameters, the output result set, and the output parameters.

BatchedUpdates Structure

Each BatchedUpdates has a list of Command objects (which must be either Insert, Update or Delete) that compose the batch.

Language Utilities

This section covers utilities available when using, creating, and manipulating the language interfaces.

Data Types

The Translator API contains an interface TypeFacility that defines data types and provides value translation facilities. This interface can be obtained from calling "getTypeFacility()" method on the "ExecutionFactory" class.

The TypeFacitlity interface has methods that support data type transformation and detection of appropriate runtime or JDBC types.  The TypeFacility.RUNTIME_TYPES and TypeFacility.RUNTIME_NAMES interfaces defines constants for all Teiid runtime data types.  All Expression instances define a data type based on this set of types.  These constants are often needed in understanding or creating language interfaces.

Language Manipulation

In Translators that support a fuller set of capabilities (those that generally are translating to a language of comparable to SQL), there is often a need to manipulate or create language interfaces to move closer to the syntax of choice.  Some utilities are provided for this purpose:

Similar to the TypeFacility, you can call "getLanguageFactory()" method on the "ExecutionFactory" to get a reference to the LanguageFactory instance for your translator.  This interface is a factory that can be used to create new instances of all the concrete language interface objects.

Some helpful utilities for working with Condition objects are provided in the LanguageUtil class.  This class has methods to combine Condition with AND or to break an Condition apart based on AND operators.  These utilities are helpful for breaking apart a criteria into individual filters that your translator can implement.

Runtime Metadata

Teiid uses a library of metadata, known as "runtime metadata" for each virtual database that is deployed in Teiid. The runtime metadata is a subset of metadata as defined by models in the Teiid models that compose the virtual database.  While builing your VDB in the Designer, you can define what called "Extension Model", that defines any number of arbitary properties on a model and its objects. At runtime, using this runtime metadata interface, you get access to those set properties defined during the design time, to define/hint any execution behavior.

Translator gets access to the RuntimeMetadata interface at the time of Excecution creation. Translators can access runtime metadata by using the interfaces defined in org.teiid.metadata package.  This package defines API representing a Schema, Table, Columns and Procedures, and ways to navigate these objects.

Metadata Objects

All the language objects extend AbstractMetadataRecord class

  • Column - returns Column metadata record

  • Table - returns a Table metadata record

  • Procedure - returns a Procedure metadata record

  • ProcedureParameter - returns a Procedure Parameter metadata record

Once a metadata record has been obtained, it is possible to use its metadata about that object or to find other related metadata.

Access to Runtime Metadata

The RuntimeMetadata interface is passed in for the creation of an "Execution". See "createExecution" method on the "ExecutionFactory" class. It provides the ability to look up metadata records based on their fully qualified names in the VDB.

The process of getting a Table’s properties is sometimes needed for translator development.  For example to get the "NameInSource" property or all extension properties:

Obtaining Metadata Properties
//getting the Table metadata from an Table is straight-forward
Table table = runtimeMetadata.getTable("table-name");
String contextName = table.getNameInSource();

//The props will contain extension properties
Map<String, String> props = table.getProperties();

Language Visitors

Framework

The API provides a language visitor framework in the org.teiid.language.visitor package.  The framework provides utilities useful in navigating and extracting information from trees of language objects.

The visitor framework is a variant of the Visitor design pattern, which is documented in several popular design pattern references.  The visitor pattern encompasses two primary operations: traversing the nodes of a graph (also known as iteration) and performing some action at each node of the graph.  In this case, the nodes are language interface objects and the graph is really a tree rooted at some node.  The provided framework allows for customization of both aspects of visiting.

The base AbstractLanguageVisitor class defines the visit methods for all leaf language interfaces that can exist in the tree.  The LanguageObject interface defines an acceptVisitor() method – this method will call back on the visit method of the visitor to complete the contract.  A base class with empty visit methods is provided as AbstractLanguageVisitor.  The AbstractLanguageVisitor is just a visitor shell – it performs no actions when visiting nodes and does not provide any iteration.

The HierarchyVisitor provides the basic code for walking a language object tree.  The HierarchyVisitor performs no action as it walks the tree – it just encapsulates the knowledge of how to walk it.  If your translator wants to provide a custom iteration that walks the objects in a special order (to exclude nodes, include nodes multiple times, conditionally include nodes, etc) then you must either extend HierarchyVisitor or build your own iteration visitor.  In general, that is not necessary.

The DelegatingHierarchyVisitor is a special subclass of the HierarchyVisitor that provides the ability to perform a different visitor’s processing before and after iteration.  This allows users of this class to implement either pre- or post-order processing based on the HierarchyVisitor.  Two helper methods are provided on DelegatingHierarchyVisitor to aid in executing pre- and post-order visitors.

Provided Visitors

The SQLStringVisitor is a special visitor that can traverse a tree of language interfaces and output the equivalent Teiid SQL.  This visitor can be used to print language objects for debugging and logging.  The SQLStringVisitor does not use the HierarchyVisitor described in the last section; it provides both iteration and processing type functionality in a single custom visitor.

The CollectorVisitor is a handy utility to collect all language objects of a certain type in a tree. Some additional helper methods exist to do common tasks such as retrieving all elements in a tree, retrieving all groups in a tree, and so on.

Writing a Visitor

Writing your own visitor can be quite easy if you use the provided facilities.  If the normal method of iterating the language tree is sufficient, then just follow these steps:

Create a subclass of AbstractLanguageVisitor.  Override any visit methods needed for your processing.  For instance, if you wanted to count the number of elements in the tree, you need only override the visit(ColumnReference) method.  Collect any state in local variables and provide accessor methods for that state.

Decide whether to use pre-order or post-order iteration. Note that visitation order is based upon syntax ordering of SQL clauses - not processing order.

Write code to execute your visitor using the utility methods on DelegatingHierarchyVisitor:

// Get object tree
LanguageObject objectTree = …

// Create your visitor initialize as necessary
MyVisitor visitor = new MyVisitor();

// Call the visitor using pre-order visitation
DelegatingHierarchyVisitor.preOrderVisit(visitor, objectTree);

// Retrieve state collected while visiting
int count = visitor.getCount();

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