Is PostgreSQL's Ltree module a good fit for threaded comments?

1. Yes and yes;
2. Hierarchy of sections in a knowledge base (one of the implementations);
3. Yes;

The definition of one of the tables in question:

                                                   Table "knowledgebase.section"           Column           |           Type           |                                  Modifiers----------------------------+--------------------------+----------------------------------------------------------------------------- section_sid                | integer                  | not null default nextval('knowledgebase.section_section_sid_seq'::regclass) section                    | character varying        | not null description                | character varying        | path                       | ltree                    | not null is_active                  | boolean                  | not null default true role_sid                   | integer                  | not null last_modified_by           | integer                  | not null creation_datetime          | timestamp with time zone | not null default now() last_modification_datetime | timestamp with time zone | not null default now() is_expanded                | boolean                  | not null default false section_idx                | tsvector                 |Indexes:    "section_sid_pkey" PRIMARY KEY, btree (section_sid)    "section_section_key" UNIQUE, btree (section)    "idxsection_idx" gist (section_idx)    "path_gist_idx" gist (path)Foreign-key constraints:    "last_modified_by_fkey" FOREIGN KEY (last_modified_by) REFERENCES "user"."role"(role_sid) ON UPDATE CASCADE ON DELETE RESTRICT    "role_sid_fkey" FOREIGN KEY (role_sid) REFERENCES "user"."role"(role_sid) ON  UPDATE CASCADE ON DELETE RESTRICTTriggers:    section_idx_update BEFORE INSERT OR UPDATE ON knowledgebase.section FOR EACH ROW EXECUTE PROCEDURE tsearch2('section_idx', 'section')

The "path" column uses the primary key as a label.

A sample of the current contents of that table (regarding the primary key and the "path" column):

  section_sid | path -------------+-------           53 | 34.53           56 | 56           55 | 29.55           35 | 35           54 | 34.54           37 | 30.37          ... | ...

I recommend anyone implementing hierarchical relationships in SQL read Joe Celko's Trees and Hierarchies in SQL for Smarties.

Traversing arbitrary depth parent child links can be very inefficient when using just a parent_id. The book describes techniques that make this access fast.

One strategy (which I happen to use) can also be found for free in this series of articles:

• Part 1 (look for the section "Trees in SQL") (Wayback link)
• Part 2 (Wayback link)
• Part 3 (Wayback link)
• Part 4 (Wayback link)

Version 8.4 of PostgreSQL will be bringing Common Table Expressions functionality into the core with WITH and WITH... RECURSIVE expressions. If you're modifying old code, you may want to wait until 8.4 is released, as then you won't have to worry about any incompatibilities between Ltree and the new core syntax. If you're working with old code, or do not want to wait for 8.4, you will probably want to make sure you write code that is easily translatable to the new syntax, especially if you're changing an old schema or designing a new one.

See also:

• The 8.4 manual entry on WITH queries and CTEs
• A writeup with examples on the 8.4 implementation of CTEs