pageinspect模块详解

PostgreSQL数据库工作学习随笔 2022-03-26

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pageinspect模块提供函数让你从低层次观察数据库页面的内容，这对于调试目的很有用。要求所有这些函数只能被超级用户使用。

通用函数

get_raw_page(relname text,fork text,blkon int) 返回bytea

如果只读取主分支信息可以写成：

get_raw_page(relname text,blkon int) 返回bytea

get_raw_page读取提及的关系中的指定块并且以一个bytea值的形式返回一个拷贝。这允许得到该块的一个单一的时间一致的拷贝。对于主数据库分支，fork应该是"main"，对于空闲空间映射应该是"fsm"，对于可见性映射应该是"vm"，对于初始化分支应该时"init"。

每个关系（relation）可能会有四种分支，分支编号分别为0，1，2，3，0号分支main为关系数据文件本体，1号分支fsm保存了main分支中空闲空间的信息，2号分支vm保存了main分支中可见性的信息，3号分支init是很少见的特殊分支，通常用于不被日志记录（unlogged）的表与索引。

postgres=# create table test (id int,name varchar);
CREATE TABLE
postgres=# insert into test values (1,'tom');
INSERT 0 1
postgres=# insert into test values (2,'lucy');
INSERT 0 1
postgres=# select * from get_raw_page('test','main',0);
-[ RECORD 1 ]+---------------------
get_raw_page | \x050000...0009746f6d
postgres=# select * from get_raw_page('test','fsm',0);
-[ RECORD 1 ]+---------------------
get_raw_page | \x050000...00000000
postgres=# select * from get_raw_page('test','vm',0);
-[ RECORD 1 ]+---------------------
get_raw_page | \x05000...0000000
postgres=# select * from get_raw_page('test','init',0);
ERROR:  could not open file "base/13593/1081011_init": No such file or directory
postgres=# select pg_relation_filepath('test');
 pg_relation_filepath
----------------------
 base/13593/1081013
postgres=# \q
[pg12@k8s2 pgdata]$ ll base/13593/1081011
1081013      1081013_fsm  1081013_vm
[pg12@k8s2 pgdata]
创建unlogged表
postgres=# create unlogged table test1 (id int);
CREATE TABLE
postgres=# insert into test1 values (1);
INSERT 0 1
postgres=# insert into test1 values (2);
INSERT 0 1
postgres=# insert into test1 values (3);
INSERT 0 1
postgres=# vacuum test1;
VACUUM
postgres=# select pg_relation_filepath('test1');
 pg_relation_filepath
----------------------
 base/13593/1081016
(1 row)
postgres=# \q
[pg12@k8s2 pgdata]$ ll base/13593/1081016
1081016       1081016_fsm   1081016_init  1081016_vm
[pg12@k8s2 pgdata]$

page_header(page bytea) 返回record

page_header显示所有PostgreSQL堆和索引页面的公共域（可以看到unlogged 创建的表没有记录wal日志，所以lsn为0/0）。

用get_raw_page获得的一个页面影像作为参数传递。

postgres=# select * from page_header(get_raw_page('test',0));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
------------+----------+-------+-------+-------+---------+----------+---------+-----------
5/4C8A8688 |        0 |     4 |    32 |  8120 |    8192 |     8192 |       4 |         0
(1 row)

postgres=#

postgres=# select * from page_header(get_raw_page('test1',0));
 lsn | checksum | flags | lower | upper | special | pagesize | version | prune_xid
-----+----------+-------+-------+-------+---------+----------+---------+-----------
0/0 |        0 |     4 |    36 |  8096 |    8192 |     8192 |       4 |         0
(1 row)

postgres=#
详见src/include/storage/bufpage.h。

任何页面通用的空间管理信息

pd_lsn - 标识此页面的最后更改的 xlog 记录。

pd_checksum - 页面校验和。

pd_flags - 标志位。

pd_lower - 到可用空间开始的偏移量，指向最后一个指针末尾。

pd_upper - 到可用空间末尾的偏移量，指向最新元组的起始位置。

pd_special - 特殊空间开始的偏移量。

pd_pagesize_version - 字节大小和页面布局版本号。

pd_prune_xid - 页面上可能可修剪的元组中最旧的 XID。

checksum域时存放在页面中的校验和，如果页面被损坏它可能是不正确的值。查看实例是否启用数据校验和（1是开启，0是关闭），如果没有启用存储这个值没有意义。

[pg12@k8s2 ~]$ pg_controldata |grep checksum
Data page checksum version:           1
[pg12@k8s2 ~]$

page_checksum(page bytea,blkon int) returns smallint

page_checksum为页面计算校验和，就像它被放置在给定块上一样。

应该将get_raw_page获得的页面映像作为参数传入。

注意校验和取决于块号，因此应该将匹配的块号传入。

用这个函数计算的校验和可以和函数page_header的结果域checksum进行比较。如果为这个实例启用了数据校验和，则两个值应该相等。

postgres=# select * from page_header(get_raw_page('tbl_test',0));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
------------+----------+-------+-------+-------+---------+----------+---------+-----------
5/4C87FD98 |    10445 |     5 |   452 |  7544 |    8192 |     8192 |       4 |         0
(1 row)

postgres=# select * from page_checksum(get_raw_page('tbl_test',0),0);
 page_checksum
---------------
10445
(1 row)

postgres=# select * from page_header(get_raw_page('tbl_test',2));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid
------------+----------+-------+-------+-------+---------+----------+---------+-----------
5/4C12DF40 |    32692 |     4 |   452 |   488 |    8192 |     8192 |       4 |         0
(1 row)

postgres=# select * from page_checksum(get_raw_page('tbl_test',2),2);
 page_checksum
---------------
32692
(1 row)

postgres=#

fsm_page_contents(page bytea) returns text

fsm_page_contents显示FSM页面的内部节点结构。

输出为多行字符串，页面中的二进制树中每个节点有一行。仅有不为0的节点会被打印。所谓的"next"指针，指向页面中下一个要返回的槽，也会被打印。

postgres=# select * from fsm_page_contents(get_raw_page('test','fsm',0));
fsm_page_contents
-------------------
0: 252           +
1: 252           +
3: 252           +
7: 252           +
15: 252          +
31: 252          +
63: 252          +
127: 252         +
255: 252         +
511: 252         +
1023: 252        +
2047: 252        +
4095: 252        +
fp_next_slot: 0  +

(1 row)

postgres=#

HEAP函数

heap_page_items(page bytea) returns setof record

heap_page_items显示一个堆页面上所有的行指针。对那些使用中的行指针，元组头部和元组原始数据也会被显示。不管元组对拷贝原始页面的MVCC快照是否可见。他们都会被显示。

用get_raw_page获得的一个堆页面映像应该作为参数传递。

postgres=# select * from heap_page_items(get_raw_page('test2',0));
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits | t_oid |   t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------+-------+------------
  1 |   8160 |        1 |     28 | 704481 |      0 |        0 | (0,1)  |           1 |       2304 |     24 |        |       | \x01000000
2 |   8128 |        1 |     28 | 704482 |      0 |        0 | (0,2)  |           1 |       2048 |     24 |        |       | \x02000000
  3 |   8096 |        1 |     28 | 704483 |      0 |        0 | (0,3)  |           1 |       2048 |     24 |        |       | \x03000000
(3 rows)

postgres=

详见src/include/storage/itemid.h和src/include/access/htup_details.h

lp_off 到元组的偏移量（从页面开始）

lp_flags 行指针状态

0 未使用

1 已使用

2 HOT 重定向

3 死元组

lp_len 元组的字节长度

t_xmin inserting xact ID

t_xmax deleting or locking xact ID

t_field3 插入或删除命令ID（t_cid)和旧版本VACUUM FULL xact ID

t_ctid current TID of this or newer tuple (or a speculative insertion token)

t_infomask2 number of attributes + various flags

t_infomask various flag bits

记录提交状态事物提交状态解析。

t_hoff sizeof header incl. bitmap, padding

t_bits bitmap of NULLs

tuple_data_split(rel_oid oid,t_data bytea,t_infomask integer,t_informask2 integer,t_bits text[,do_detoast bool)] returns bytea[]

tuple_data_split以后端内部的相同方式将元组数据拆解成属性。

postgres=# select tuple_data_split('test'::regclass,t_data,t_infomask,t_infomask2,t_bits) from heap_page_items(get_raw_page('test',0));
        tuple_data_split
---------------------------------
 {"\\x01000000","\\x09746f6d"}
 {"\\x02000000","\\x0b6c756379"}
(2 rows)

postgres=#

heap_page_item_attrs(page bytea,rel_oid regclass [,do_detoast boool]) returns setof record

heap_page_item_attrs等效于heap_page_items，不过它会把元组原始数据返回为属性的数组，如果do_detoast为真（默认为false），这些数据会被反toast。

应该把用get_raw_page得到的一个堆页面映像作为参数传入。

postgres=# select * from heap_page_item_attrs(get_raw_page('test2',0),'test2'::regclass);
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits | t_oid |     t_attrs
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------+-------+-----------------
  1 |   8160 |        1 |     28 | 704481 |      0 |        0 | (0,1)  |           1 |       2304 |     24 |        |       | {"\\x01000000"}
2 |   8128 |        1 |     28 | 704482 |      0 |        0 | (0,2)  |           1 |       2304 |     24 |        |       | {"\\x02000000"}
  3 |   8096 |        1 |     28 | 704483 |      0 |        0 | (0,3)  |           1 |       2304 |     24 |        |       | {"\\x03000000"}
(3 rows)

postgres=# s

B树函数

bt_metap(relname text) returns record

bt_metap返回关于一个B树索引元页的信息。

postgres=# select * from bt_metap('test2_pkey');
-[ RECORD 1 ]-----------+-------
magic                   | 340322
version                 | 4
root                    | 1
level                   | 0
fastroot                | 1
fastlevel               | 0
oldest_xact             | 0
last_cleanup_num_tuples | 3

postgres=#
详见src/include/access/nbtxlog.

bt_page_stats(relname text,blkno int) returns record

bt_page_stats返回有关B-树索引单一页面的总计信息。

postgres=# select * from bt_page_stats('test2_pkey',1);
-[ RECORD 1 ]-+-----
blkno         | 1
type          | l
live_items    | 3
dead_items    | 0
avg_item_size | 16
page_size     | 8192
free_size     | 8088
btpo_prev     | 0
btpo_next     | 0
btpo          | 0
btpo_flags    | 3

postgres=#

bt_page_items(relname text,blkon int) returns setof record

bt_page_items返回一个B-树索引页面上的所有细节信息，也可以把页面以bytea值的形式传递给bt_page_items。

可以通过bt_page_items来学习HOT。

postgres=# select * from bt_page_items('test2_pkey',1);
 itemoffset | ctid  | itemlen | nulls | vars |          data
------------+-------+---------+-------+------+-------------------------
          1 | (0,1) |      16 | f     | f    | 01 00 00 00 00 00 00 00
2 | (0,2) |      16 | f     | f    | 02 00 00 00 00 00 00 00
          3 | (0,3) |      16 | f     | f    | 03 00 00 00 00 00 00 00
(3 rows)

postgres=# select * from bt_page_items(get_raw_page('test2_pkey',1));
 itemoffset | ctid  | itemlen | nulls | vars |          data
------------+-------+---------+-------+------+-------------------------
          1 | (0,1) |      16 | f     | f    | 01 00 00 00 00 00 00 00
2 | (0,2) |      16 | f     | f    | 02 00 00 00 00 00 00 00
          3 | (0,3) |      16 | f     | f    | 03 00 00 00 00 00 00 00
(3 rows)

postgres=#

BRIN函数

brin_page_type(page bytea) returns text

brin_page_type返回一个给定的BRIN索引页面的页面类型，如果该页面不是一个合法的BRIN页面则抛出错误。

postgres=# select brin_page_type(get_raw_page('test2_pkey',0));
 brin_page_type
----------------
 unknown (00)
(1 row)
                                          ^
postgres=# create index brin_test2_idx on test2 using brin (name);
CREATE INDEX
postgres=# select brin_page_type(get_raw_page('brin_test2_idx',0));
 brin_page_type
----------------
 meta
(1 row)

postgres=#

brin_metapage_info(page bytea) returns record

brin_metapage_info返回有关一个BRIN索引页面的详细信息。

postgres=# select * from brin_metapage_info(get_raw_page('brin_test2_idx',0));
   magic    | version | pagesperrange | lastrevmappage
------------+---------+---------------+----------------
 0xA8109CFA |       1 |           128 |              1
(1 row)

postgres=#

brin_revmap_data(page bytea) returns setof tid

brin_revmap_data返回一个BRIN索引范围映射页面中元组标识符的表。


postgres=# select * from brin_revmap_data(get_raw_page('brin_test2_idx',1)) limit 5;
 pages
-------
 (2,1)
 (0,0)
 (0,0)
 (0,0)
 (0,0)
(5 rows)

postgres=#

brin_page_items(page bytea,index oid) returns setof record

brin_page_items返回存在BRIN数据页面中存储的数据。

postgres=# select * from brin_page_items(get_raw_page('brin_test2_idx',2),'brin_test2_idx');
 itemoffset | blknum | attnum | allnulls | hasnulls | placeholder |     value
------------+--------+--------+----------+----------+-------------+---------------
1 |      0 |      1 | f        | t        | f           | {lucy .. tom}
(1 row)

postgres=#
详见src/include/access/brin_tuple.h

GIN函数

gin_metapage_info(page bytea) returns record

gin_metapage_info返回有关一个GIN索引元页的信息。

postgres=# select * from gin_metapage_info(get_raw_page('gin_tbl_idx',0));
-[ RECORD 1 ]----+-----------
pending_head     | 4294967295
pending_tail     | 4294967295
tail_free_size   | 0
n_pending_pages  | 0
n_pending_tuples | 0
n_total_pages    | 1151
n_entry_pages    | 1134
n_data_pages     | 16
n_entries        | 4624
version          | 2

postgres=#

gin_page_opaque_info(page bytea) returns record

gin_page_opaque_info返回有关一个GIN索引不透明区域的信息，如页面类型等。

postgres=# select * from gin_page_opaque_info(get_raw_page('gin_tbl_idx',2));
 rightlink  | maxoff |         flags
------------+--------+------------------------
 4294967295 |      0 | {data,leaf,compressed}
(1 row)

postgres=#

gin_leafpage_items(page bytea) returns setof record

gin_leafpage_items返回有关储存在一个GIN叶子页面中的数据信息。

postgres=# SELECT first_tid, nbytes, tids[0:5] AS some_tids FROM gin_leafpage_items(get_raw_page('gin_tbl_idx', 2));
 first_tid | nbytes |                         some_tids
-----------+--------+------------------------------------------------------------
 (0,101)   |    376 | {"(0,101)","(0,107)","(1,17)","(1,24)","(1,72)"}
 (50,78)   |    376 | {"(50,78)","(50,86)","(51,11)","(51,22)","(51,29)"}
 (100,5)   |    376 | {"(100,5)","(100,59)","(100,74)","(100,98)","(101,8)"}
 (150,68)  |    376 | {"(150,68)","(150,77)","(150,88)","(150,104)","(150,107)"}
 (199,23)  |    375 | {"(199,23)","(199,25)","(199,26)","(199,28)","(199,34)"}
 (250,13)  |    376 | {"(250,13)","(250,17)","(250,35)","(250,36)","(250,41)"}
 (297,88)  |    376 | {"(297,88)","(297,103)","(297,105)","(298,16)","(298,39)"}
 (347,80)  |    376 | {"(347,80)","(348,7)","(348,8)","(348,21)","(348,41)"}
 (396,79)  |    376 | {"(396,79)","(396,96)","(396,104)","(397,9)","(397,19)"}
 (444,58)  |    376 | {"(444,58)","(444,62)","(444,72)","(444,98)","(445,26)"}
 (498,69)  |    376 | {"(498,69)","(498,76)","(498,78)","(498,86)","(498,87)"}
 (547,81)  |    375 | {"(547,81)","(548,17)","(548,34)","(548,45)","(548,74)"}
 (594,8)   |    376 | {"(594,8)","(594,24)","(594,61)","(594,63)","(594,82)"}
 (644,98)  |    376 | {"(644,98)","(645,6)","(645,14)","(645,19)","(645,26)"}
 (693,84)  |    376 | {"(693,84)","(693,95)","(693,99)","(693,102)","(694,3)"}
 (741,87)  |    376 | {"(741,87)","(741,89)","(741,106)","(741,107)","(742,12)"}
 (789,62)  |    376 | {"(789,62)","(789,75)","(789,102)","(789,103)","(790,5)"}
 (840,44)  |    376 | {"(840,44)","(840,107)","(841,19)","(841,21)","(841,39)"}
 (888,2)   |    375 | {"(888,2)","(888,5)","(888,8)","(888,16)","(888,21)"}
(19 rows)

postgres=#

HASH函数

hash_page_type(page bytea) returns text

hash_page_type返回给定的hash索引页面的页面类型。

postgres=# create index hash_tbl_idx on tbl_test using hash (info);
CREATE INDEX
postgres=# select * from hash_page_type(get_raw_page('hash_tbl_idx',0));
 hash_page_type
----------------
 metapage
(1 row)

postgres=#

hash_page_stats(page bytea) returns setof record

hash_page_stats返回有关一个HASH索引的桶页或者溢出页的信息。

postgres=# select * from hash_page_stats(get_raw_page('hash_tbl_idx',1));
-[ RECORD 1 ]---+-----------
live_items      | 201
dead_items      | 0
page_size       | 8192
free_size       | 4128
hasho_prevblkno | 511
hasho_nextblkno | 4294967295
hasho_bucket    | 0
hasho_flag      | 2
hasho_page_id   | 65408
postgres=#

hash_page_items(page bytea) returns setof record

hash_page_items返返回有关一个HASH索引的桶页或者溢出页中存储的数据的信息。

postgres=# select * from hash_page_items(get_raw_page('hash_tbl_idx',1)) limit 6;
itemoffset |   ctid    |   data
------------+-----------+----------
1 | (587,81)  |  6690304
2 | (599,40)  | 30260224
3 | (568,84)  | 42152960
4 | (112,12)  | 45416960
5 | (322,79)  | 50068480
6 | (793,103) | 53645824
(6 rows)

postgres=# select * from hash_page_items(get_raw_page('hash_tbl_idx',2)) limit 6;
itemoffset |   ctid    |   data
------------+-----------+-----------
1 | (684,51)  |  53047809
2 | (361,72)  |  69544449
3 | (348,103) | 100082177
4 | (505,63)  | 107412993
5 | (771,48)  | 117727233
6 | (525,87)  | 141176833
(6 rows)

postgres=#

hash_bitmap_info(index oid,blkno int) returns record

hash_bitmap_info返回HASH所以一个特定溢出页在位图页中的位的状态（没搞懂这个溢出页场景，以后有机会遇到了再补上）。

postgres=# select * from hash_bitmap_info('hash_test2_idx',0);
ERROR:  invalid overflow block number 0
postgres=#

hash_metapage_info(page bytea) returns record

hash_metapage_info返回一个HASH索引的元页中存放的信息。

postgres=# select * from hash_metapage_info(get_raw_page('hash_test2_idx',0));
-[ RECORD 1 ]--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
magic     | 105121344
version   | 4
ntuples   | 3
ffactor   | 307
bsize     | 8152
bmsize    | 4096
bmshift   | 15
maxbucket | 1
highmask  | 3
lowmask   | 1
ovflpoint | 1
firstfree | 0
nmaps     | 1
procid    | 400
spares    | {0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}
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pageinspect模块详解

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