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Disclaimer: I am relatively new to PostgreSQL.

I'm wondering how to optimize a query that does 2 INNER JOINs. My scenario is fairly simple:

Select Posts with a photo (Posts.photo IS NOT NULL) and a Hashtag that has the name 'dead' (Hashtags.name = 'dead').

The associations are as follows:

Posts <- PostHashtags -> Hashtags

Posts.id    = PostHashtags.postId (FK)
Hashtags.id = PostHashtags.hashtagId (FK)

Here is the query:

SELECT
  "Posts".*,
  "hashtags"."id" AS "hashtags.id",
  "hashtags"."count" AS "hashtags.count",
  "hashtags"."name" AS "hashtags.name",
  "hashtags"."createdAt" AS "hashtags.createdAt",
  "hashtags"."updatedAt" AS "hashtags.updatedAt",
  "hashtags"."objectId" AS "hashtags.objectId",
  "hashtags"."_etl" AS "hashtags._etl",
  "hashtags.PostHashtag"."id" AS "hashtags.PostHashtag.id",
  "hashtags.PostHashtag"."createdAt" AS "hashtags.PostHashtag.createdAt",
  "hashtags.PostHashtag"."updatedAt" AS "hashtags.PostHashtag.updatedAt",
  "hashtags.PostHashtag"."postId" AS "hashtags.PostHashtag.postId",
  "hashtags.PostHashtag"."hashtagId" AS "hashtags.PostHashtag.hashtagId",
  "hashtags.PostHashtag"."objectId" AS "hashtags.PostHashtag.objectId",
  "hashtags.PostHashtag"."_etl" AS "hashtags.PostHashtag._etl"

FROM (
  SELECT
    "Posts"."id",
    "Posts"."note",
    "Posts"."photo",
    "Posts"."createdAt",
    "user"."id" AS "user.id",
    "user"."name" AS "user.name"
  FROM "Posts" AS "Posts"

  INNER JOIN "Users" AS "user" ON "Posts"."userId" = "user"."id"

  WHERE "Posts"."photo" IS NOT NULL
  AND (
    SELECT "PostHashtags"."id" FROM "PostHashtags" AS "PostHashtags"
    INNER JOIN "Hashtags" AS "Hashtag" ON "PostHashtags"."hashtagId" = "Hashtag"."id"
    WHERE "Posts"."id" = "PostHashtags"."postId"
    LIMIT 1
  ) IS NOT NULL

  ORDER BY "Posts"."createdAt" DESC LIMIT 10
) AS "Posts"

INNER JOIN (
  "PostHashtags" AS "hashtags.PostHashtag"
  INNER JOIN "Hashtags" AS "hashtags" ON "hashtags"."id" = "hashtags.PostHashtag"."hashtagId"
)

ON "Posts"."id" = "hashtags.PostHashtag"."postId"
AND "hashtags"."name" = 'dead'

ORDER BY "Posts"."createdAt" DESC;

EXPLAIN results:

Nested Loop  (cost=886222912.89..886223769.55 rows=1 width=277)
  Join Filter: ("hashtags.PostHashtag"."postId" = "Posts".id)
  ->  Limit  (cost=886220835.39..886220835.42 rows=10 width=189)
        ->  Sort  (cost=886220835.39..886220988.88 rows=61394 width=189)
              Sort Key: "Posts"."createdAt"
              ->  Nested Loop  (cost=0.42..886219508.69 rows=61394 width=189)
                    ->  Seq Scan on "Posts"  (cost=0.00..885867917.51 rows=78196 width=177)
                          Filter: ((photo IS NOT NULL) AND ((SubPlan 1) IS NOT NULL))
                          SubPlan 1
                            ->  Limit  (cost=0.42..815.70 rows=1 width=4)
                                  ->  Nested Loop  (cost=0.42..815.70 rows=1 width=4)
                                        ->  Seq Scan on "PostHashtags"  (cost=0.00..811.25 rows=1 width=8)
                                              Filter: ("Posts".id = "postId")
                                        ->  Index Only Scan using "Hashtags_pkey" on "Hashtags" "Hashtag"  (cost=0.42..4.44 rows=1 width=4)
                                              Index Cond: (id = "PostHashtags"."hashtagId")
                    ->  Index Scan using "Users_pkey" on "Users" "user"  (cost=0.42..4.49 rows=1 width=16)
                          Index Cond: (id = "Posts"."userId")
  ->  Materialize  (cost=2077.50..2933.89 rows=1 width=88)
        ->  Hash Join  (cost=2077.50..2933.89 rows=1 width=88)
              Hash Cond: ("hashtags.PostHashtag"."hashtagId" = hashtags.id)
              ->  Seq Scan on "PostHashtags" "hashtags.PostHashtag"  (cost=0.00..721.00 rows=36100 width=40)
              ->  Hash  (cost=2077.49..2077.49 rows=1 width=48)
                    ->  Seq Scan on "Hashtags" hashtags  (cost=0.00..2077.49 rows=1 width=48)
                          Filter: ((name)::text = 'dead'::text)

This query has been simplified slightly. It also performs OUTER JOINS on other data related to Posts, which is why the SELECT must be performed on Posts instead of, say, PostHashtags.

Any help in translating the EXPLAIN to a useful index would be greatly appreciated.

My ideas:

  1. Build an index on Posts.photo, but should it be a partial index WHERE "photo" IS NOT NULL?
  2. Build a UNIQUE index on Hashtags.name.

I'm not sure if those are necessarily the bottlenecks, though.

  • 1
    The output of EXPLAIN (BUFFERS, ANALYZE) would be more helpful. Plus your version of Postgres. What do you get for: SELECT count(*) AS ct, count(photo) AS pct FROM "Posts"? Most importantly: Your description does not match what the query does exactly: from the latest 10 posts with photo and any tags, display only those with the tag 'dead'. Is that intented or is LIMIT 10 in the subquery a debugging residue? Changes the nature of the query. – Erwin Brandstetter Jan 16 '15 at 0:47
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Also consider the first answer.

Query

This does what your current query currently does, just simpler and faster:

SELECT p.id, p.note, p.photo, p."createdAt",
  u.id           AS "user.id",
  u.name         AS "user.name",
  h.id           AS "hashtags.id",
  h.count        AS "hashtags.count",
  h.name         AS "hashtags.name",
  h."createdAt"  AS "hashtags.createdAt",
  h."updatedAt"  AS "hashtags.updatedAt",
  h."objectId"   AS "hashtags.objectId",
  h._etl         AS "hashtags._etl",
  ph.id          AS "hashtags.PostHashtag.id",
  ph."createdAt" AS "hashtags.PostHashtag.createdAt",
  ph."updatedAt" AS "hashtags.PostHashtag.updatedAt",
  ph."postId"    AS "hashtags.PostHashtag.postId",
  ph."hashtagId" AS "hashtags.PostHashtag.hashtagId",
  ph."objectId"  AS "hashtags.PostHashtag.objectId",
  ph._etl        AS "hashtags.PostHashtag._etl"
FROM (
    SELECT id, note, photo, "createdAt", "userId"
    FROM   "Posts" p
    WHERE  photo IS NOT NULL
    AND    EXISTS (
        SELECT 1
        FROM   "PostHashtags" ph
        WHERE  ph."postId" = p.id
        )
    ORDER  BY p."createdAt" DESC
    LIMIT  10
   ) p
JOIN   "PostHashtags" ph ON ph."postId" = p.id
JOIN   "Hashtags"     h  ON h.id = ph."hashtagId"
JOIN   "Users"        u  ON u.id = p."userId"
WHERE  h.name = 'dead'
ORDER  BY p."createdAt" DESC;

The EXISTS semi-join should be faster than your subquery construct. I am assuming that the column "PostHashtags".id is the PK and cannot be NULL by itself. Also, if referential integrity is enforced by a FK constraint, no need to join to "Hashtags" in this test.

Indexes

Partial index on Posts

CREATE INDEX posts_foo_idx ON "Posts" ("createdAt", id)
WHERE photo IS NOT NULL;

Note the columns: ("createdAt", id). Postgres will the latest posts, I expect an index scan on posts_foo_idx from the top, followed by a test for matching entries in PostHashtags using id with the next index.

UNIQUE index on PostHashtags

This time we need the index with "postId" first.

The rest is mostly like in the first answer.

  • I have never seen those multiple "FROM"s - is that some kind of specialty of Postgres? Or was it meant to be a JOIN? – cslotty Oct 25 '16 at 16:02
  • 1
    @cslotty: Nope, that was just a typo that went unnoticed - until now. Thanks, fixed. – Erwin Brandstetter Oct 25 '16 at 22:37
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Also consider the alternative answer.

Query

This does what your description says, not what your query currently does (see comment):

Select Posts with a photo and a Hashtag that has the name 'dead'

.. limited to the latest 10 qualifying posts:

SELECT
  p.id,
  p.note,
  p.photo,
  p."createdAt",
  u.id           AS "user.id",
  u.name         AS "user.name",
  h.id           AS "hashtags.id",
  h.count        AS "hashtags.count",
  h.name         AS "hashtags.name",
  h."createdAt"  AS "hashtags.createdAt",
  h."updatedAt"  AS "hashtags.updatedAt",
  h."objectId"   AS "hashtags.objectId",
  h._etl         AS "hashtags._etl",
  ph.id          AS "hashtags.PostHashtag.id",
  ph."createdAt" AS "hashtags.PostHashtag.createdAt",
  ph."updatedAt" AS "hashtags.PostHashtag.updatedAt",
  ph."postId"    AS "hashtags.PostHashtag.postId",
  ph."hashtagId" AS "hashtags.PostHashtag.hashtagId",
  ph."objectId"  AS "hashtags.PostHashtag.objectId",
  ph._etl        AS "hashtags.PostHashtag._etl"
FROM   "Hashtags"     h
JOIN   "PostHashtags" ph ON ph."hashtagId" = h.id
JOIN   "Posts"        p  ON p.id = ph."postId"
JOIN   "Users"        u  ON u.id = p."userId"
WHERE  h.name = 'dead'
AND    p.photo IS NOT NULL
ORDER  BY p."createdAt" DESC
LIMIT  10;

If you enforce referential integrity between Posts and Users with a FK constraint, make that
LEFTJOIN "Users"
This way, Postgres can pick the latest ten rows before considering users.

The various subqueries and parentheses in your joins were not helpful.

Indexes

Both your ideas are good. Plus at least one more.

Partial index on Posts

CREATE INDEX posts_foo_idx ON "Posts" (id, "createdAt")
WHERE photo IS NOT NULL;

Note the columns: (id, "createdAt"). My expectation is that Postgres will start with the most selective predicate on Hashtags.name and work its way via PostHashtags to Posts. Then the used index posts_foo_idx already provides createdAt, which is needed for ORDER BY / LIMIT.

The partial index only makes sense if a big percentage of the table has photo IS NOT NULL. Else, make it a full index.

UNIQUE index on Hashtags.name.

CREATE UNIQUE INDEX hashtags_foo_idx ON "Hashtags" (name);

UNIQUE index on PostHashtags

Assuming each post can only be associated with each hashtag once.

ALTER TABLE "PostHashtags" ADD CONSTRAINT ph_uni UNIQUE ("hashtagId", "postId");

This could be your PK. Maybe it is your PK already, but with reversed column order: ("postId", "hashtagId"). You probably should have both variants. For my query you need this one with "hashtagId" first. For your query, you'd need it the other way round. Consider:

Aside

I suggest you reconsider your naming convention. Use legal, lower case identifiers to avoid the quoting mess and much potential for errors.

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