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From "Daniel John Debrunner (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-742) Use a single result BooleanDataValue for a boolean expression in class generation
Date Fri, 02 Dec 2005 01:31:32 GMT
Use a single result BooleanDataValue for a boolean expression in class generation
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         Key: DERBY-742
         URL: http://issues.apache.org/jira/browse/DERBY-742
     Project: Derby
        Type: Sub-task
  Components: SQL  
    Reporter: Daniel John Debrunner
 Assigned to: Daniel John Debrunner 
    Priority: Minor


BinaryLogicalOperatorNode.generateExpression() creates a BooleanDataValue for the result of
the expression, this is to avoid object generation during runtime.

The code is logically like (ignoring the short circuit logic) for the SQL <left> AND
<right>

BooleanDataValue   e34; // instance field for the result

 e34 = <left>.and( <right>,  e34);

The and method on BooleanDataValue will create a new BooleanDataValue if the passed in e34
is null, otherwise re-use it.
On the first execution e34 will be null, and thus the 'and' method will create an object and
it will get stored in e34.
[ Ignore for the moment the inefficiency of setting e34 everytime to the same value on every
subsequent execution. ]
[ This approach is standard practice in the generated code and elsewhere and ensures that
 object allocation within a Derby ]
[ statement execution is a fixed cost, and not linear with the number of rows scanned. ]

This issue is about what happens when multiple boolean expressions get combined, as in the
query in DERBY-732. For each AND/OR
a new BooleanDataValue field is created to hold the result of the intrermediate step, e.g.
for the SQL ( <A> AND <B> ) AND <C>


BooleanDataValue   e34; // instance field for the result of A and B
BooleanDataValue   e35; // instance field for the final result

 e34 = <A>.and( <B>,  e34);
 e35 = <e34>.and(<C>, e35);

But since this is a single expression, of the same type up and down the tree, why not  have
a single BooleanDataValue for the result

BooleanDataValue   e36; // instance field for the final result

 e36 = <A>.and( <B>,  e36);
 e36 = <e36>.and(<C>, e36);

or

e36 = new BooleanDataValue(); // through data value factory
<A>.and( <B>,  e36);
<e36>.and(<C>, e36);

The query for DERBY-732 has over 500 BooleanDataValue   fields, which I think are all intermediate
result holders.

Would need to document that the and and or methods return the result passed in if it is not
null, which I think is the case for
all DataValueDescriptor methods that take a result parameter.







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