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An XQuery expression is evaluated in Query execution encompasses multiple steps:
Each of the steps allows for numerous optimizations, some of which Some rewritings are described in this article.
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This article is part of the [[XQuery|XQuery Portal]]. Optimizations are presented It presents some of the optimizations that speed up the execution time and reduce memory consumption. The text will be regularly extended with further examplesof queries.
=Introduction=
# At parse time, the '''Parsing''': The query input string – an XQuery main module – is transformed to executable code. The result is a tree representation, called the ''abstract syntax tree'' (AST).# At compile time, the '''Compilation''': The syntax tree is decorated with additional information (type information, expression properties); expressions . Expressions (nodes) in the tree are relocated, simplified, or pre-evaluated:## . Logical optimizations are performed that do not rely on external information.# '''context-independentOptimization'''. They can be applied no matter which data will be processed later on.## Physical optimizations rely on : The dynamic context information, such as database statistics or is incorporated: Referenced databases are opened and analyzed; queries are rewritten to use available indexes; accumulative and statistical operations (counts, summations, min/max, distinct values) are pre-evaluated; XPath expressions are simplified, based on the existence of steps.# At evaluation time, the '''Evaluation''': The resulting expression tree code is processedexecuted.# '''Printing''': The results are returned to the user. Some expression (such as simple loops) can be evaluated query result is serialized and presented in iterative mannera format that is either human-readable, whereas others (such as sort operations) need to or can be fully evaluated before the first result is availablefurther processed by an API.
If you run a query on [[Command-Line_Options#Standalone|command-line]], you can use {{Code|-V}} to output detailed query information. In the [[GUI]], you can enable the Info View panel.
=Logical OptimizationsCompilation=
==Pre-Evaluation==
* <code>true#0 and true#0</code> must raise an error; it cannot be simplified to <code>true#0</code>
=Physical OptimizationsOptimization=
Some physical optimizations are also presented in the article on [[Indexes|index structures]].
If the accessed database is not known at compile time, or if you want to give a predicate preference to another one, you can [[Indexes#Enforce Rewritings|enforce index rewritings]].
=Evaluation-Time Optimizations=
==Comparisons==
