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authorAkihiko Odaki <nekomanma@pixiv.co.jp>2020-08-25 20:39:35 +0900
committerGitHub <noreply@github.com>2020-08-25 13:39:35 +0200
commit41eeb9ebaa65abe3fcbab60847b69d2469726d8a (patch)
treef4ea8b1eee79722b5f0eb12f0b4067a58288c99f
parent9cadd40cf4e52a299ba828e84b6a4fddaf976afb (diff)
Use Status.group instead of Status.distinct in HashQueryService (#14662)
DISTINCT clause removes duplicated records according to all the selected
attributes. In reality, it can remove duplicated records only looking at
statuses.id, but the clause confuses the query planner and yields
insufficient performance.
The behavior is also problematic if the scope produced by HashQueryService
is used to query columns without id (using pluck method, for example). The
scope is expected to contain unique statuses, but the uniquness will be
evaluated with some arbitrary columns other than id.

GROUP BY clause resolves those problem by explicitly specifying the
column to take into account for the record distinction.

A workaround for the problem of DISTINCT clause in
Api::V1::Timelines::TagController is no longer necessary and removed.
-rw-r--r--app/controllers/api/v1/timelines/tag_controller.rb4
-rw-r--r--app/services/hashtag_query_service.rb2
2 files changed, 2 insertions, 4 deletions
diff --git a/app/controllers/api/v1/timelines/tag_controller.rb b/app/controllers/api/v1/timelines/tag_controller.rb
index 2d6ad5a80..62f34d3f7 100644
--- a/app/controllers/api/v1/timelines/tag_controller.rb
+++ b/app/controllers/api/v1/timelines/tag_controller.rb
@@ -33,9 +33,7 @@ class Api::V1::Timelines::TagController < Api::BaseController
       )
 
       if truthy_param?(:only_media)
-        # `SELECT DISTINCT id, updated_at` is too slow, so pluck ids at first, and then select id, updated_at with ids.
-        status_ids = statuses.joins(:media_attachments).distinct(:id).pluck(:id)
-        statuses.where(id: status_ids)
+        statuses.joins(:media_attachments)
       else
         statuses
       end
diff --git a/app/services/hashtag_query_service.rb b/app/services/hashtag_query_service.rb
index 196de0639..0bdf60221 100644
--- a/app/services/hashtag_query_service.rb
+++ b/app/services/hashtag_query_service.rb
@@ -8,7 +8,7 @@ class HashtagQueryService < BaseService
     all  = tags_for(params[:all])
     none = tags_for(params[:none])
 
-    Status.distinct
+    Status.group(:id)
           .as_tag_timeline(tags, account, local)
           .tagged_with_all(all)
           .tagged_with_none(none)