Query and order by number of matches in JSON array

Question

Using JSON arrays in a jsonb column in Postgres 9.4 and Rails, I can set up a scope that returns all rows containing any elements from an array passed to the scope method - like so:

scope :tagged, ->(tags) {
  where(["data->'tags' ?| ARRAY[:tags]", { tags: tags }])
}

I'd also like to order the results based on the number of matched elements in the array.

I appreciate I might need to step outside the confines of ActiveRecord to do this, so a vanilla Postgres SQL answer is helpful too, but bonus points if it can be wrapped up in ActiveRecord so it can be a chain-able scope.

As requested, here's an example table. (Actual schema is far more complicated but this is all I'm concerned about.)

 id |               data                
----+-----------------------------------
  1 | {"tags": ["foo", "bar", "baz"]}
  2 | {"tags": ["bish", "bash", "baz"]}
  3 |
  4 | {"tags": ["foo", "foo", "foo"]}

The use case is to find related content based on tags. More matching tags are more relevant, hence results should be ordered by the number of matches. In Ruby I'd have a simple method like this:

Page.tagged(['foo', 'bish', 'bash', 'baz']).all

Which should return the pages in the following order: 2, 1, 4.

Answer 1

Your arrays contain only primitive values, nested documents would be more complicated.

Query

Unnest the JSON arrays of found rows with jsonb_array_elements_text() in a LATERAL join and count matches:

SELECT *
FROM  (
   SELECT *
   FROM   tbl
   WHERE  data->'tags' ?| ARRAY['foo', 'bar']
   ) t
, LATERAL (
   SELECT count(*) AS ct
   FROM   jsonb_array_elements_text(t.data->'tags') a(elem)
   WHERE  elem = ANY (ARRAY['foo', 'bar'])  -- same array parameter
   ) ct
ORDER  BY ct.ct DESC;  -- more expressions to break ties?

Alternative with INSTERSECT. It's one of the rare occasions that we can make use of this basic SQL feature:

SELECT *
FROM  (
   SELECT *
   FROM   tbl
   WHERE  data->'tags' ?| '{foo, bar}'::text[]  -- alt. syntax w. array
   ) t
, LATERAL (
   SELECT count(*) AS ct
   FROM  (
      SELECT * FROM jsonb_array_elements_text(t.data->'tags')
      INTERSECT ALL
      SELECT * FROM unnest('{foo, bar}'::text[])  -- same array literal
      ) i
   ) ct
ORDER  BY ct.ct DESC;

Note a subtle difference: This consumes each element when matched, so it does not count unmatched duplicates in data->'tags' like the first variant does. For details see the demo below.

Also demonstrating an alternative way to pass the array parameter: as array literal: '{foo, bar}'. This may be simpler to handle for some clients:

• PostgreSQL: Issue with passing array to procedure

Or you could create a server side search function taking a VARIADIC parameter and pass a variable number of plain text values:

• Passing multiple values in single parameter

Related:

• Check if key exists in a JSON with PL/pgSQL?

Index

Be sure to have a functional GIN index to support the jsonb existence operator ?|:

CREATE INDEX tbl_dat_gin ON tbl USING gin (data->'tags');

• Index for finding an element in a JSON array
• What's the proper index for querying structures in arrays in Postgres jsonb?

Nuances with duplicates

Clarification as per request in the comment. Say, we have a JSON array with two duplicated tags (4 total):

jsonb '{"tags": ["foo", "bar", "foo", "bar"]}'

And search with an SQL array parameter including both tags, one of them duplicated (3 total):

'{foo, bar, foo}'::text[]

Consider the results of this demo:

SELECT *
FROM  (SELECT jsonb '{"tags":["foo", "bar", "foo", "bar"]}') t(data)

, LATERAL (
   SELECT count(*) AS ct
   FROM   jsonb_array_elements_text(t.data->'tags') e
   WHERE  e = ANY ('{foo, bar, foo}'::text[])
   ) ct

, LATERAL (
   SELECT count(*) AS ct_intsct_all
   FROM  (
      SELECT * FROM jsonb_array_elements_text(t.data->'tags')
      INTERSECT ALL
      SELECT * FROM unnest('{foo, bar, foo}'::text[])
      ) i
   ) ct_intsct_all

, LATERAL (
   SELECT count(DISTINCT e) AS ct_dist
   FROM   jsonb_array_elements_text(t.data->'tags') e
   WHERE  e = ANY ('{foo, bar, foo}'::text[])
   ) ct_dist

, LATERAL (
   SELECT count(*) AS ct_intsct
   FROM  (
      SELECT * FROM jsonb_array_elements_text(t.data->'tags')
      INTERSECT
      SELECT * FROM unnest('{foo, bar, foo}'::text[])
      ) i
   ) ct_intsct;

Result:

data                                     | ct | ct_intsct_all | ct_dist | ct_intsct
-----------------------------------------+----+---------------+---------+----------
'{"tags": ["foo", "bar", "foo", "bar"]}' | 4  | 3             | 2       | 2

Comparing elements in the JSON array to elements in the array parameter:

• 4 tags match any of the search elements: ct.
• 3 tags in the set intersect (can be matched element-to-element): ct_intsct_all.
• 2 distinct matching tags can be identified: ct_dist or ct_intsct.

If you don't have dupes or if you don't care to exclude them, use one of the first two techniques. The other two are a bit slower (besides the different result), because they have to check for dupes.