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 16493lob_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...3lob_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 16492lo_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 16520lob_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;VACUUMlob_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 | 0lob_test=# \lo_import ‘/tmp/randoms‘;lo_import 16521lob_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 16521lo_unlink 16521lob_test=# select pg_relation_size(‘pg_largeobject‘); pg_relation_size------------------ 4235264(1 row)lob_test=# vacuum full pg_largeobject;VACUUMlob_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/