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695 lines
23 KiB
Plaintext
695 lines
23 KiB
Plaintext
<!-- doc/src/sgml/ltree.sgml -->
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<sect1 id="ltree" xreflabel="ltree">
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<title>ltree</title>
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<indexterm zone="ltree">
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<primary>ltree</primary>
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</indexterm>
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<para>
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This module implements a data type <type>ltree</> for representing
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labels of data stored in a hierarchical tree-like structure.
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Extensive facilities for searching through label trees are provided.
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</para>
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<sect2>
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<title>Definitions</title>
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<para>
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A <firstterm>label</firstterm> is a sequence of alphanumeric characters
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and underscores (for example, in C locale the characters
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<literal>A-Za-z0-9_</> are allowed). Labels must be less than 256 bytes
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long.
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</para>
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<para>
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Examples: <literal>42</>, <literal>Personal_Services</>
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</para>
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<para>
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A <firstterm>label path</firstterm> is a sequence of zero or more
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labels separated by dots, for example <literal>L1.L2.L3</>, representing
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a path from the root of a hierarchical tree to a particular node. The
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length of a label path must be less than 65kB, but keeping it under 2kB is
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preferable. In practice this is not a major limitation; for example,
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the longest label path in the DMOZ catalog (<ulink
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url="http://www.dmoz.org"></ulink>) is about 240 bytes.
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</para>
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<para>
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Example: <literal>Top.Countries.Europe.Russia</literal>
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</para>
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<para>
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The <filename>ltree</> module provides several data types:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<type>ltree</type> stores a label path.
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</para>
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</listitem>
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<listitem>
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<para>
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<type>lquery</type> represents a regular-expression-like pattern
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for matching <type>ltree</> values. A simple word matches that
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label within a path. A star symbol (<literal>*</>) matches zero
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or more labels. For example:
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<synopsis>
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foo <lineannotation>Match the exact label path <literal>foo</></lineannotation>
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*.foo.* <lineannotation>Match any label path containing the label <literal>foo</></lineannotation>
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*.foo <lineannotation>Match any label path whose last label is <literal>foo</></lineannotation>
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</synopsis>
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</para>
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<para>
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Star symbols can also be quantified to restrict how many labels
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they can match:
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<synopsis>
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*{<replaceable>n</>} <lineannotation>Match exactly <replaceable>n</> labels</lineannotation>
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*{<replaceable>n</>,} <lineannotation>Match at least <replaceable>n</> labels</lineannotation>
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*{<replaceable>n</>,<replaceable>m</>} <lineannotation>Match at least <replaceable>n</> but not more than <replaceable>m</> labels</lineannotation>
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*{,<replaceable>m</>} <lineannotation>Match at most <replaceable>m</> labels — same as </lineannotation> *{0,<replaceable>m</>}
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</synopsis>
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</para>
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<para>
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There are several modifiers that can be put at the end of a non-star
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label in <type>lquery</> to make it match more than just the exact match:
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<synopsis>
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@ <lineannotation>Match case-insensitively, for example <literal>a@</> matches <literal>A</></lineannotation>
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* <lineannotation>Match any label with this prefix, for example <literal>foo*</> matches <literal>foobar</></lineannotation>
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% <lineannotation>Match initial underscore-separated words</lineannotation>
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</synopsis>
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The behavior of <literal>%</> is a bit complicated. It tries to match
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words rather than the entire label. For example
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<literal>foo_bar%</> matches <literal>foo_bar_baz</> but not
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<literal>foo_barbaz</>. If combined with <literal>*</>, prefix
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matching applies to each word separately, for example
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<literal>foo_bar%*</> matches <literal>foo1_bar2_baz</> but
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not <literal>foo1_br2_baz</>.
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</para>
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<para>
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Also, you can write several possibly-modified labels separated with
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<literal>|</> (OR) to match any of those labels, and you can put
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<literal>!</> (NOT) at the start to match any label that doesn't
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match any of the alternatives.
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</para>
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<para>
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Here's an annotated example of <type>lquery</type>:
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<programlisting>
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Top.*{0,2}.sport*@.!football|tennis.Russ*|Spain
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a. b. c. d. e.
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</programlisting>
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This query will match any label path that:
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</para>
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<orderedlist numeration="loweralpha">
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<listitem>
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<para>
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begins with the label <literal>Top</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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and next has zero to two labels before
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</para>
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</listitem>
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<listitem>
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<para>
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a label beginning with the case-insensitive prefix <literal>sport</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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then a label not matching <literal>football</literal> nor
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<literal>tennis</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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and then ends with a label beginning with <literal>Russ</literal> or
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exactly matching <literal>Spain</literal>.
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</para>
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</listitem>
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</orderedlist>
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</listitem>
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<listitem>
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<para><type>ltxtquery</type> represents a full-text-search-like
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pattern for matching <type>ltree</> values. An
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<type>ltxtquery</type> value contains words, possibly with the
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modifiers <literal>@</>, <literal>*</>, <literal>%</> at the end;
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the modifiers have the same meanings as in <type>lquery</>.
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Words can be combined with <literal>&</> (AND),
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<literal>|</> (OR), <literal>!</> (NOT), and parentheses.
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The key difference from
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<type>lquery</> is that <type>ltxtquery</type> matches words without
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regard to their position in the label path.
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</para>
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<para>
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Here's an example <type>ltxtquery</type>:
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<programlisting>
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Europe & Russia*@ & !Transportation
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</programlisting>
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This will match paths that contain the label <literal>Europe</literal> and
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any label beginning with <literal>Russia</literal> (case-insensitive),
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but not paths containing the label <literal>Transportation</literal>.
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The location of these words within the path is not important.
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Also, when <literal>%</> is used, the word can be matched to any
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underscore-separated word within a label, regardless of position.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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Note: <type>ltxtquery</> allows whitespace between symbols, but
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<type>ltree</> and <type>lquery</> do not.
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</para>
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</sect2>
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<sect2>
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<title>Operators and Functions</title>
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<para>
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Type <type>ltree</> has the usual comparison operators
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<literal>=</>, <literal><></literal>,
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<literal><</>, <literal>></>, <literal><=</>, <literal>>=</>.
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Comparison sorts in the order of a tree traversal, with the children
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of a node sorted by label text. In addition, the specialized
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operators shown in <xref linkend="ltree-op-table"> are available.
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</para>
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<table id="ltree-op-table">
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<title><type>ltree</> Operators</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Operator</entry>
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<entry>Returns</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><type>ltree</> <literal>@></> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>is left argument an ancestor of right (or equal)?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal><@</> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>is left argument a descendant of right (or equal)?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>~</> <type>lquery</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>lquery</> <literal>~</> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>?</> <type>lquery[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match any <type>lquery</> in array?</entry>
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</row>
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<row>
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<entry><type>lquery[]</> <literal>?</> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match any <type>lquery</> in array?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>@</> <type>ltxtquery</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match <type>ltxtquery</>?</entry>
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</row>
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<row>
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<entry><type>ltxtquery</> <literal>@</> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> match <type>ltxtquery</>?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>||</> <type>ltree</></entry>
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<entry><type>ltree</type></entry>
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<entry>concatenate <type>ltree</> paths</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>||</> <type>text</></entry>
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<entry><type>ltree</type></entry>
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<entry>convert text to <type>ltree</> and concatenate</entry>
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</row>
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<row>
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<entry><type>text</> <literal>||</> <type>ltree</></entry>
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<entry><type>ltree</type></entry>
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<entry>convert text to <type>ltree</> and concatenate</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>@></> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain an ancestor of <type>ltree</>?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal><@</> <type>ltree[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain an ancestor of <type>ltree</>?</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal><@</> <type>ltree</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain a descendant of <type>ltree</>?</entry>
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</row>
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<row>
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<entry><type>ltree</> <literal>@></> <type>ltree[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain a descendant of <type>ltree</>?</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>~</> <type>lquery</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain any path matching <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>lquery</> <literal>~</> <type>ltree[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain any path matching <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>?</> <type>lquery[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> array contain any path matching any <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>lquery[]</> <literal>?</> <type>ltree[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does <type>ltree</> array contain any path matching any <type>lquery</>?</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>@</> <type>ltxtquery</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain any path matching <type>ltxtquery</>?</entry>
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</row>
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<row>
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<entry><type>ltxtquery</> <literal>@</> <type>ltree[]</></entry>
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<entry><type>boolean</type></entry>
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<entry>does array contain any path matching <type>ltxtquery</>?</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>?@></> <type>ltree</></entry>
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<entry><type>ltree</type></entry>
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<entry>first array entry that is an ancestor of <type>ltree</>; NULL if none</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>?<@</> <type>ltree</></entry>
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<entry><type>ltree</type></entry>
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<entry>first array entry that is a descendant of <type>ltree</>; NULL if none</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>?~</> <type>lquery</></entry>
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<entry><type>ltree</type></entry>
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<entry>first array entry that matches <type>lquery</>; NULL if none</entry>
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</row>
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<row>
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<entry><type>ltree[]</> <literal>?@</> <type>ltxtquery</></entry>
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<entry><type>ltree</type></entry>
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<entry>first array entry that matches <type>ltxtquery</>; NULL if none</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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The operators <literal><@</literal>, <literal>@></literal>,
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<literal>@</literal> and <literal>~</literal> have analogues
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<literal>^<@</>, <literal>^@></>, <literal>^@</>,
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<literal>^~</literal>, which are the same except they do not use
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indexes. These are useful only for testing purposes.
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</para>
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<para>
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The available functions are shown in <xref linkend="ltree-func-table">.
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</para>
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<table id="ltree-func-table">
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<title><type>ltree</> Functions</title>
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<tgroup cols="5">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Return Type</entry>
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<entry>Description</entry>
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<entry>Example</entry>
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<entry>Result</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><function>subltree(ltree, int start, int end)</function><indexterm><primary>subltree</primary></indexterm></entry>
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<entry><type>ltree</type></entry>
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<entry>subpath of <type>ltree</> from position <parameter>start</> to
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position <parameter>end</>-1 (counting from 0)</entry>
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<entry><literal>subltree('Top.Child1.Child2',1,2)</literal></entry>
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<entry><literal>Child1</literal></entry>
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</row>
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<row>
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<entry><function>subpath(ltree, int offset, int len)</function><indexterm><primary>subpath</primary></indexterm></entry>
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<entry><type>ltree</type></entry>
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<entry>subpath of <type>ltree</> starting at position
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<parameter>offset</>, length <parameter>len</>.
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If <parameter>offset</> is negative, subpath starts that far from the
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end of the path. If <parameter>len</> is negative, leaves that many
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labels off the end of the path.</entry>
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<entry><literal>subpath('Top.Child1.Child2',0,2)</literal></entry>
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<entry><literal>Top.Child1</literal></entry>
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</row>
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<row>
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<entry><function>subpath(ltree, int offset)</function></entry>
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<entry><type>ltree</type></entry>
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<entry>subpath of <type>ltree</> starting at position
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<parameter>offset</>, extending to end of path.
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If <parameter>offset</> is negative, subpath starts that far from the
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end of the path.</entry>
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<entry><literal>subpath('Top.Child1.Child2',1)</literal></entry>
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<entry><literal>Child1.Child2</literal></entry>
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</row>
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<row>
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<entry><function>nlevel(ltree)</function><indexterm><primary>nlevel</primary></indexterm></entry>
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<entry><type>integer</type></entry>
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<entry>number of labels in path</entry>
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<entry><literal>nlevel('Top.Child1.Child2')</literal></entry>
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<entry><literal>3</literal></entry>
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</row>
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<row>
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<entry><function>index(ltree a, ltree b)</function><indexterm><primary>index</primary></indexterm></entry>
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<entry><type>integer</type></entry>
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<entry>position of first occurrence of <parameter>b</> in
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<parameter>a</>; -1 if not found</entry>
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<entry><literal>index('0.1.2.3.5.4.5.6.8.5.6.8','5.6')</literal></entry>
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<entry><literal>6</literal></entry>
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</row>
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<row>
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<entry><function>index(ltree a, ltree b, int offset)</function></entry>
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<entry><type>integer</type></entry>
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<entry>position of first occurrence of <parameter>b</> in
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<parameter>a</>, searching starting at <parameter>offset</>;
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negative <parameter>offset</> means start <parameter>-offset</>
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labels from the end of the path</entry>
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<entry><literal>index('0.1.2.3.5.4.5.6.8.5.6.8','5.6',-4)</literal></entry>
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<entry><literal>9</literal></entry>
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</row>
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<row>
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<entry><function>text2ltree(text)</function><indexterm><primary>text2ltree</primary></indexterm></entry>
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<entry><type>ltree</type></entry>
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<entry>cast <type>text</> to <type>ltree</></entry>
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<entry><literal></literal></entry>
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<entry><literal></literal></entry>
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</row>
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<row>
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<entry><function>ltree2text(ltree)</function><indexterm><primary>ltree2text</primary></indexterm></entry>
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<entry><type>text</type></entry>
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<entry>cast <type>ltree</> to <type>text</></entry>
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<entry><literal></literal></entry>
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<entry><literal></literal></entry>
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</row>
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<row>
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<entry><function>lca(ltree, ltree, ...)</function><indexterm><primary>lca</primary></indexterm></entry>
|
|
<entry><type>ltree</type></entry>
|
|
<entry>lowest common ancestor, i.e., longest common prefix of paths
|
|
(up to 8 arguments supported)</entry>
|
|
<entry><literal>lca('1.2.2.3','1.2.3.4.5.6')</literal></entry>
|
|
<entry><literal>1.2</literal></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><function>lca(ltree[])</function></entry>
|
|
<entry><type>ltree</type></entry>
|
|
<entry>lowest common ancestor, i.e., longest common prefix of paths</entry>
|
|
<entry><literal>lca(array['1.2.2.3'::ltree,'1.2.3'])</literal></entry>
|
|
<entry><literal>1.2</literal></entry>
|
|
</row>
|
|
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Indexes</title>
|
|
<para>
|
|
<filename>ltree</> supports several types of indexes that can speed
|
|
up the indicated operators:
|
|
</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
B-tree index over <type>ltree</>:
|
|
<literal><</>, <literal><=</>, <literal>=</>,
|
|
<literal>>=</>, <literal>></literal>
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
GiST index over <type>ltree</>:
|
|
<literal><</>, <literal><=</>, <literal>=</>,
|
|
<literal>>=</>, <literal>></>,
|
|
<literal>@></>, <literal><@</>,
|
|
<literal>@</>, <literal>~</>, <literal>?</literal>
|
|
</para>
|
|
<para>
|
|
Example of creating such an index:
|
|
</para>
|
|
<programlisting>
|
|
CREATE INDEX path_gist_idx ON test USING GIST (path);
|
|
</programlisting>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
GiST index over <type>ltree[]</>:
|
|
<literal>ltree[] <@ ltree</>, <literal>ltree @> ltree[]</>,
|
|
<literal>@</>, <literal>~</>, <literal>?</literal>
|
|
</para>
|
|
<para>
|
|
Example of creating such an index:
|
|
</para>
|
|
<programlisting>
|
|
CREATE INDEX path_gist_idx ON test USING GIST (array_path);
|
|
</programlisting>
|
|
<para>
|
|
Note: This index type is lossy.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Example</title>
|
|
|
|
<para>
|
|
This example uses the following data (also available in file
|
|
<filename>contrib/ltree/ltreetest.sql</> in the source distribution):
|
|
</para>
|
|
|
|
<programlisting>
|
|
CREATE TABLE test (path ltree);
|
|
INSERT INTO test VALUES ('Top');
|
|
INSERT INTO test VALUES ('Top.Science');
|
|
INSERT INTO test VALUES ('Top.Science.Astronomy');
|
|
INSERT INTO test VALUES ('Top.Science.Astronomy.Astrophysics');
|
|
INSERT INTO test VALUES ('Top.Science.Astronomy.Cosmology');
|
|
INSERT INTO test VALUES ('Top.Hobbies');
|
|
INSERT INTO test VALUES ('Top.Hobbies.Amateurs_Astronomy');
|
|
INSERT INTO test VALUES ('Top.Collections');
|
|
INSERT INTO test VALUES ('Top.Collections.Pictures');
|
|
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy');
|
|
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Stars');
|
|
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Galaxies');
|
|
INSERT INTO test VALUES ('Top.Collections.Pictures.Astronomy.Astronauts');
|
|
CREATE INDEX path_gist_idx ON test USING GIST (path);
|
|
CREATE INDEX path_idx ON test USING BTREE (path);
|
|
</programlisting>
|
|
|
|
<para>
|
|
Now, we have a table <structname>test</> populated with data describing
|
|
the hierarchy shown below:
|
|
</para>
|
|
|
|
<literallayout class="monospaced">
|
|
Top
|
|
/ | \
|
|
Science Hobbies Collections
|
|
/ | \
|
|
Astronomy Amateurs_Astronomy Pictures
|
|
/ \ |
|
|
Astrophysics Cosmology Astronomy
|
|
/ | \
|
|
Galaxies Stars Astronauts
|
|
</literallayout>
|
|
|
|
<para>
|
|
We can do inheritance:
|
|
<screen>
|
|
ltreetest=> SELECT path FROM test WHERE path <@ 'Top.Science';
|
|
path
|
|
------------------------------------
|
|
Top.Science
|
|
Top.Science.Astronomy
|
|
Top.Science.Astronomy.Astrophysics
|
|
Top.Science.Astronomy.Cosmology
|
|
(4 rows)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
Here are some examples of path matching:
|
|
<screen>
|
|
ltreetest=> SELECT path FROM test WHERE path ~ '*.Astronomy.*';
|
|
path
|
|
-----------------------------------------------
|
|
Top.Science.Astronomy
|
|
Top.Science.Astronomy.Astrophysics
|
|
Top.Science.Astronomy.Cosmology
|
|
Top.Collections.Pictures.Astronomy
|
|
Top.Collections.Pictures.Astronomy.Stars
|
|
Top.Collections.Pictures.Astronomy.Galaxies
|
|
Top.Collections.Pictures.Astronomy.Astronauts
|
|
(7 rows)
|
|
|
|
ltreetest=> SELECT path FROM test WHERE path ~ '*.!pictures@.*.Astronomy.*';
|
|
path
|
|
------------------------------------
|
|
Top.Science.Astronomy
|
|
Top.Science.Astronomy.Astrophysics
|
|
Top.Science.Astronomy.Cosmology
|
|
(3 rows)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
Here are some examples of full text search:
|
|
<screen>
|
|
ltreetest=> SELECT path FROM test WHERE path @ 'Astro*% & !pictures@';
|
|
path
|
|
------------------------------------
|
|
Top.Science.Astronomy
|
|
Top.Science.Astronomy.Astrophysics
|
|
Top.Science.Astronomy.Cosmology
|
|
Top.Hobbies.Amateurs_Astronomy
|
|
(4 rows)
|
|
|
|
ltreetest=> SELECT path FROM test WHERE path @ 'Astro* & !pictures@';
|
|
path
|
|
------------------------------------
|
|
Top.Science.Astronomy
|
|
Top.Science.Astronomy.Astrophysics
|
|
Top.Science.Astronomy.Cosmology
|
|
(3 rows)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
Path construction using functions:
|
|
<screen>
|
|
ltreetest=> SELECT subpath(path,0,2)||'Space'||subpath(path,2) FROM test WHERE path <@ 'Top.Science.Astronomy';
|
|
?column?
|
|
------------------------------------------
|
|
Top.Science.Space.Astronomy
|
|
Top.Science.Space.Astronomy.Astrophysics
|
|
Top.Science.Space.Astronomy.Cosmology
|
|
(3 rows)
|
|
</screen>
|
|
</para>
|
|
|
|
<para>
|
|
We could simplify this by creating a SQL function that inserts a label
|
|
at a specified position in a path:
|
|
<screen>
|
|
CREATE FUNCTION ins_label(ltree, int, text) RETURNS ltree
|
|
AS 'select subpath($1,0,$2) || $3 || subpath($1,$2);'
|
|
LANGUAGE SQL IMMUTABLE;
|
|
|
|
ltreetest=> SELECT ins_label(path,2,'Space') FROM test WHERE path <@ 'Top.Science.Astronomy';
|
|
ins_label
|
|
------------------------------------------
|
|
Top.Science.Space.Astronomy
|
|
Top.Science.Space.Astronomy.Astrophysics
|
|
Top.Science.Space.Astronomy.Cosmology
|
|
(3 rows)
|
|
</screen>
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Transforms</title>
|
|
|
|
<para>
|
|
Additional extensions are available that implement transforms for
|
|
the <type>ltree</type> type for PL/Python. The extensions are
|
|
called <literal>ltree_plpythonu</literal>, <literal>ltree_plpython2u</literal>,
|
|
and <literal>ltree_plpython3u</literal>
|
|
(see <xref linkend="plpython-python23"> for the PL/Python naming
|
|
convention). If you install these transforms and specify them when
|
|
creating a function, <type>ltree</type> values are mapped to Python lists.
|
|
(The reverse is currently not supported, however.)
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Authors</title>
|
|
|
|
<para>
|
|
All work was done by Teodor Sigaev (<email>teodor@stack.net</email>) and
|
|
Oleg Bartunov (<email>oleg@sai.msu.su</email>). See
|
|
<ulink url="http://www.sai.msu.su/~megera/postgres/gist/"></ulink> for
|
|
additional information. Authors would like to thank Eugeny Rodichev for
|
|
helpful discussions. Comments and bug reports are welcome.
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|