ltree — hierarchical tree-like data type

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.

Definitions

A label is a sequence of alphanumeric characters and underscores, and hyphens. Valid alphanumeric character ranges are dependent on the database locale. For example, in C locale the characters A-Za-z0-9_ are allowed. Labels must be no more than 1000 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 path foo
*.foo.*     Match any label path containing the label foo
*.foo       Match any label path whose last label is foo

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 example a@ matches A
*           Match any label with this prefix, for example foo* matches foobar
%           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: \
a. begins with the label *Top* \
b. and next has zero to two labels before \
c. a label beginning with the case-insensitive prefix *sport* \
d. then has one or more labels, none of which match *football* nor *tennis* \
e. 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.

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 below are available.

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
lquery ~ ltree → boolean

Does ltree match lquery?

ltree ? lquery[] → boolean
lquery[] ? ltree → boolean

Does ltree match any lquery in array?

ltree @ ltxtquery → boolean
ltxtquery @ ltree → boolean

Does ltree match ltxtquery?

ltree || ltree → ltree

Concatenates ltree paths.

ltree || text → ltree
text || ltree → ltree

Converts text to ltree and concatenates.

ltree[] @> ltree → boolean
ltree <@ ltree[] → boolean

Does array contain an ancestor of ltree?

ltree[] <@ ltree → boolean
ltree @> ltree[] → boolean

Does array contain a descendant of ltree?

ltree[] ~ lquery → boolean
lquery ~ ltree[] → boolean

Does array contain any path matching lquery?

ltree[] ? lquery[] → boolean
lquery[] ? ltree[] → boolean

Does ltree array contain any path matching any lquery?

ltree[] @ ltxtquery → boolean
ltxtquery @ ltree[] → boolean

Does array contain any path matching ltxtquery?

ltree[] ?@> ltree → ltree

Returns first array entry that is an ancestor of ltree, or NULL if none.

ltree[] ?<@ ltree → ltree

Returns first array entry that is a descendant of ltree, or NULL if none.

ltree[] ?~ lquery → ltree

Returns first array entry that matches lquery, or NULL if none.

ltree[] ?@ ltxtquery → ltree

Returns first array entry that matches ltxtquery, or 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 ltree functions are shown below.

subltree ( ltree, start integer, end integer ) → ltree

Returns subpath of ltree from position start to position end-1 (counting from 0).

Example:

subltree('Top.Child1.Child2', 1, 2) → Child1

subpath ( ltree, offset integer, len integer ) → ltree

Returns subpath of ltree starting at position offset, with 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.

Example:

subpath('Top.Child1.Child2', 0, 2) → Top.Child1

subpath ( ltree, offset integer ) → ltree

Returns 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.

Example:

subpath('Top.Child1.Child2', 1) → Child1.Child2

nlevel ( ltree ) → integer

Returns number of labels in path.

Example:

nlevel('Top.Child1.Child2') → 3

index ( a ltree, b ltree ) → integer

Returns position of first occurrence of b in a, or -1 if not found.

Example:

index('0.1.2.3.5.4.5.6.8.5.6.8', '5.6') → 6

index ( a ltree, b ltree, смещение integer ) → integer

Returns position of first occurrence of b in a, or -1 if not found. The search starts at position offset; negative offset means start -offset labels from the end of the path.

Example:

index('0.1.2.3.5.4.5.6.8.5.6.8', '5.6', -4) → 9

text2ltree ( text ) → ltree

Casts text to ltree.

ltree2text ( ltree ) → text

Casts ltree to text.

lca ( ltree [, ltree [, ... ]] ) → ltree

Computes longest common ancestor of paths (up to 8 arguments are supported).

Example:

lca('1.2.3', '1.2.3.4.5.6') → 1.2

lca ( ltree[] ) → ltree

Computes longest common ancestor of paths in array.

Example:

lca(array['1.2.3'::ltree,'1.2.3.4']) → 1.2

Indexes

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.

Example

This example uses the following data:

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 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)

Authors

All work was done by Teodor Sigaev (teodor@stack.net) and Oleg Bartunov (oleg@sai.msu.su). 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.

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