F.22. ltree
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## F.22. 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.22.1. 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.

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. 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

  Star symbols can also 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}

  There are several modifiers that can be put at the end of a non-star label in `lquery` 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 labels separated with `|` (OR) to match any of those labels, and you can put `!` (NOT) at the start to match any label that doesn't match any of the alternatives.

  Here's an annotated example of `lquery`:

      Top.*{0,2}.sport*@.!football|tennis.Russ*|Spain
      a.  b.     c.      d.               e.

  This query will match any label path that:

  1. begins with the label `Top`

  2. and next has zero to two labels before

  3. a label beginning with the case-insensitive prefix `sport`

  4. then a label not matching `football` nor `tennis`

  5. 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.

### F.22.2. 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.14 are available.

Table F.14. `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.15.

Table F.15. `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.22.3. Indexes

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

- B-tree index over `ltree`: `<`, `<=`, `=`, `>=`, `>`

- GiST index over `ltree`: `<`, `<=`, `=`, `>=`, `>`, `@>`, `<@`, `@`, `~`, `?`

  Example of creating such an index:

      CREATE INDEX path_gist_idx ON test USING GIST (path);

- GiST index over `ltree[]`: `ltree[] <@ ltree`, `ltree @> ltree[]`, `@`, `~`, `?`

  Example of creating such an index:

      CREATE INDEX path_gist_idx ON test USING GIST (array_path);

  Note: This index type is lossy.

### F.22.4. 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.22.5. Transforms

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 45.1 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.)

### F.22.6. 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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