Postgresql – Recursive CTE to find unique slug


I have an article table where I want the slug to be unique.

CREATE TABLE article (
   title char(50) NOT NULL,
   slug  char(50) NOT NULL

When the user enters a title e.g. News on Apple, I want to check the database to see if a corresponding slug exists e.g. news-on-apple. If it does, I'll suffix a numerical value until I find a unique one e.g. news-on-apple-1. Can that be achieved with a recursive CTE query instead of doing recursion in my ORM. Is there a good ballpark number where I should stop recursing and error out. I can imagine people using the same title a 1000 times which would result in 1000 queries just to create 1 article.

It's possible that my understanding of recursive CTE is incorrect and there's no better way to find a unique slug. Please suggest any alternatives.

Best Answer

First off, you do not want to use char(50). Use varchar(50) or just text. Read more:

Assuming the following rules:

  • Basic slugs never end with a dash.
  • Duplicate slugs are suffixed with a dash and a sequential number (-123).

Note that all of the following methods are subject to a race conditions: concurrent operations might identify the same "free" name for the next slug.
To defend against it, you can impose a UNIQUE constraint on slug and be prepared to repeat an INSERT upon duplicate key violation or you to take out a write lock on the table at the start of the transaction.

If you glue the suffix to the basic slug name with a dash and allow basic slugs to end in separate numbers, the specification is a tiny bit ambiguous (see comments). I suggest a unique delimiter of your choice instead (which is otherwise disallowed).

Efficient rCTE

  input AS (SELECT 'news-on-apple'::text AS slug)  -- input basic slug here once
, cte   AS (
   SELECT slug || '-' AS slug  -- append '-' once, if basic slug exists
        , 1 as suffix          -- start with suffix 1
   FROM   article
   JOIN   input USING (slug)

   SELECT c.slug, c.suffix + 1  -- increment by 1 ...
   FROM   cte     c
   JOIN   article a ON a.slug = c.slug || c.suffix  -- ... if slug-n already exists
SELECT slug || suffix AS slug
FROM   cte
ORDER  BY suffix DESC  -- pick the last (free) one
)  -- parentheses required
UNION  ALL  -- if the basic slug wasn't taken, fall back to that
SELECT slug FROM input

Better performance without rCTE

If you worry about thousands of slugs competing for the same slug or generally want to optimize performance, I'd consider a different, faster approach.

WITH input AS (SELECT 'news-on-apple'::text  AS slug
                    , 'news-on-apple-'::text AS slug1)  -- input basic slug here
SELECT i.slug
FROM   input        i
LEFT   JOIN article a USING (slug)
WHERE  a.slug IS NULL  -- doesn't exist yet.

(  -- parentheses required
SELECT i.slug1 || COALESCE(right(a.slug, length(i.slug1) * -1)::int + 1, 1)
FROM   input        i
LEFT   JOIN article a ON a.slug LIKE (i.slug1 || '%')  -- match up to last "-"
                     AND right(a.slug, length(i.slug1) * -1) ~ '^\d+$' -- suffix numbers only
ORDER  BY right(a.slug, length(i.slug1) * -1)::int DESC
  • If the basic slug isn't taken yet, the more expensive second SELECT is never executed - same as above, but much more important here. Check with EXPLAIN ANALYZE, Postgres is smart that way with LIMIT queries. Related:

  • Check for the leading string and the suffix separately, so the LIKE expression can use a basic btree index with text_pattern_ops like

    CREATE INDEX article_slug_idx ON article (slug text_pattern_ops);

    Detailed explanation:

  • Convert the suffix to integer before you apply max(). Numbers in text representation don't work.

Optimize performance

To get the optimum, consider storing the suffix separated from the basic slug and concatenate the slug as needed: concat_ws('-' , slug, suffix::text) AS slug

CREATE TABLE article (
   article_id serial PRIMARY KEY
 , title text NOT NULL
 , slug  text NOT NULL
 , suffix int

The query for a new slug then becomes:

    || COALESCE((
          SELECT '-'::text || (max(suffix) + 1)::text
          FROM   article a
          WHERE  a.slug = i.slug), '') As slug
FROM  (SELECT 'news-on-apple'::text AS slug) i  -- input basic slug here

Ideally supported with a unique index on (slug, suffix).

Query for list of slugs

In any version of Postgres you can provide rows in a VALUES expression.

FROM   article
     ('slug-foo'::text, 1)
   ) u(slug,suffix) USING (slug,suffix);

You can also use IN with a set of row-type expressions Which is shorter:

FROM   article
WHERE (slug,suffix) IN (('slug-foo', 1), ('slug-bar',7));

Details under this related question (as commented below):

For long lists, the JOIN to a VALUES expression is typically faster.

In Postgres 9.4 (released today!) you can also use the new variant of unnest() to unnest multiple arrays in parallel.

Given an array of basic slugs and a corresponding array of suffixes (as per comment):

FROM   article
JOIN   unnest('{slug-foo,slug-bar}'::text[]
            , '{1,7}'::int[]) AS u(slug,suffix) USING (slug,suffix);