Re: Unexpected expensive index scan - Mailing list pgsql-performance

From: Jake Nielsen    Sent: Tuesday, September 27, 2016 5:22 PM

the query

SELECT * FROM SyncerEvent WHERE ID > 12468 AND propogatorId NOT IN ('"d8130ab9!-66d0!-4f13!-acec!-a9556362f0ad"') AND conflicted != 1 AND userId = '57dc984f1c87461c0967e228' ORDER BY ID LIMIT 4000;^

On Tue, Sep 27, 2016 at 5:02 PM, Jake Nielsen <jake.k.nielsen@gmail.com> wrote:

I've got a query that takes a surprisingly long time to run, and I'm having a really rough time trying to figure it out.

 

Before I get started, here are the specifics of the situation:

 

Here is the table that I'm working with (apologies for spammy indices, I've been throwing shit at the wall)

                            Table "public.syncerevent"

    Column    |  Type   |                        Modifiers                         

--------------+---------+----------------------------------------------------------

 id           | bigint  | not null default nextval('syncerevent_id_seq'::regclass)

 userid       | text    | 

 event        | text    | 

 eventid      | text    | 

 originatorid | text    | 

 propogatorid | text    | 

 kwargs       | text    | 

 conflicted   | integer | 

Indexes:

    "syncerevent_pkey" PRIMARY KEY, btree (id)

    "syncereventidindex" UNIQUE, btree (eventid)

    "anothersyncereventidindex" btree (userid)

    "anothersyncereventidindexwithascending" btree (userid, id)

    "asdfasdgasdf" btree (userid, id DESC)

    "syncereventuseridhashindex" hash (userid)

 

To provide some context, as per the wiki, 

there are 3,290,600 rows in this table. 

It gets added to frequently, but never deleted from. 

The "kwargs" column often contains mid-size JSON strings (roughly 30K characters on average)

As of right now, the table has 53 users in it. About 20% of those have a negligible number of events, but the rest of the users have a fairly even smattering.

 

EXPLAIN (ANALYZE, BUFFERS) says:

                                                                          QUERY PLAN                                                                          

--------------------------------------------------------------------------------------------------------------------------------------------------------------

 Limit  (cost=0.43..1218.57 rows=4000 width=615) (actual time=3352.390..3403.572 rows=4000 loops=1)  Buffers: shared hit=120244 read=160198

   ->  Index Scan using syncerevent_pkey on syncerevent  (cost=0.43..388147.29 rows=1274560 width=615) (actual time=3352.386..3383.100 rows=4000 loops=1)

         Index Cond: (id > 12468)

         Filter: ((propogatorid <> '"d8130ab9!-66d0!-4f13!-acec!-a9556362f0ad"'::text) AND (conflicted <> 1) AND (userid = '57dc984f1c87461c0967e228'::text))

         Rows Removed by Filter: 1685801

         Buffers: shared hit=120244 read=160198

 Planning time: 0.833 ms

 Execution time: 3407.633 ms

(9 rows)

If it matters/interests you, here is my underlying confusion:

From some internet sleuthing, I've decided that having a table per user (which would totally make this problem a non-issue) isn't a great idea. Because there is a file per table, having a table per user would not scale. My next thought was partial indexes (which would also totally help), but since there is also a table per index, this really doesn't side-step the problem. My rough mental model says: If there exists a way that a table-per-user scheme would make this more efficient, then there should also exist an index that could achieve the same effect (or close enough to not matter). I would think that "userid = '57dc984f1c87461c0967e228'" could utilize at least one of the two indexes on the userId column, but clearly I'm not understanding something.

Any help in making this query more efficient would be greatly appreciated, and any conceptual insights would be extra awesome.

Thanks for reading.

-Jake