F.21. ltree

This module implements a data type ltree for representing labels of data stored in a hierarchical tree-like structure. Extensive facilities for searching through label trees are provided.

F.21.1. Definitions

A label is a sequence of alphanumeric characters and underscores (for example, in C locale the characters A-Za-z0-9_ are allowed). Labels must be less than 256 bytes long.

Examples: 42, Personal_Services

A label path is a sequence of zero or more labels separated by dots, for example L1.L2.L3, representing a path from the root of a hierarchical tree to a particular node. The length of a label path must be less than 65Kb, but keeping it under 2Kb is preferable. In practice this is not a major limitation; for example, the longest label path in the DMOZ catalogue (http://www.dmoz.org) is about 240 bytes.

Example: Top.Countries.Europe.Russia

The ltree module provides several data types:

Note: ltxtquery allows whitespace between symbols, but ltree and lquery do not.

F.21.2. Operators and Functions

Type ltree 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 in Table F-12 are available.

Table F-12. ltree Operators

Operator Returns Description
ltree @> ltree boolean is left argument an ancestor of right (or equal)?
ltree <@ ltree boolean is left argument a descendant of right (or equal)?
ltree ~ lquery boolean does ltree match lquery?
lquery ~ ltree boolean does ltree match lquery?
ltree ? lquery[] boolean does ltree match any lquery in array?
lquery[] ? ltree boolean does ltree match any lquery in array?
ltree @ ltxtquery boolean does ltree match ltxtquery?
ltxtquery @ ltree boolean does ltree match ltxtquery?
ltree || ltree ltree concatenate ltree paths
ltree || text ltree convert text to ltree and concatenate
text || ltree ltree convert text to ltree and concatenate
ltree[] @> ltree boolean does array contain an ancestor of ltree?
ltree <@ ltree[] boolean does array contain an ancestor of ltree?
ltree[] <@ ltree boolean does array contain a descendant of ltree?
ltree @> ltree[] boolean does array contain a descendant of ltree?
ltree[] ~ lquery boolean does array contain any path matching lquery?
lquery ~ ltree[] boolean does array contain any path matching lquery?
ltree[] ? lquery[] boolean does ltree array contain any path matching any lquery?
lquery[] ? ltree[] boolean does ltree array contain any path matching any lquery?
ltree[] @ ltxtquery boolean does array contain any path matching ltxtquery?
ltxtquery @ ltree[] boolean does array contain any path matching ltxtquery?
ltree[] ?@> ltree ltree first array entry that is an ancestor of ltree; NULL if none
ltree[] ?<@ ltree ltree first array entry that is a descendant of ltree; NULL if none
ltree[] ?~ lquery ltree first array entry that matches lquery; NULL if none
ltree[] ?@ ltxtquery ltree first array entry that matches ltxtquery; NULL if none

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 in Table F-13.

Table F-13. ltree Functions

Function Return Type Description Example Result
subltree(ltree, int start, int end) ltree subpath of ltree from position start to position end-1 (counting from 0) subltree('Top.Child1.Child2',1,2) Child1
subpath(ltree, int offset, int len) ltree subpath of ltree starting at position offset, length len. If offset is negative, subpath starts that far from the end of the path. If len is negative, leaves that many labels off the end of the path. subpath('Top.Child1.Child2',0,2) Top.Child1
subpath(ltree, int offset) ltree subpath of ltree starting at position offset, extending to end of path. If offset is negative, subpath starts that far from the end of the path. subpath('Top.Child1.Child2',1) Child1.Child2
nlevel(ltree) integer number of labels in path nlevel('Top.Child1.Child2') 3
index(ltree a, ltree b) integer position of first occurrence of b in a; -1 if not found index('0.1.2.3.5.4.5.6.8.5.6.8','5.6') 6
index(ltree a, ltree b, int offset) integer position of first occurrence of b in a, searching starting at offset; negative offset means start -offset labels from the end of the path index('0.1.2.3.5.4.5.6.8.5.6.8','5.6',-4) 9
text2ltree(text) ltree cast text to ltree
ltree2text(ltree) text cast ltree to text
lca(ltree, ltree, ...) ltree lowest common ancestor, i.e., longest common prefix of paths (up to 8 arguments supported) lca('1.2.2.3','1.2.3.4.5.6') 1.2
lca(ltree[]) ltree lowest common ancestor, i.e., longest common prefix of paths lca(array['1.2.2.3'::ltree,'1.2.3']) 1.2

F.21.3. Indexes

ltree supports several types of indexes that can speed up the indicated operators:

F.21.4. Example

This example uses the following data (also available in file contrib/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);

Now, we have a table test 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 a 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.21.5. Authors

All work was done by Teodor Sigaev () and Oleg Bartunov (). See http://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.

copyright  ©  December 21 2014 sean dreilinger url: http://durak.org/sean/pubs/software/postgresql/ltree.html