//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();
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-classes, namely:
Insert- also represents an upsert, see the isUpsert flag.
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 https://docs.jboss.org/teiid/16.0.0/apidocs/.
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 BulkCommand, 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.
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
NamedTable– represents a single Table
Join– has a left and right
TableReferenceand information on the join between the items
DerivedTable– represents a table defined by an inline
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 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).
QueryExpression can be a
Select describing the expressions (typically
ColumnReference`s) 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).
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 clause contains named
QueryExpressions held by
WithItems that can be referenced as tables in the main
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
Update will have a single
NamedTable specifying the table being updated and list of
SetClause entries that specify
Expression pairs for the update. The Update may optionally provide a criteria
Condition specifying which rows should be updated.
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 has zero or more
Argument objects. The
Argument objects describe the input parameters, the output result set, and the output parameters.
BatchedUpdates has a list of
Command objects (which must be either
Delete) that compose the batch.
This section covers utilities available when using, creating, and manipulating the language interfaces.
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.
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.
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. Extension metadata may be associated via the OPTIONS clause. 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.
All the language objects extend
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:
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.
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.
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.
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.
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.
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 `ColumnReference`s 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
ColumnReference`s 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();