Re: VACUUM produces odd freespace values

On 18 September 2010 02:00, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> 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  <bruce(at)momjian(dot)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)
>

That's odd. When I delete val 2, the freespace goes back up in 9.0rc1
(attached).

--
Thom Brown
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