tags_count only need to join on the taggable's table if using STI

[hengwoon]

The INNER JOIN on the taggable's table name is only necessary if the taggable is using single table inheritance. Otherwise, the IN subquery generated in generate_tagging_scope_in_clause, which retrieves a list of the taggable's IDs based on the current scope, is sufficient. tagging_conditions also already includes making sure that taggable_type is the one that corresponds to the taggable.

The removal of the extra inner join in non-STI cases improves performance quite significantly for tag counts.

Take into consideration Project.tag_counts_on(:tags), and Project.where("client is not null").tag_counts_on(:tag)

Current:

SELECT  tags.*, taggings.tags_count AS count 
FROM "tags" 
JOIN (
  SELECT taggings.tag_id, COUNT(taggings.tag_id) AS tags_count 
  FROM "taggings" 
  INNER JOIN projects ON projects.id = taggings.taggable_id 
  WHERE (taggings.taggable_type = 'Project' AND taggings.context = 'tags') 
  AND (taggings.taggable_id IN(
    SELECT projects.id FROM "projects"
  )) 
  GROUP BY taggings.tag_id 
  HAVING COUNT(taggings.tag_id) > 0
) AS taggings ON taggings.tag_id = tags.id

SELECT  tags.*, taggings.tags_count AS count 
FROM "tags" 
JOIN (
  SELECT taggings.tag_id, COUNT(taggings.tag_id) AS tags_count 
  FROM "taggings" 
  INNER JOIN projects ON projects.id = taggings.taggable_id 
  WHERE (taggings.taggable_type = 'Project' AND taggings.context = 'tags') 
  AND (taggings.taggable_id IN(
    SELECT projects.id FROM "projects" where client is not null
  )) 
  GROUP BY taggings.tag_id 
  HAVING COUNT(taggings.tag_id) > 0
) AS taggings ON taggings.tag_id = tags.id

With this change:

SELECT  tags.*, taggings.tags_count AS count 
FROM "tags" 
JOIN (
  SELECT taggings.tag_id, COUNT(taggings.tag_id) AS tags_count 
  FROM "taggings"
  WHERE (taggings.taggable_type = 'Project' AND taggings.context = 'tags') 
  AND (taggings.taggable_id IN(
    SELECT projects.id FROM "projects"
  )) 
  GROUP BY taggings.tag_id 
  HAVING COUNT(taggings.tag_id) > 0
) AS taggings ON taggings.tag_id = tags.id

SELECT  tags.*, taggings.tags_count AS count 
FROM "tags" 
JOIN (
  SELECT taggings.tag_id, COUNT(taggings.tag_id) AS tags_count 
  FROM "taggings" 
  WHERE (taggings.taggable_type = 'Project' AND taggings.context = 'tags') 
  AND (taggings.taggable_id IN(
    SELECT projects.id FROM "projects" where client is not null
  )) 
  GROUP BY taggings.tag_id 
  HAVING COUNT(taggings.tag_id) > 0
) AS taggings ON taggings.tag_id = tags.id

Fixes #724

[seuros]

Can you add a test case and the change log entry ?

[hengwoon]

I've updated the changelog as well as added a test case and modified the version.