From: Guillaume Cottenceau <gc@mnc.ch>
To: pgsql-performance@postgresql.org
Subject: 9.2.1 & index-only scans : abnormal heap fetches after VACUUM FULL
Date: Thu, 29 Nov 2012 12:33:50 +0100
Message-ID: <m3mwy0myn5.fsf@mnc.ch> (raw)
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I am toying around with 9.2.1, trying to measure/determine how
index-only scans can improve our performance.
A small script which is attached to this mail, shows that as long
as the table has been VACUUM FULL'd, there is a unusual high
amount of heap fetches. It is strange that the visibilitymap_test
predicate fails in these situations, is the visibility map
somehow trashed in this situation? It should not, or at least the
documentation[1] should state it (my understanding is that vacuum
full does *more* than vacuum, but nothing less) (note to usual
anti vacuum full trollers: I know you hate vacuum full).
Using pg 9.2.1 compiled from sources, almost standard
configuration except shared_buffers at 512M, effective_cache_size
at 1536M, random_page_cost at 2, and vacuum delays increased.
Please find complete logs attached, and selected logs below:
After table creation + analyze:
Index Only Scan using i on ta (cost=0.00..156991.10 rows=2018667 width=4) (actual time=0.034..336.443 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
After vacuum:
Index Only Scan using i on ta (cost=0.00..50882.62 rows=2018667 width=4) (actual time=0.014..193.120 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
After vacuum analyze:
Index Only Scan using i on ta (cost=0.00..50167.13 rows=1990353 width=4) (actual time=0.015..193.035 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
After vacuum full:
Index Only Scan using i on ta (cost=0.00..155991.44 rows=1990333 width=4) (actual time=0.042..364.412 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
^^^^^^^ uh uh, looking bad
After vacuum full analyze:
Index Only Scan using i on ta (cost=0.00..157011.85 rows=2030984 width=4) (actual time=0.025..365.657 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
After vacuum:
Index Only Scan using i on ta (cost=0.00..51192.45 rows=2031000 width=4) (actual time=0.015..192.520 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
Thanks for any comments/hints,
Ref:
[1] http://www.postgresql.org/docs/9.1/static/sql-vacuum.html
--
Guillaume Cottenceau
--
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http://www.postgresql.org/mailpref/pgsql-performanceAttachments:
vacfull.sql (application/octet-stream, 715B) downloadfulloutput.log (text/plain, 3.5K) download
DROP TABLE ta;
psql:/tmp/vacfull.sql:3: ERROR: table "ta" does not exist
CREATE TABLE ta (ca int, cb int, cc int);
CREATE TABLE
INSERT INTO ta VALUES (generate_series(1, 5), generate_series(1, 10000000), generate_series(1, 10000000));
INSERT 0 10000000
ANALYZE ta;
ANALYZE
CREATE INDEX i ON ta (ca, cb, cc);
CREATE INDEX
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..156991.10 rows=2018667 width=4) (actual time=0.034..336.443 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
Total runtime: 385.023 ms
(4 rows)
VACUUM ta;
VACUUM
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..50882.62 rows=2018667 width=4) (actual time=0.014..193.120 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
Total runtime: 241.079 ms
(4 rows)
VACUUM ANALYZE ta;
VACUUM
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..50167.13 rows=1990353 width=4) (actual time=0.015..193.035 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
Total runtime: 241.101 ms
(4 rows)
VACUUM FULL ta;
VACUUM
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..155991.44 rows=1990333 width=4) (actual time=0.042..364.412 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
Total runtime: 412.715 ms
(4 rows)
VACUUM FULL ANALYZE ta;
VACUUM
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..157011.85 rows=2030984 width=4) (actual time=0.025..365.657 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 2000000
Total runtime: 414.223 ms
(4 rows)
VACUUM ta;
VACUUM
EXPLAIN ANALYZE SELECT cb FROM ta WHERE ca = 1 ORDER BY cb;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Only Scan using i on ta (cost=0.00..51192.45 rows=2031000 width=4) (actual time=0.015..192.520 rows=2000000 loops=1)
Index Cond: (ca = 1)
Heap Fetches: 0
Total runtime: 240.918 ms
(4 rows)
DROP TABLE ta;
DROP TABLE
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To: pgsql-performance@postgresql.org
Cc: gc@mnc.ch
Subject: Re: 9.2.1 & index-only scans : abnormal heap fetches after VACUUM FULL
In-Reply-To: <m3mwy0myn5.fsf@mnc.ch>
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