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=Nondeterminism=
{{Announce|Updated with Version 11}}: Renamed from {{Code|non-deterministic}} to {{Code|nondeterministic}}.
In [https://www.w3.org/TR/xpath-functions-31/#dt-deterministic XQuery], deterministic functions are “guaranteed to produce ·identical· results from repeated calls within a single ·execution scope· if the explicit and implicit arguments are identical”. In BaseX, many extension functions are nondeterministic or side-effecting. If an expression is internally flagged as nondeterministic, various optimizations that might change their execution order will not be applied.
<syntaxhighlight lang="xquery">
(: QUERY A... :)
let $n := 456
for $i in 1 to 2
return $n
(: ...will be optimized to :)
for $i in 1 to 2
return 456
(: QUERY B will not be rewritten :)
let $n := random:integer()
for $i in 1 to 2
return $n
</syntaxhighlight>
In some cases, functions may contain nondeterministic code, but the query compiler may not be able to detect this statically. See the following example:
<syntaxhighlight lang="xquery">
for $read in (file:read-text#1, file:read-binary#1)
let $ignored := nondeterministic $read('input.file')
return ()
</syntaxhighlight>
Two nondeterministic functions will be bound to <code>$read</code>, and the result of the function call will be bound to <code>$ignored</code>. As the variable is not referenced in the subsequent code, the let clause would usually be discarded by the compiler. In the given query, however, execution will be enforced because of the BaseX-specific {{Code|nondeterministic}} keyword.
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In analogy with option declarations and pragmas, locks can also set via annotations. See [[Transaction Management#Annotations|Transaction Management]] for details and examples.
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