Plan uses wrong index, preferring to scan pkey index instead - Mailing list pgsql-performance
| From | Yuri Kunde Schlesner |
|---|---|
| Subject | Plan uses wrong index, preferring to scan pkey index instead |
| Date | |
| Msg-id | 1416093360.655916.191452721.03160196@webmail.messagingengine.com Whole thread Raw |
| Responses |
Re: Plan uses wrong index, preferring to scan pkey index instead
(Tom Lane <tgl@sss.pgh.pa.us>)
|
| List | pgsql-performance |
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 intermittent performance problems. I've tracked it down to a query that takes a very long time (around 4 minutes) to complete when 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..154410.341 rows=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=1 loops=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 do a sequential scan of the backlog_pkey index, filtering rows by bufferid, instead of directly using the backlog_bufferid_idx index that directly maps to the operation being made by the query. I was advised on IRC to try dropping the 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..63.814 rows=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=1 loops=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 messages posted 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 throwing Postgres' 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
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