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https://issues.apache.org/jira/browse/JENA-630?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13964130#comment-13964130
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Andy Seaborne commented on JENA-630:
------------------------------------
Fixed example - I messed up as I refined the example.
The main point is that subexpressions get rewrittern multiple times - once per
depth into the experssion and the work is not used.
The output shows (+ 1 3) being rewrittern twice. {{ExprFunction2: ( 1 + 3 )}}
and the NodeValues are visitied twice. A deeper expression will larger numbers
of copies.
A sub-point is that expressions are copied even on no change, and that causes
he next level up to be copied, (ExprTransformCopy). This is a certain amount of
object churn which might be reduced. That's what i started looking for - the
fact that there is repeated unnecessary copy going on was a later discovery.
{noformat}
Expr expr = SSE.parseExpr("(+ 1 (* 2 3))") ;
Expr expr2 = ExprTransformer.transform(new ExprTransformConstantFold(),
expr) ;
System.out.println("DONE") ;
{noformat}
{noformat}
ExprFunction2: ( 2 * 3 )
NodeValue: 2
NodeValue: 3
ExprFunction2: ( 1 + ( 2 * 3 ) )
NodeValue: 1
ExprFunction2: ( 2 * 3 )
NodeValue: 2
NodeValue: 3
DONE
{noformat}
{{ExprFunction2: ( 2 * 3 )}} is called twice
{{"(+ 1 (* (+ 5 6 ) (+ 8 9)))"}} causes three calls to each of {{( 8 + 9 )}}
and {{( 5 + 6 )}}, two calls to {{( ( 5 + 6 ) * ( 8 + 9 ) )}} and one call to
{{( 1 + ( ( 5 + 6 ) * ( 8 + 9 ) ) )}}
Contrast with:
{noformat}
Expr expr3 = expr.copySubstitute(BindingFactory.root(), true) ;
{noformat}
which calls the sub-expressions once.
To do partial constant collapse will require bottom up evaluation. At the
moment, calling copySubstitute at ever level causes sub-expressions to be
folded, the results not used then called again when the next level up is
handled.
> Do constant folding as part of query optimisation
> -------------------------------------------------
>
> Key: JENA-630
> URL: https://issues.apache.org/jira/browse/JENA-630
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ
> Affects Versions: Jena 2.11.1
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Fix For: Jena 2.11.2
>
>
> Currently Jena does not automatically simplify expressions by constant
> folding even though the function API actually has support for this already
> baked into it.
> This issue will track work to integrate a transform for this into the
> standard optimiser.
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