PostgreSQL的大对象以及空间使用 (2)

在上一篇文章中，展示了插入大对象后，pg_largeobject表中有多少大对象使用空间。

让我们再深入研究一下：

该表有2个大对象（总共1024个记录）：

lob_test=# select pg_relation_size('pg_largeobject');
pg_relation_size
------------------
          1441792
(1 row)

再来添加一个随机填充的文件：

lob_test=# lo_import '/tmp/randoms';
lo_import 16493
lob_test=# select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          2842624
(1 row)

lob_test=# select oid, * from  pg_largeobject_metadata;
  oid  | lomowner | lomacl
-------+----------+--------
 16491 |       10 |
 16492 |       10 |
 16493 |       10 |
(3 rows)

不出所料，因为不能压缩随机字符序列，所以大小又增加了171个块（有关解释，请参阅我的上一篇文章）

如果您阅读了Frits Hoogland(https://fritshoogland.wordpress.com/category/postgresql/)撰写的一系列不错的博客文章，则应该了解pageinspect扩展名和t_infomask 16位掩码。

让我们安装它并检查pg_largeobjects页面的内容：

lob_test=# select * from page_header(get_raw_page('pg_largeobject',0));
     lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
-------------+----------+-------+-------+-------+---------+----------+---------+-----------
 18/38004C10 |        0 |     0 |   452 |   488 |    8192 |     8192 |       4 |         0
(1 row)

-- same result (lower 452, upper 488) for blocks 1...3

lob_test=# select * from page_header(get_raw_page('pg_largeobject',4));
     lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
-------------+----------+-------+-------+-------+---------+----------+---------+-----------
 18/380179F8 |        0 |     0 |   360 |  2144 |    8192 |     8192 |       4 |         0
(1 row)


lob_test=# select * from page_header(get_raw_page('pg_largeobject',5));
     lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
-------------+----------+-------+-------+-------+---------+----------+---------+-----------
 18/381386E0 |        0 |     0 |    36 |  1928 |    8192 |     8192 |       4 |         0
(1 row)-- same result for the remaining blocks

已经知道的一些数字，但是我们更宁愿将所有部分放在一起分析。我们知道：page header占用24个字节，并且行指针为每个元组使用4个字节。

前4个页的偏移量较低，为452个字节，这意味着我们有（452-24）/ 4 = 107个元组。

第5页（page number是4）的lower为360：（360-24）/ 4 = 84元组。

其余页面的lower为36：（36-24）/ 4 = 3个元组。

让我们检查是否正确：

lob_test=# select generate_series as page,
 (select count(*) from heap_page_items(get_raw_page('pg_largeobject',generate_series)))  as tuples
 from generate_series(0,5);
 page | tuples
------+--------
    0 |    107
    1 |    107
    2 |    107
    3 |    107
    4 |     84
    5 |      3
(6 rows)

现在，让我们删除那1Mb的文件，并再次检查空间:

lob_test=# lo_unlink 16492
lo_unlink 16492
 
 
lob_test=# select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          2842624
(1 row)
 
lob_test=# select oid, * from  pg_largeobject_metadata;
  oid  | lomowner | lomacl
-------+----------+--------
 16491 |       10 |
 16493 |       10 |
(2 rows)
 
lob_test=# select generate_series as pageno, (select count(*) from heap_page_items(get_raw_page('pg_largeobject',generate_series))  ) from generate_series(0,12);
 pageno | count
--------+-------
      0 |   107
      1 |   107
      2 |   107
      3 |   107
      4 |    84
      5 |     3
      6 |     3
      7 |     3
      8 |     3
      9 |     3
     10 |     3
     11 |     3
     12 |     3

该空间仍在使用，并且元组仍然在那里。 但是，我们可以通过检查t_xmax的有效性来检查不再使用的元组。实际上，根据文档，如果XMAX无效，则该行是最新版本：

[…] a tuple is the latest version of its row iff XMAX is invalid or t_ctid points to itself (in which case, if XMAX is valid, the tuple is either locked or deleted). […]

 (from htup_details.h lines 87-89).

我们必须对第12位(2048，或0x0800)检查信息进行检查。

#define HEAP_XMAX_INVALID       0x0800  /* t_xmax invalid/aborted */

lob_test=# select generate_series as pageno, 
  (select count(*) from heap_page_items(get_raw_page('pg_largeobject',generate_series))
  where t_infomask::bit(16) & x'0800'::bit(16) = x'0800'::bit(16)) from generate_series(0,12);
 pageno | count
--------+-------
      0 |   107
      1 |   107
      2 |   107
      3 |   107
      4 |    84
      5 |     0
      6 |     0
      7 |     0
      8 |     0
      9 |     0
     10 |     0
     11 |     0
     12 |     0

大对象被分割成压缩的chunk，其内部行为与常规行相同!

如果我们导入另一个lob，我们会看到空间没有被重用:

lob_test=# lo_import '/tmp/randoms';
lo_import 16520
lob_test=# select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          4235264
(1 row)

将元组标记为可重用是vacuum的工作:

lob_test=# vacuum pg_largeobject;
VACUUM

lob_test=# select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          4235264
(1 row)

常规的vacuum不会释放空出空间，但空间现在可以重复使用:

lob_test=# select generate_series as pageno,
 (select count(*) from heap_page_items(get_raw_page('pg_largeobject',generate_series))
 where t_infomask::bit(16) & x'0800'::bit(16) = x'0800'::bit(16)) from generate_series(0,12);
 pageno | count
--------+-------
      0 |   107
      1 |   107
      2 |   107
      3 |   107
      4 |    84
      5 |     0
      6 |     0
      7 |     0
      8 |     0
      9 |     0
     10 |     0
     11 |     0
     12 |     0

lob_test=# lo_import '/tmp/randoms';
lo_import 16521
lob_test=#

lob_test=#  select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          4235264
(1 row)

-- same size as before!

lob_test=#  select generate_series as pageno, 
(select count(*) from heap_page_items(get_raw_page('pg_largeobject',generate_series)) 
 where t_infomask::bit(16) & x'0800'::bit(16) = x'0800'::bit(16)) from generate_series(0,12);
 pageno | count
--------+-------
      0 |   107
      1 |   107
      2 |   107
      3 |   107
      4 |    84
      5 |     3
      6 |     3
      7 |     3
      8 |     3
      9 |     3
     10 |     3
     11 |     3
     12 |     3

如果我们再次unlink 这个lob对象，执行full vacuum，空闲就被释放了:

lob_test=# lo_unlink 16521
lo_unlink 16521
lob_test=#  select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          4235264
(1 row)

lob_test=# vacuum full pg_largeobject;
VACUUM
lob_test=#  select pg_relation_size('pg_largeobject');
 pg_relation_size
------------------
          2842624
(1 row)

　　

原文：

http://www.ludovicocaldara.net/dba/pgsql-lo-space-usage-part-2/