pageinspect
pageinspect
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.
Functions
get_raw_page(relname text, fork text, blkno int) returns bytea
get_raw_page
get_raw_page reads the specified block of the named
relation 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, 'fsm' for the free space map,
'vm' for the visibility map, or 'init'
for the initialization fork.
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
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 | 0
The 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
heap_page_items shows all line pointers on a heap
page. For those line pointers that are in use, tuple headers as well
as tuple raw data 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.
tuple_data_split(rel_oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]
tuple_data_split
tuple_data_split splits tuple data into attributes
in the same way as backend internals.
test=# SELECT tuple_data_split('pg_class'::regclass, t_data, t_infomask, t_infomask2, t_bits) FROM heap_page_items(get_raw_page('pg_class', 0));
This function should be called with the same arguments as the return
attributes of heap_page_items.
If do_detoast is true,
attribute that will be detoasted as needed. Default value is
false.
heap_page_item_attrs(rel_oid, t_data bytea, [, do_detoast bool]) returns bytea[]
heap_page_item_attrs
heap_page_item_attrs is equivalent to
heap_page_items except that it returns
tuple raw data as an array of attributes that can optionally
be detoasted by do_detoast which is
false by default.
A heap page image obtained with get_raw_page should
be passed as argument. For example:
test=# SELECT * FROM heap_page_item_attrs(get_raw_page('pg_class', 0), 'pg_class'::regclass);
bt_metap(relname text) returns record
bt_metap
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
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
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
In a B-tree leaf page, ctid points to a heap tuple.
In an internal page, the block number part of ctid
points to another page in the index itself, while the offset part
(the second number) is ignored and is usually 1.
Note that the first item on any non-rightmost page (any page with
a non-zero value in the btpo_next field) is the
page's high key
, meaning its data
serves as an upper bound on all items appearing on the page, while
its ctid field is meaningless. Also, on non-leaf
pages, the first real data item (the first item that is not a high
key) is a minus infinity
item, with no actual value
in its data field. Such an item does have a valid
downlink in its ctid field, however.
brin_page_type(page bytea) returns text
brin_page_type
brin_page_type returns the page type of the given
BRIN index page, or throws an error if the page is
not a valid BRIN page. For example:
test=# SELECT brin_page_type(get_raw_page('brinidx', 0));
brin_page_type
----------------
meta
brin_metapage_info(page bytea) returns record
brin_metapage_info
brin_metapage_info returns assorted information
about a BRIN index metapage. For example:
test=# SELECT * FROM brin_metapage_info(get_raw_page('brinidx', 0));
magic | version | pagesperrange | lastrevmappage
------------+---------+---------------+----------------
0xA8109CFA | 1 | 4 | 2
brin_revmap_data(page bytea) returns setof tid
brin_revmap_data
brin_revmap_data returns the list of tuple
identifiers in a BRIN index range map page.
For example:
test=# SELECT * FROM brin_revmap_data(get_raw_page('brinidx', 2)) limit 5;
pages
---------
(6,137)
(6,138)
(6,139)
(6,140)
(6,141)
brin_page_items(page bytea, index oid) returns setof record
brin_page_items
brin_page_items returns the data stored in the
BRIN data page. For example:
test=# SELECT * FROM brin_page_items(get_raw_page('brinidx', 5),
'brinidx')
ORDER BY blknum, attnum LIMIT 6;
itemoffset | blknum | attnum | allnulls | hasnulls | placeholder | value
------------+--------+--------+----------+----------+-------------+--------------
137 | 0 | 1 | t | f | f |
137 | 0 | 2 | f | f | f | {1 .. 88}
138 | 4 | 1 | t | f | f |
138 | 4 | 2 | f | f | f | {89 .. 176}
139 | 8 | 1 | t | f | f |
139 | 8 | 2 | f | f | f | {177 .. 264}
The returned columns correspond to the fields in the
BrinMemTuple and BrinValues structs.
See src/include/access/brin_tuple.h for details.
gin_metapage_info(page bytea) returns record
gin_metapage_info
gin_metapage_info returns information about
a GIN index metapage. For example:
test=# SELECT * FROM gin_metapage_info(get_raw_page('gin_index', 0));
-[ RECORD 1 ]----+-----------
pending_head | 4294967295
pending_tail | 4294967295
tail_free_size | 0
n_pending_pages | 0
n_pending_tuples | 0
n_total_pages | 7
n_entry_pages | 6
n_data_pages | 0
n_entries | 693
version | 2
gin_page_opaque_info(page bytea) returns record
gin_page_opaque_info
gin_page_opaque_info returns information about
a GIN index opaque area, like the page type.
For example:
test=# SELECT * FROM gin_page_opaque_info(get_raw_page('gin_index', 2));
rightlink | maxoff | flags
-----------+--------+------------------------
5 | 0 | {data,leaf,compressed}
(1 row)
gin_leafpage_items(page bytea) returns setof record
gin_leafpage_items
gin_leafpage_items returns information about
the data stored in a GIN leaf page. For example:
test=# SELECT first_tid, nbytes, tids[0:5] as some_tids
FROM gin_leafpage_items(get_raw_page('gin_test_idx', 2));
first_tid | nbytes | some_tids
-----------+--------+----------------------------------------------------------
(8,41) | 244 | {"(8,41)","(8,43)","(8,44)","(8,45)","(8,46)"}
(10,45) | 248 | {"(10,45)","(10,46)","(10,47)","(10,48)","(10,49)"}
(12,52) | 248 | {"(12,52)","(12,53)","(12,54)","(12,55)","(12,56)"}
(14,59) | 320 | {"(14,59)","(14,60)","(14,61)","(14,62)","(14,63)"}
(167,16) | 376 | {"(167,16)","(167,17)","(167,18)","(167,19)","(167,20)"}
(170,30) | 376 | {"(170,30)","(170,31)","(170,32)","(170,33)","(170,34)"}
(173,44) | 197 | {"(173,44)","(173,45)","(173,46)","(173,47)","(173,48)"}
(7 rows)
fsm_page_contents(page bytea) returns text
fsm_page_contents
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.