| java.lang.Object
 org.apache.derby.impl.sql.execute.SqlXmlExecutor
| SqlXmlExecutor | | public class SqlXmlExecutor (Code) | | This class is really just an execution time "utility" that
makes calls to methods on the XMLDataValue interface. Instances
of this class are generated at execution time by the various
Derby XML operators--one instance for each row in the target
result set--and then the appropriate operator call is made on
that instance (see, for example, the generateExpression() methods
in UnaryOperatorNode and BinaryOperatorNode). When an instance
of this class is instantiated, one of the arguments that can be
provided is an id that is used to retrieve an already-constructed
(from compilation time) instance of SqlXmlUtil from the current
Activation. When it comes time to execute the operator, this class
just makes the appropriate call on the received XMLDataValue object
and passes in the SqlXmlUtil, from which the XMLDataValue can
retrieve compile-time objects. The XMLDataValue can also make
calls to various XML-specific utilities on the SqlXmlUtil
object.
Let's take an example. Assume the statement that the user
wants to execute is:
select id from xtable
where XMLEXISTS('/simple' PASSING BY REF xcol)
At compilation time we will compile the expression "/simple"
and store the compiled version of the query into an instance
of SqlXmlUtil. Then we will save that instance of SqlXmlUtil
as an object in the statement activation, from which we will
receive an id that can be used later to retrieve the object
(i.e. to retrieve the SqlXmlUtil). Then, for *each* row
in xtable, we'll generate the following:
boolean result =
(new SqlXmlExecutor(activation, compileTimeObjectId)).
XMLExists("/simple", xcol);
In other words, for each row we create a new instance of
this class and call "XMLExists" on that instance. Then,
as seen below, we retrieve the SqlXmlUtil from the activation
and pass that into a call to "XMLExists" on the XML value
itself (i.e. xcol). XMLDataValue.XMLExists() then uses the
methods and objects (which include the compiled query
expression for "/simple") defined on SqlXmlUtil to complete
the operation.
Okay, so why do we use this execution-time SqlXmlExecutor class
instead of just generating a call to XMLDataValue.XMLExists()
directly? The reason is that we only want to compile the XML
query expression once per statement--and where possible we'd
also like to only generate re-usable XML-specific objects
once per statement, as well. If instead we generated a call to
XMLDataValue.XMLExists() directly for each row, then we would
have to either pass in the expression string and have XMLDataValue
compile it, or we would have to compile the expression string
and then pass the compiled object into XMLDataValue--in either
case, we'd end up compiling the XML query expression (and creating
the corresponding XML-specific objects) once for each row in
the target result set. By using the "saveObject" functionality
in Activation along with this SqlXmlExecutor class, we make
it so that we only have to compile the XML query expression and
create XML-specific objects once (at compile time), and then
we can re-use those objects for every row in the target
result set. Yes, we're still creating an instance of this
class (SqlXmlExecutor) once per row, and yes we have to fetch
the appropriate SqlXmlUtil object once per row, but this is
still going to be cheaper than having to re-compile the query
expression and re-create XML objects for every row.
So in short, this class allows us to improve the execution-time
performance of XML operators by allowing us to create XML-
specific objects and compile XML query expressions once per
statement, instead of once per row.
One final note: the reason this class is in this package
instead of the types package is that, in order to retrieve
the compile-time objects, we have to use the "getSavedObject()"
method on the Activation. But the Activation class is part
of the SQL layer (org.apache.derby.iapi.sql.Activation) and
we want to keep the types layer independent of the SQL layer
because the types can be used during recovery before the SQL
system has booted. So the next logical choices were the compile
package (impl.sql.compile) or the execution package; of those,
the execution package seems more appropriate since this
class is only instantiated and used during execution, not
during compilation.
|
| Constructor Summary | |
| public | SqlXmlExecutor(Activation activation, int utilId, boolean preserveWS)
Constructor 1: Used for XMLPARSE op. | | public | SqlXmlExecutor(int targetTypeId, int targetMaxWidth)
Constructor 2: Used for XMLSERIALIZE op. | | public | SqlXmlExecutor(Activation activation, int utilId)
Constructor 3: Used for XMLEXISTS/XMLQUERY ops. |
| Method Summary | |
| public BooleanDataValue | XMLExists(StringDataValue xExpr, XMLDataValue xmlContext)
Make the call to perform an XMLEXISTS operation on the
received XML data value.
Parameters:
xExpr - Query expression to be evaluated
Parameters:
xmlContext - Context node against which to evaluatethe expression. | | public XMLDataValue | XMLParse(StringDataValue xmlText, XMLDataValue result)
Make the call to perform an XMLPARSE operation on the
received XML string and store the result in the received
XMLDataValue (or if it's null, create a new one).
Parameters:
xmlText - String to parse
Parameters:
result - XMLDataValue in which to store the result The received XMLDataValue with its content set tocorrespond to the received xmlText, if the text constitutesa valid XML document. | | public XMLDataValue | XMLQuery(StringDataValue xExpr, XMLDataValue xmlContext, XMLDataValue result)
Make the call to perform an XMLQUERY operation on the
received XML data value and store the result in the
received result holder (or, if it's null, create a
new one). | | public StringDataValue | XMLSerialize(XMLDataValue xmlVal, StringDataValue result)
Make the call to perform an XMLSERIALIZE operation on the
received XML data value and store the result in the received
StringDataValue (or if it's null, create a new one).
Parameters:
xmlVal - XML value to serialize
Parameters:
result - StringDataValue in which to store the result A serialized (to string) version of this XML object,in the form of a StringDataValue object. |
| SqlXmlExecutor | | public SqlXmlExecutor(Activation activation, int utilId, boolean preserveWS)(Code) | | Constructor 1: Used for XMLPARSE op.
Parameters:
activation - Activation from which to retrieve saved objects
Parameters:
utilId - Id by which we find saved objects in activation
Parameters:
preserveWS - Whether or not to preserve whitespace |
| SqlXmlExecutor | | public SqlXmlExecutor(int targetTypeId, int targetMaxWidth)(Code) | | Constructor 2: Used for XMLSERIALIZE op.
Parameters:
targetTypeId - The string type to which we want to serialize.
Parameters:
targetMaxWidth - The max width of the target type. |
| SqlXmlExecutor | | public SqlXmlExecutor(Activation activation, int utilId)(Code) | | Constructor 3: Used for XMLEXISTS/XMLQUERY ops.
Parameters:
activation - Activation from which to retrieve saved objects
Parameters:
utilId - Id by which we find saved objects in activation |
| XMLExists | | public BooleanDataValue XMLExists(StringDataValue xExpr, XMLDataValue xmlContext) throws StandardException(Code) | | Make the call to perform an XMLEXISTS operation on the
received XML data value.
Parameters:
xExpr - Query expression to be evaluated
Parameters:
xmlContext - Context node against which to evaluatethe expression. True if evaluation of the query expressionagainst xmlContext returns at least one item; unknown ifeither the xml value is NULL; false otherwise. |
| XMLParse | | public XMLDataValue XMLParse(StringDataValue xmlText, XMLDataValue result) throws StandardException(Code) | | Make the call to perform an XMLPARSE operation on the
received XML string and store the result in the received
XMLDataValue (or if it's null, create a new one).
Parameters:
xmlText - String to parse
Parameters:
result - XMLDataValue in which to store the result The received XMLDataValue with its content set tocorrespond to the received xmlText, if the text constitutesa valid XML document. If the received XMLDataValue isnull, then create a new one and set its content tocorrespond to the received xmlText. |
| XMLQuery | | public XMLDataValue XMLQuery(StringDataValue xExpr, XMLDataValue xmlContext, XMLDataValue result) throws StandardException(Code) | | Make the call to perform an XMLQUERY operation on the
received XML data value and store the result in the
received result holder (or, if it's null, create a
new one).
Parameters:
xExpr - Query expression to be evaluated
Parameters:
xmlContext - Context node against which to evaluatethe expression.
Parameters:
result - XMLDataValue in which to store the result The received XMLDataValue with its content set toresult of evaulating the query expression against xmlContext.If the received XMLDataValue is null, then create a new oneand set its content to correspond to the received xmlText. |
| XMLSerialize | | public StringDataValue XMLSerialize(XMLDataValue xmlVal, StringDataValue result) throws StandardException(Code) | | Make the call to perform an XMLSERIALIZE operation on the
received XML data value and store the result in the received
StringDataValue (or if it's null, create a new one).
Parameters:
xmlVal - XML value to serialize
Parameters:
result - StringDataValue in which to store the result A serialized (to string) version of this XML object,in the form of a StringDataValue object. |
|
|