Looks like my client mangled (reflowed) the command outputs, so I
requoted them in this message sent as plain text. In case it happens
again I also posted them to a pastebin here:
https://gist.github.com/yuriks/e86fb0c3cefb8d348c34
--yuriks
On Sat, Nov 15, 2014, at 09:16 PM, Yuri Kunde Schlesner wrote:
> Hi all,
>
> Excuse me if I made any mistakes, as this is my first time posting to a mailing list.
>
> I'm a user of Quassel, a IRC client that uses postgres a backing store for IRC logs and am running into heavy
intermittentperformance problems. I've tracked it down to a query that takes a very long time (around 4 minutes) to
completewhen its data isn't cached.
>
> This is the layout of the table being queried and EXPLAIN ANALYZE result for the problematic query:
>
> quassel=> \d backlog
> Table "public.backlog"
> Column | Type | Modifiers
> -----------+-----------------------------+-------------------------------------------------------------
> messageid | integer | not null default nextval('backlog_messageid_seq'::regclass)
> time | timestamp without time zone | not null
> bufferid | integer | not null
> type | integer | not null
> flags | integer | not null
> senderid | integer | not null
> message | text |
> Indexes:
> "backlog_pkey" PRIMARY KEY, btree (messageid)
> "backlog_bufferid_idx" btree (bufferid, messageid DESC)
> Foreign-key constraints:
> "backlog_bufferid_fkey" FOREIGN KEY (bufferid) REFERENCES buffer(bufferid) ON DELETE CASCADE
> "backlog_senderid_fkey" FOREIGN KEY (senderid) REFERENCES sender(senderid) ON DELETE SET NULL
>
> quassel=> explain (analyze, buffers) SELECT messageid, time, type, flags, sender, message
> FROM backlog
> LEFT JOIN sender ON backlog.senderid = sender.senderid
> WHERE bufferid = 39
> ORDER BY messageid DESC LIMIT 10;
> QUERY PLAN
>
---------------------------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.72..37.78 rows=10 width=102) (actual time=154410.353..154410.424 rows=10 loops=1)
> Buffers: shared hit=13952 read=19244
> -> Nested Loop Left Join (cost=0.72..145800.61 rows=39345 width=102) (actual time=154410.350..154410.414 rows=10
loops=1)
> Buffers: shared hit=13952 read=19244
> -> Index Scan Backward using backlog_pkey on backlog (cost=0.43..63830.21 rows=39345 width=62) (actual
time=154410.327..154410.341rows=10 loops=1)
> Filter: (bufferid = 39)
> Rows Removed by Filter: 1248320
> Buffers: shared hit=13921 read=19244
> -> Index Scan using sender_pkey on sender (cost=0.29..2.07 rows=1 width=48) (actual time=0.005..0.005
rows=1loops=10)
> Index Cond: (backlog.senderid = senderid)
> Buffers: shared hit=31
> Total runtime: 154410.477 ms
> (12 rows)
>
> This plan is consistently chosen, even after ANALYZEing and REINDEXing the table. It looks like Postgres is opting to
doa sequential scan of the backlog_pkey index, filtering rows by bufferid, instead of directly using the
backlog_bufferid_idxindex that directly maps to the operation being made by the query. I was advised on IRC to try
droppingthe backlog_pkey index to force Postgres to use the correct one, and that uses a better plan:
>
> quassel=> begin;
> BEGIN
> quassel=> alter table backlog drop constraint backlog_pkey;
> ALTER TABLE
> quassel=> explain analyze SELECT messageid, time, type, flags, sender, message
> FROM backlog
> LEFT JOIN sender ON backlog.senderid = sender.senderid
> WHERE bufferid = 39
> ORDER BY messageid DESC LIMIT 10;
> QUERY PLAN
>
------------------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.72..40.50 rows=10 width=102) (actual time=63.826..162.134 rows=10 loops=1)
> -> Nested Loop Left Join (cost=0.72..156518.91 rows=39345 width=102) (actual time=63.823..162.126 rows=10
loops=1)
> -> Index Scan using backlog_bufferid_idx on backlog (cost=0.43..74548.51 rows=39345 width=62) (actual
time=63.798..63.814rows=10 loops=1)
> Index Cond: (bufferid = 39)
> -> Index Scan using sender_pkey on sender (cost=0.29..2.07 rows=1 width=48) (actual time=8.532..9.825
rows=1loops=10)
> Index Cond: (backlog.senderid = senderid)
> Total runtime: 162.377 ms
> (7 rows)
>
> quassel=> rollback;
> ROLLBACK
>
> (This query was also run with empty caches.) bufferid=39 in particular has this issue because it hasn't had any
messagesposted to for a long time, so scanning backlog upwards will take a long time to gather 10 messages from it. In
contrast,most other bufferid's have their messages interleaved on the last entries of backlog. I believe this might be
throwingPostgres' estimates off.
>
> Does anyone know if there's any tweaking I can do in Postgres so that it uses the appropriate plan?
>
> Info about my setup:
> PostgreSQL 9.3.5 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.9.1, 64-bit
> Arch Linux, PostgreSQL installed from the official repositories, running inside a Xen HVM VPS.
> Connecting to PostgreSQL using psql via UNIX socket.
> Changed options: (locale-related ones omitted)
> listen_addresses =
> max_stack_depth = 2MB
> shared_buffers = 256MB (Issue is also present with default value)
> Total RAM: 1GB
>
>
> Thanks,
> --yuriks