| F.31. ltree — hierarchical tree-like data type | ||||
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F.31. ltree — hierarchical tree-like data type#
This module implements a data typeltree for representing labels of data stored in a hierarchical tree-like structure. Extensive facilities for searching through label trees are provided.
This module is considered“trusted”, that is, it can be installed by non-superusers who haveCREATE privilege on the current database.
F.31.1. Definitions#
Alabel is a sequence of alphanumeric characters, underscores, and hyphens. Valid alphanumeric character ranges are dependent on the database locale. For example, in C locale, the charactersA-Za-z0-9_- are allowed. Labels must be no more than 1000 characters long.
Examples:42,Personal_Services
Alabel path is a sequence of zero or more labels separated by dots, for exampleL1.L2.L3, representing a path from the root of a hierarchical tree to a particular node. The length of a label path cannot exceed 65535 labels.
Example:Top.Countries.Europe.Russia
Theltree module provides several data types:
ltreestores a label path.lqueryrepresents a regular-expression-like pattern for matchingltreevalues. A simple word matches that label within a path. A star symbol (*) matches zero or more labels. These can be joined with dots to form a pattern that must match the whole label path. For example:fooMatch the exact label path
foo*.foo.*Match any label path containing the labelfoo*.fooMatch any label path whose last label isfooBoth star symbols and simple words can be quantified to restrict how many labels they can match:
*{n}Match exactlynlabels*{n,}Match at leastnlabels*{n,m}Match at leastnbut not more thanmlabels*{,m}Match at mostmlabels — same as*{0,m}foo{n,m}Match at leastnbut not more thanmoccurrences offoofoo{,}Match any number of occurrences offoo, including zeroIn the absence of any explicit quantifier, the default for a star symbol is to match any number of labels (that is,
{,}) while the default for a non-star item is to match exactly once (that is,{1}).There are several modifiers that can be put at the end of a non-star
lqueryitem to make it match more than just the exact match:@Match case-insensitively, for example
a@matchesA*Match any label with this prefix, for examplefoo*matchesfoobar%Match initial underscore-separated wordsThe behavior of
%is a bit complicated. It tries to match words rather than the entire label. For examplefoo_bar%matchesfoo_bar_bazbut notfoo_barbaz. If combined with*, prefix matching applies to each word separately, for examplefoo_bar%*matchesfoo1_bar2_bazbut notfoo1_br2_baz.Also, you can write several possibly-modified non-star items separated with
|(OR) to match any of those items, and you can put!(NOT) at the start of a non-star group to match any label that doesn't match any of the alternatives. A quantifier, if any, goes at the end of the group; it means some number of matches for the group as a whole (that is, some number of labels matching or not matching any of the alternatives).Here's an annotated example of
lquery:Top.*{0,2}.sport*@.!football|tennis{1,}.Russ*|Spaina. b. c. d. e.This query will match any label path that:
begins with the label
Topand next has zero to two labels before
a label beginning with the case-insensitive prefix
sportthen has one or more labels, none of which match
footballnortennisand then ends with a label beginning with
Russor exactly matchingSpain.
ltxtqueryrepresents a full-text-search-like pattern for matchingltreevalues. Anltxtqueryvalue contains words, possibly with the modifiers@,*,%at the end; the modifiers have the same meanings as inlquery. Words can be combined with&(AND),|(OR),!(NOT), and parentheses. The key difference fromlqueryis thatltxtquerymatches words without regard to their position in the label path.Here's an example
ltxtquery:Europe & Russia*@ & !Transportation
This will match paths that contain the label
Europeand any label beginning withRussia(case-insensitive), but not paths containing the labelTransportation. The location of these words within the path is not important. Also, when%is used, the word can be matched to any underscore-separated word within a label, regardless of position.
Note:ltxtquery allows whitespace between symbols, butltree andlquery do not.
F.31.2. Operators and Functions#
Typeltree has the usual comparison operators=,<>,<,>,<=,>=. Comparison sorts in the order of a tree traversal, with the children of a node sorted by label text. In addition, the specialized operators shown inTable F.17 are available.
Table F.17. ltree Operators
Operator Description |
|---|
Is left argument an ancestor of right (or equal)? |
Is left argument a descendant of right (or equal)? |
Does |
Does |
Does |
Concatenates |
Converts text to |
Does array contain an ancestor of |
Does array contain a descendant of |
Does array contain any path matching |
Does |
Does array contain any path matching |
Returns first array entry that is an ancestor of |
Returns first array entry that is a descendant of |
Returns first array entry that matches |
Returns first array entry that matches |
The operators<@,@>,@ and~ have analogues^<@,^@>,^@,^~, which are the same except they do not use indexes. These are useful only for testing purposes.
The available functions are shown inTable F.18.
Table F.18. ltree Functions
F.31.3. Indexes#
ltree supports several types of indexes that can speed up the indicated operators:
B-tree index over
ltree:<,<=,=,>=,>Hash index over
ltree:=GiST index over
ltree(gist_ltree_opsopclass):<,<=,=,>=,>,@>,<@,@,~,?gist_ltree_opsGiST opclass approximates a set of path labels as a bitmap signature. Its optional integer parametersiglendetermines the signature length in bytes. The default signature length is 8 bytes. The length must be a positive multiple ofintalignment (4 bytes on most machines)) up to 2024. Longer signatures lead to a more precise search (scanning a smaller fraction of the index and fewer heap pages), at the cost of a larger index.Example of creating such an index with the default signature length of 8 bytes:
CREATE INDEX path_gist_idx ON test USING GIST (path);
Example of creating such an index with a signature length of 100 bytes:
CREATE INDEX path_gist_idx ON test USING GIST (path gist_ltree_ops(siglen=100));
GiST index over
ltree[](gist__ltree_opsopclass):ltree[] <@ ltree,ltree @> ltree[],@,~,?gist__ltree_opsGiST opclass works similarly togist_ltree_opsand also takes signature length as a parameter. The default value ofsigleningist__ltree_opsis 28 bytes.Example of creating such an index with the default signature length of 28 bytes:
CREATE INDEX path_gist_idx ON test USING GIST (array_path);
Example of creating such an index with a signature length of 100 bytes:
CREATE INDEX path_gist_idx ON test USING GIST (array_path gist__ltree_ops(siglen=100));
Note: This index type is lossy.
F.31.4. Example#
This example uses the following data (also available in filecontrib/ltree/ltreetest.sql in the source distribution):
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);CREATE INDEX path_hash_idx ON test USING HASH (path); Now, we have a tabletest populated with data describing the hierarchy shown below:
Top / | \ Science Hobbies Collections / | \ Astronomy Amateurs_Astronomy Pictures / \ |Astrophysics Cosmology Astronomy / | \ Galaxies Stars Astronauts
We can do inheritance:
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)
Here are some examples of path matching:
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)
Here are some examples of full text search:
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)
Path construction using functions:
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)
We could simplify this by creating an SQL function that inserts a label at a specified position in a path:
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)
F.31.5. Transforms#
Theltree_plpython3u extension implements transforms for theltree type for PL/Python. If installed and specified when creating a function,ltree values are mapped to Python lists. (The reverse is currently not supported, however.)
Caution
It is strongly recommended that the transform extension be installed in the same schema asltree. Otherwise there are installation-time security hazards if a transform extension's schema contains objects defined by a hostile user.
F.31.6. Authors#
All work was done by Teodor Sigaev (<teodor@stack.net>) and Oleg Bartunov (<oleg@sai.msu.su>). Seehttp://www.sai.msu.su/~megera/postgres/gist/ for additional information. Authors would like to thank Eugeny Rodichev for helpful discussions. Comments and bug reports are welcome.