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.
This module is considered “trusted”, that is, it can be installed by non-superusers who have CREATE privilege on the current database.
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 characters 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 cannot exceed 65535 labels.
Example: Top.Countries.Europe.Russia
The ltree module provides several data types:
ltree stores a label path.
lquery represents a regular-expression-like pattern for matching ltree values. 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:
foo Match the exact label pathfoo*.foo.* Match any label path containing the labelfoo*.foo Match any label path whose last label isfoo
Both star symbols and simple words can be quantified to restrict how many labels they can match:
*{n} Match exactly n labels
*{n,} Match at least n labels
*{n,m} Match at least n but not more than m labels
*{,m} Match at most m labels — same as *{0,m}
foo{n,m} Match at least n but not more than m occurrences of foo
foo{,} Match any number of occurrences of foo, including zero
In 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 lquery item to make it match more than just the exact match:
@ Match case-insensitively, for examplea@matchesA* Match any label with this prefix, for examplefoo*matchesfoobar% Match initial underscore-separated words
The behavior of % is a bit complicated. It tries to match words rather than the entire label. For example foo_bar% matches foo_bar_baz but not foo_barbaz. If combined with *, prefix matching applies to each word separately, for example foo_bar%* matches foo1_bar2_baz but not foo1_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*|Spain
a. b. c. d. e.
This query will match any label path that:
begins with the label Top
and next has zero to two labels before
a label beginning with the case-insensitive prefix sport
then has one or more labels, none of which match football nor tennis
and then ends with a label beginning with Russ or exactly matching Spain.
ltxtquery represents a full-text-search-like pattern for matching ltree values. An ltxtquery value contains words, possibly with the modifiers @, *, % at the end; the modifiers have the same meanings as in lquery. Words can be combined with & (AND), | (OR), ! (NOT), and parentheses. The key difference from lquery is that ltxtquery matches 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 Europe and any label beginning with Russia (case-insensitive), but not paths containing the label Transportation. 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, but ltree and lquery do not.
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.13 are available.
Table F.13. 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 in Table F.14.
Table F.14. ltree Functions
ltree supports several types of indexes that can speed up the indicated operators:
B-tree index over ltree: <, <=, =, >=, >
GiST index over ltree (gist_ltree_ops opclass): <, <=, =, >=, >, @>, <@, @, ~, ?
gist_ltree_ops GiST opclass approximates a set of path labels as a bitmap signature. Its optional integer parameter siglen determines the signature length in bytes. The default signature length is 8 bytes. The length must be a positive multiple of int alignment (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_ops opclass): ltree[] <@ ltree, ltree @> ltree[], @, ~, ?
gist__ltree_ops GiST opclass works similarly to gist_ltree_ops and also takes signature length as a parameter. The default value of siglen in gist__ltree_ops is 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.
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 ~ '*[email protected].*';
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)
Additional extensions are available that implement transforms for the ltree type for PL/Python. The extensions are called ltree_plpythonu, ltree_plpython2u, and ltree_plpython3u (see Section 46.1 for the PL/Python naming convention). If you install these transforms and specify them when creating a function, ltree values are mapped to Python lists. (The reverse is currently not supported, however.)
It is strongly recommended that the transform extensions be installed in the same schema as ltree. Otherwise there are installation-time security hazards if a transform extension's schema contains objects defined by a hostile user.
All work was done by Teodor Sigaev (<[email protected]>) and Oleg Bartunov (<[email protected]>). 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.
If you see anything in the documentation that is not correct, does not match your experience with the particular feature or requires further clarification, please use this form to report a documentation issue.