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This article is part of the [[XQuery|XQuery Portal]]. It lists some optimizations Optimizations are presented that are performed by BaseX to speed up queries the execution time and reduce memory consumption. The text will be regularly extended with further examples.

# At compile time, the syntax tree is decorated with additional information (type information, expression properties); expressions are relocated, simplified, or pre-evaluated:## Logical optimizations are ''context-independent''. They can be applied no matter which data will be processed later on.## Physical optimizations rely on context information, such as database statistics or available indexes.

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Subsequent rewritings might result in query plans that differ a lot from the original query. As this might complicate debugging, you can disable function inling during development by setting {{Option|INLINELIMIT}} to {{Code|0}}. ==Paths== Due to the compact syntax of XPath, it can make a big difference if a slash is added or omitted in a path expression. A classical example is the double slash {{Code|//}}, which is a shortcut for {{Code|descendant-or-node()/}}. If the query is evaluated without optimizations, all nodes of a document are gathered, and for each of them, the next step is evaluated. This leads to a potentially huge number of duplicate node tree traversals, most of which are redundant, as all duplicate nodes will be removed at the end anyway. In most cases, paths with a double slash can be rewritten to descendant steps… <syntaxhighlight lang="xquery">(: equivalent queries, with identical syntax trees :)doc('addressbook.xml')//city,doc('addressbook.xml')/descendant-or-node()/child::city (: rewritten to :)doc('addressbook.xml')/descendant::city</syntaxhighlight> …unless the last step does not contain a positional predicate: <syntaxhighlight lang="xquery">doc('addressbook.xml')//city[1]</syntaxhighlight> As the positional test refers to the city child step, a rewritten query would yield different steps. Paths may contain predicates that will be evaluated again by a later axis step. Such predicates are either shifted down or discarded: <syntaxhighlight lang="xquery">(: equivalent query :)a[Optionb]/b[c/d]/c (: rewritten to :)a/b/c[d]</syntaxhighlight> Names of nodes can be specified via name tests or predicates. If names are e.g. supplied via external variables, the predicates can often be dissolved: <syntaxhighlight lang="xquery">declare variable $name external := 'city';db:open('addressbook')/descendant::*[name() = $name] (: rewritten to :)db:open('addressbook')/descendant::city</syntaxhighlight> ==FLWOR Rewritings== FLWOR expressions are central to XQuery and the most complex constructs the language offers. Numerous optimizations have been realized to improve the execution time: * Nested FLWOR expressions are flattened.* {{Code|for}} clauses with single items are rewritten to {{Code|let}} clauses.* {{Code|let}} clauses that are iterated multiple times are lifted up.* Expressions of {{Code|let}} clauses are inlined.* Unused variables are removed.* {{Code|where}} clauses are rewritten to predicates.* {{Code|if}} expressions in the return clause are rewritten to {{Code|where}} clauses.* The last {{Code|for}} clause is merged into the {{Code|return}} clause and rewritten to a [[XQuery_3.0|Simple_Map_Operator|INLINELIMITsimple map]] expression. Various of these rewriting are demonstrated in the following example: <syntaxhighlight lang="xquery">for $a in 1 to 5for $b in 2where $a > 3let $c := $a + $breturn $c (: for is rewritten to let :)for $a in 1 to 5let $b := 2where $a > 3let $c := $a + $breturn $c (: let is lifted up :)let $b := 2for $a in 1 to 5where $a > 3let $c := $a + $breturn $c (: the where expression is rewritten to a predicate :)let $b := 2for $a in 1 to 5[. > 3]let $c := $a + $breturn $c (: $b is inlined :)for $a in 1 to 5[. > 3]let $c := $a + 2return $c (: $c is inlined :)for $a in 1 to 5[. > 3] return $a + 2 (: the remaining clauses are merged and rewritten to a simple map :)(1 to 5)[. > 3] ! (. + 2)</syntaxhighlight> ==Static Typing== If the type of a value is known at compile time, type checks can be removed. In the example below, the static information that {{Code|$i}} will always reference items of type {{Code|xs:integer}} can be utilized to simplify the expression: <syntaxhighlight lang="xquery">for $i in 1 to 5return typeswitch($i) case xs:numeric return 'number' default return 'string' (: rewritten to :)for $i in 1 to 5return 'number'</syntaxhighlight> ==Pure Logic== If expressions can often be simplified: <syntaxhighlight lang="xquery">for $a in ('a', '')return $a[boolean(if(.) then true() else false())] (: rewritten to :)for $a in ('a', '')return $a[boolean(.)] (: rewritten to :)for $a in ('a', '')return $a[.] (: rewritten to :)('a', '')[.]</syntaxhighlight> Boolean algebra (and set theory) comes with a set of laws that can all be applied to XQuery expressions. {| class="wikitable"|- valign="top"| Expression| Rewritten expression| Rule|- valign="top"| <code>$a + 0</code>, <code>$a * 1</code>| <code>$a</code>| Identity|- valign="top"| <code>$a * 0</code>| <code>0</code>| Annihilator|- valign="top"| <code>$a and $a</code>| <code>$a</code>| Idempotence|- valign="top"| <code>$a and ($a or $b)</code>| <code>$a</code>| Absorption|- valign="top"| <code>($a and $b) or ($a and $c)</code>| <code>$a and ($b or $c)</code>| Distributivity|- valign="top"| <code>$a or not($a)</code>| <code>true()</code>| Tertium non datur|- valign="top"| <code>not($a) and not($b)</code>| <code>not($a or $b)</code>| De Morgan|} It is not sufficient to apply the rules to arbitrary input. Examples: * If the operands are no boolean values, a conversion is enforced: <code>$string and $string</code> is rewritten to <code>boolean($string)</code>.* <code>xs:double('NaN') * 0</code> yields <code>NaN</code> instead of <code>0</code>* <code>true#0 and true#0</code> must raise an error; it cannot be simplified to <code>true#0</code> =Physical Optimizations= Some physical optimizations are also presented in the article on [[Indexes|index structures]]. ==Database Statistics== In each database, metadata is stored that can be utilized by the query optimizer to speed up or even skip query evaluation: ;Count element nodes The number of elements that are found for a specific path need not be evaluated sequentially. Instead, the count can directly be retrieved from the database statistics: <syntaxhighlight lang="xquery">count(/mondial/country) (: rewritten to :)231</syntaxhighlight> ;Return distinct values The distinct values for specific names and paths can also be fetched from the database metadata, provided that the number does not exceed the maximum number of distinct values (see {{Option|MAXCATS}} for more information): <syntaxhighlight lang="xquery">distinct-values(//religions) (: rewritten to :)('Muslim', 'Roman Catholic', 'Albanian Orthodox', ...)</syntaxhighlight> ==Index Rewritings== A major feature of BaseX is the ability to rewrite all kinds of query patterns for index access. The following queries are all equivalent. They will be rewritten to exactly the same query that will eventually access the text index of a <code>factbook.xml</code> database instance (the file included in our full distributions): <syntaxhighlight lang="xquery">declare context item := db:open('factbook');declare variable $DB := 'factbook'; //name[. = 'Shenzhen'],//name[data() = 'Shenzhen'],//name[./text() = 'Shenzhen'],//name[text()[. = 'Shenzhen']],//name[string() = 'Shenzhen'],//name[string() = 'Shen' || 'zhen'],//name[./data(text()/string()) = 'Shenzhen'],//name[text() ! data() ! string() = 'Shenzhen'], .//name[. = 'Shenzhen'],//*[local-name() = 'name'][data() = 'Shenzhen'], db:open('factbook')//name[. = 'Shenzhen'],db:open($DB)//name[. = 'Shenzhen'], for $name in //name[text() = 'Shenzhen']return $name, for $name in //namereturn $name[text() = 'Shenzhen'], for $name in //namereturn if($name/text() = 'Shenzhen') then $name else (), for $name in //namewhere $name/text() = 'Shenzhen'return $name, for $name in //namewhere $name/text()[. = 'Shenzhen']return $name, for $node in //*where data($node) = 'Shenzhen'where name($node) = 'name'return $node, (: rewritten to :)db:text('factbook', 'Shenzhen')/parent::name</syntaxhighlight> Multiple element names and query strings can be supplied in a path: <syntaxhighlight lang="xquery">//*[(ethnicgroups, religions)/text() = ('Jewish', 'Muslim')] (: rewritten to :)db:text('factbook', ('Jewish', 'Muslim'))/(parent::*:ethnicgroups | parent::*:religions)/parent::*</syntaxhighlight> If multiple candidates for index access are found, the database statistics (if available) are consulted to choose the cheapest candidate: <syntaxhighlight lang="xquery">/mondial/country [religions = 'Muslim'] (: yields 77 results :) [ethnicgroups = 'Greeks'] (: yields 2 results :)  (: rewritten to :)db:text('factbook', 'Greeks')/parent::ethnicgroups/parent::country[religions = 'Muslim']</syntaxhighlight> If index access is possible within more complex FLWOR expressions, only the paths will be rewritten: <syntaxhighlight lang="xquery">for $country in //countrywhere $country/ethnicgroups = 'German'order by $country/name[1]return element { replace($country/@name, ' ', '') } {}, (: rewritten to :)for $country in db:text('factbook', 'German')/parent::ethnicgroups/parent::countryorder by $country/name[1]return element { replace($country/@name, ' ', '') } {}</syntaxhighlight> The [https://projects.cwi.nl/xmark/ XMark XML Benchmark] comes with sample auction data and a bunch of queries, some of which are suitable for index rewritings: ;XMark Query 1 <syntaxhighlight lang="xquery">let $auction := doc('xmark')return for $b in $auction/site/people/person[@id = 'person0']return $b/name/text() (: rewritten to :)db:attribute('xmark', 'person0')/self::attribute(id)/parent::person/name/text()</syntaxhighlight> ;XMark Query 8 <syntaxhighlight lang="xquery">let $auction := doc('xmark')return for $p in $auction/site/people/person let $a := for $t in $auction/site/closed_auctions/closed_auction where $t/buyer/@person = $p/@id return $t return <item person='{ $p/name/text() }'>{ count($a) }</item>, (: rewritten to :)db:open('xmark')/site/people/person ! <item person='{ name/text() }'>{ count( db:attribute('xmark', @id)/self::attribute(person)/parent::buyer/parent::closed_auction )}</item></syntaxhighlight> 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== In many cases, the amount of data to be processed is only known after the query has been compiled. Moreover, the data that is looped through expressions may change. In those cases, the best optimizations needs to be chosen at runtime. If sequences of items are compared against each other, a dynamic hash index will be generated, and the total number of comparisons can be significantly reduced. In the following example, <code>count($input1) * count($input2)</code> comparisons would need to be made without the intermediate index structure: <syntaxhighlight lang="xquery">let $input1 := file:read-text-lines('huge1.txt')let $input2 := file:read-text-lines('huge2.txt')return $input1[not(. = $input2)]</syntaxhighlight> =Changelog= Introduced with Version 9.4.