Bruce Momjian wrote:
> Can anyone explain why VACUUM after INSERT shows steadily decreasing
> freespace, while DELETE of the same rows does not decrease consistently?
>
> Specifically, after one row is inserted I see:
>
> SELECT pg_freespace('mvcc_demo');
> pg_freespace
> --------------
> (0,8128)
> (1 row)
>
> but after inserting two more rows and deleting those two rows, I see:
>
> SELECT pg_freespace('mvcc_demo');
> pg_freespace
> --------------
> (0,8096)
> (1 row)
>
> Seems that value should be '(0,8128)'. Is it the unused line pointers
> that are causing this?
>
> Another odd thing --- if I change the second VACUUM to VACUUM FULL I
> see:
>
> VACUUM FULL mvcc_demo;
> VACUUM
> SELECT pg_freespace('mvcc_demo');
> pg_freespace
> --------------
> (0,0)
> (1 row)
>
> There is still a row in the table, so why is there no free space
> reported? I realize after VACUUM FULL that only the last page has
> freespace --- do we assume that will be used as default for the next
> addition and just not bother with the free space map? --- makes sense if
> we do that. Does this happen because cluster creates a new relfilenode?
>
> I am attaching the init script, the SQL query script, and the results I
> obtained against our CVS HEAD.
Sorry. Attached is trimmed-down result file that shows just the
problem.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
00-init.sql
------------------
This script is designed to run in a database called test
and requires installation of /contrib/pageinspect and
/contrib/pg_freespacemap.
You are now connected to database "test" as user "postgres".
DROP TABLE IF EXISTS mvcc_demo;
DROP TABLE
CREATE TABLE mvcc_demo (val INTEGER);
CREATE TABLE
DROP VIEW IF EXISTS mvcc_demo_page0;
DROP VIEW
CREATE VIEW mvcc_demo_page0 AS
SELECT '(0,' || lp || ')' AS ctid,
CASE lp_flags
WHEN 0 THEN 'Unused'
WHEN 1 THEN 'Normal'
WHEN 2 THEN 'Redirect to ' || lp_off
WHEN 3 THEN 'Dead'
END,
t_xmin::text::int8 AS xmin,
t_xmax::text::int8 AS xmax,
t_ctid
FROM heap_page_items(get_raw_page('mvcc_demo', 0))
ORDER BY lp;
CREATE VIEW
31-vacuum-freesp.sql
------------------
TRUNCATE mvcc_demo;
TRUNCATE TABLE
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0 rows)
INSERT INTO mvcc_demo VALUES (1);
INSERT 0 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8128)
(1 row)
INSERT INTO mvcc_demo VALUES (2);
INSERT 0 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8096)
(1 row)
INSERT INTO mvcc_demo VALUES (3);
INSERT 0 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8064)
(1 row)
DELETE FROM mvcc_demo WHERE val = 3;
DELETE 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8096)
(1 row)
DELETE FROM mvcc_demo WHERE val = 2;
DELETE 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0,8096)
(1 row)
SELECT * FROM mvcc_demo_page0;
ctid | case | xmin | xmax | t_ctid
-------+--------+------+------+--------
(0,1) | Normal | 1339 | 0 | (0,1)
(0,2) | Unused | | |
(0,3) | Unused | | |
(3 rows)
DELETE FROM mvcc_demo WHERE val = 1;
DELETE 1
VACUUM mvcc_demo;
VACUUM
SELECT pg_freespace('mvcc_demo');
pg_freespace
--------------
(0 rows)
VACUUM mvcc_demo;
VACUUM
SELECT pg_relation_size('mvcc_demo');
pg_relation_size
------------------
0
(1 row)
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