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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
The pageinspect module provides functions that allow you to inspect the contents of database pages at a low level, which is useful for debugging purposes. All of these functions may be used only by superusers.
Note: XCONLY: The following description applies only to Postgres-XC.
Functions of this module returns information about connecting Coordinators locally. To get information of a Datanode, you can connect to the Datanode using psql directly. In this case, statements will be handled by local transaction control without GTM, you will have warnings and the visibility could be somewhat inconsistent.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
get_raw_page(relname text, fork text, blkno int) returns bytea get_raw_page reads the specified block of the named
table and returns a copy as a bytea value. This allows a
single time-consistent copy of the block to be obtained.
fork should be 'main' for
the main data fork, or 'fsm' for the free space map,
or 'vm' for the visibility map.
get_raw_page(relname text, blkno int) returns bytea A shorthand version of get_raw_page, for reading
from the main fork. Equivalent to
get_raw_page(relname, 'main', blkno)
page_header(page bytea) returns record page_header shows fields that are common to all
PostgreSQL heap and index pages.
A page image obtained with get_raw_page should be
passed as argument. For example:
test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
lsn | checksum | flags | lower | upper | special | pagesize | version | prune_xid
-----------+----------+--------+-------+-------+---------+----------+---------+-----------
0/24A1B50 | 1 | 1 | 232 | 368 | 8192 | 8192 | 4 | 0The returned columns correspond to the fields in the PageHeaderData struct. See src/include/storage/bufpage.h for details.
heap_page_items(page bytea) returns setof record heap_page_items shows all line pointers on a heap
page. For those line pointers that are in use, tuple headers are also
shown. All tuples are shown, whether or not the tuples were visible to
an MVCC snapshot at the time the raw page was copied.
A heap page image obtained with get_raw_page should
be passed as argument. For example:
test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));See src/include/storage/itemid.h and src/include/access/htup_details.h for explanations of the fields returned.
bt_metap(relname text) returns record bt_metap returns information about a B-tree
index's metapage. For example:
test=# SELECT * FROM bt_metap('pg_cast_oid_index');
-[ RECORD 1 ]-----
magic | 340322
version | 2
root | 1
level | 0
fastroot | 1
fastlevel | 0
bt_page_stats(relname text, blkno int) returns record bt_page_stats returns summary information about
single pages of B-tree indexes. For example:
test=# SELECT * FROM bt_page_stats('pg_cast_oid_index', 1);
-[ RECORD 1 ]-+-----
blkno | 1
type | l
live_items | 256
dead_items | 0
avg_item_size | 12
page_size | 8192
free_size | 4056
btpo_prev | 0
btpo_next | 0
btpo | 0
btpo_flags | 3
bt_page_items(relname text, blkno int) returns setof record bt_page_items returns detailed information about
all of the items on a B-tree index page. For example:
test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
itemoffset | ctid | itemlen | nulls | vars | data
------------+---------+---------+-------+------+-------------
1 | (0,1) | 12 | f | f | 23 27 00 00
2 | (0,2) | 12 | f | f | 24 27 00 00
3 | (0,3) | 12 | f | f | 25 27 00 00
4 | (0,4) | 12 | f | f | 26 27 00 00
5 | (0,5) | 12 | f | f | 27 27 00 00
6 | (0,6) | 12 | f | f | 28 27 00 00
7 | (0,7) | 12 | f | f | 29 27 00 00
8 | (0,8) | 12 | f | f | 2a 27 00 00
fsm_page_contents(page bytea) returns text fsm_page_contents shows the internal node structure
of a FSM page. The output is a multiline string, with one line per
node in the binary tree within the page. Only those nodes that are not
zero are printed. The so-called "next" pointer, which points to the
next slot to be returned from the page, is also printed.
See src/backend/storage/freespace/README for more information on the structure of an FSM page.