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The Since {{Version|8.6}}, the distinct values are also stored for elements and attributes of numeric type. Various queries will be evaluated much faster if an up-to-date path index is applied to rewrite descendant available (as can be observed when opening the [[GUI#Visualizations|Info View]]): * Descendant steps will be rewritten to multiple child steps. Child steps can be are evaluated faster, as fewer nodes have to be accessedtraversed:
Updates in BaseX Generally, update operations are generally very fastin BaseX. By default, because the index structures will be invalidated by updates. As ; as a result, subsequent queries that benefit from index structures may be executed more slowly than before the updateslow down after updates. There are different alternatives to cope with this:
=Changelog=* Updated: [[#Name Index|Name Index]], [[#Path Index|Path Index]]
→Enforce Rewritings
==Name Index==
The name index contains all element and attribute references to the names of a database, all elements and the fixed-size index ids are stored attributes in the main database table. If a database is updated, new names are automatically added. Furthermore, the index is enriched with It contains some basic statistical information, such as the distinct (categorical) or minimum and maximum values number of its elements and attributes. The maximum number occurrence of categories to store per a name can be changed via [[Options#MAXCATS|MAXCATS]]. The index are discarded after [[#Updates|database updates]].
The name index is e.g. applied to pre-evaluate discard location steps that will never yield results:
<pre class="brush:xquery">
The contents of the name indexes can be directly accessed with the XQuery functions [[Index Module#index:element-names|index:element-names]] and [[Index Module#index:attribute-names|index:attribute-names]].
If a database is updated, new names will be added incrementally, but the statistical information will get out-dated.
==Path Index==
The path index (which is also called ''path summary'' or ''data guide'') stores all distinct paths of the documents in the database. It contains similar additional statistical information , such as the name indexnumber of occurrence of a path, its distinct string values, and the minimum/maximum of numeric values. The index are discarded after [[#Updatesmaximum number of distinct values to store per name can be changed via {{Option|database updates]]MAXCATS}}.
<pre class="brush:xquery">
</pre>
* The paths statistics are e.g. used to pre-evaluate the {{Code|fn:count}} functionwill be pre-evaluated by looking up the number in the index:
<pre class="brush:xquery">
count(doc(: will 'factbook')//country)</pre> * The distinct values of elements or attributes can be rewritten and pre-evaluated by looked up in the path index as well: <pre class="brush:)xquery"> countdistinct-values( docdb:open('factbook')//country religions)
</pre>
The contents of the path index can be directly accessed with the XQuery function [[Index Module#index:facets|index:facets]].
If a database is updated, the statistics in the path index will be invalidated.
==Document Index==
Matching text nodes can be directly requested from the index with the XQuery function {{Function|Database|db:text}}. The index contents can be accessed via {{Function|Index|index:text}}.
The {{Option|UPDINDEX}} option can be activated to keep this index up-to-date., for example: <pre class="brush:xquery">db:optimize( 'mydb', true(), map { 'updindex':true(), 'textindex': true(), 'textinclude':'id' })</pre>
===Range Queries===
Attribute nodes can directly be retrieved from the index with the XQuery functions {{Function|Database|db:attribute}} and {{Function|Database|db:attribute-range}}. The index contents can be accessed with {{Function|Index|index:attributes}}.
The [[Options#UPDINDEX{{Option|UPDINDEX]] }} option can be activated to keep this index up-to-date.
==Token Index==
With {{Command|CREATE INDEX}} and {{Function|Database|db:optimize}}, new selective indexing options will ba applied to an existing database.
==Enforce Rewritings==
In various cases, existing index structures will not be utilized by the query optimizer. This is usually the case if the name of the database is not a static string (e.g., because it is bound to a variable or passed on as argument of a function call). Furthermore, several candidates for index rewritings may exist, and the query optimizer may decide for a rewriting that turns out to be suboptimal.
With {{Version|9.0}}, the {{Option|ENFORCEINDEX}} option was introduced, which allows you to enforce certain index rewritings. While the option can be globally enabled, it is usually better to supply it as [[XQuery Extensions#Pragmas|Pragma]]. Two examples:
* In the query below, 10 databases will be addressed. If it is known in advance that these databases contain an up-to-date text index, the index rewriting can be enforced as follows:
<pre class="brush:xquery">
(# db:enforceindex #) {
for $n in 1 to 10
let $db := 'persons' || $n
return db:open($db)//person[name/text() = 'John']
}
</pre>
* The following query contains two predicates that may both be rewritten for index access. If the automatically chosen rewriting is known not to be optimal, another index rewriting can enforced by surrounding the specific expression with the pragma:
<pre class="brush:xquery">
db:open('factbook')//country
[(# db:enforceindex #) {
@population > '10000000' and
@population < '10999999'
}]
[religions/text() = 'Protestant']
</pre>
Please note that:
* The option should only be enabled if the addressed databases exist, have all required index structures and are up-to-date (otherwise, you will be given an error message).
* The enforcement of index rewritings gives you no guarantee that an index will be used. There are many expressions that cannot be rewritten for index access.
* If you have more than one enforce pragma in a single path expression, only the first will be considered.
=Custom Index Structures=
=Updates=
* After the execution of one or more update operations, the {{Command|OPTIMIZE}} command or the {{Function|Database|db:optimize}} function can be called to rebuild the index structures.
* The {{Option|UPDINDEX}} option can be activated before creating or optimizing the database. As a result, the text, attribute and token indexes will be incrementally updated after each database update. Please note that incremental updates are not available for the full-text index and database statistics. This is also explains why the UPTODATE flag, which is e.g. displayed via {{Command|INFO DB}} or {{Function|Database|db:info}}, will be set to {{Code|false}} until the database will be optimized again (various optimizations won’t be triggered. For example, count(//item) can be extremely fast if all meta data is up-to-date. After an update operation, the database will need to be sequentially traversed to find all item elements (because the database meta data are not reliable anymore).* The {{Option|AUTOOPTIMIZE}} option can be enabled before creating or optimizing the database. All outdated index structures and statistics will then be recreated after each database update. This option should only be used done for small and medium-sized databases.
* Both options can be used side by side: {{Option|UPDINDEX}} will take care that the value index structures will be updated as part of the actual update operation. {{Option|AUTOOPTIMIZE}} will update the remaining data structures (full-text index, database statistics).
=Changelog=
;Version 9.0
* Added: [[#Enforce Rewritings|Enforce Rewritings]]
;Version 8.4
;Version 8.4
