pageinspectpageinspect
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.
General Functionsget_raw_page(relname text, fork text, blkno bigint) returns byteaget_raw_pageget_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 bigint) 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 recordpage_headerpage_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 | 0 | 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.
The checksum field is the checksum stored in
the page, which might be incorrect if the page is somehow corrupted. If
data checksums are not enabled for this instance, then the value stored
is meaningless.
page_checksum(page bytea, blkno bigint) returns smallintpage_checksumpage_checksum computes the checksum for the page, as if
it was located at the given block.
A page image obtained with get_raw_page should be
passed as argument. For example:
test=# SELECT page_checksum(get_raw_page('pg_class', 0), 0);
page_checksum
---------------
13443
Note that the checksum depends on the block number, so matching block
numbers should be passed (except when doing esoteric debugging).
The checksum computed with this function can be compared with
the checksum result field of the
function page_header. If data checksums are
enabled for this instance, then the two values should be equal.
fsm_page_contents(page bytea) returns textfsm_page_contentsfsm_page_contents shows the internal node structure
of an FSM page. For example:
test=# SELECT fsm_page_contents(get_raw_page('pg_class', 'fsm', 0));
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.
Heap Functionsheap_page_items(page bytea) returns setof recordheap_page_itemsheap_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.
The heap_tuple_infomask_flags function can be
used to unpack the flag bits of t_infomask
and t_infomask2 for heap tuples.
tuple_data_split(rel_oid oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]tuple_data_splittuple_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,
attributes will be detoasted as needed. Default value is
false.
heap_page_item_attrs(page bytea, rel_oid regclass [, do_detoast bool]) returns setof recordheap_page_item_attrsheap_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);
heap_tuple_infomask_flags(t_infomask integer, t_infomask2 integer) returns recordheap_tuple_infomask_flagsheap_tuple_infomask_flags decodes the
t_infomask and
t_infomask2 returned by
heap_page_items into a human-readable
set of arrays made of flag names, with one column for all
the flags and one column for combined flags. For example:
test=# SELECT t_ctid, raw_flags, combined_flags
FROM heap_page_items(get_raw_page('pg_class', 0)),
LATERAL heap_tuple_infomask_flags(t_infomask, t_infomask2)
WHERE t_infomask IS NOT NULL OR t_infomask2 IS NOT NULL;
This function should be called with the same arguments as the return
attributes of heap_page_items.
Combined flags are displayed for source-level macros that take into
account the value of more than one raw bit, such as
HEAP_XMIN_FROZEN.
See src/include/access/htup_details.h for
explanations of the flag names returned.
B-Tree Functionsbt_metap(relname text) returns recordbt_metapbt_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 | 4
root | 1
level | 0
fastroot | 1
fastlevel | 0
last_cleanup_num_delpages | 0
last_cleanup_num_tuples | 230
allequalimage | f
bt_page_stats(relname text, blkno bigint) returns recordbt_page_statsbt_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 | 224
dead_items | 0
avg_item_size | 16
page_size | 8192
free_size | 3668
btpo_prev | 0
btpo_next | 0
btpo_level | 0
btpo_flags | 3
bt_page_items(relname text, blkno bigint) returns setof recordbt_page_itemsbt_page_items returns detailed information about
all of the items on a B-tree index page. For example:
test=# SELECT itemoffset, ctid, itemlen, nulls, vars, data, dead, htid, tids[0:2] AS some_tids
FROM bt_page_items('tenk2_hundred', 5);
itemoffset | ctid | itemlen | nulls | vars | data | dead | htid | some_tids
------------+-----------+---------+-------+------+-------------------------+------+--------+---------------------
1 | (16,1) | 16 | f | f | 30 00 00 00 00 00 00 00 | | |
2 | (16,8292) | 616 | f | f | 24 00 00 00 00 00 00 00 | f | (1,6) | {"(1,6)","(10,22)"}
3 | (16,8292) | 616 | f | f | 25 00 00 00 00 00 00 00 | f | (1,18) | {"(1,18)","(4,22)"}
4 | (16,8292) | 616 | f | f | 26 00 00 00 00 00 00 00 | f | (4,18) | {"(4,18)","(6,17)"}
5 | (16,8292) | 616 | f | f | 27 00 00 00 00 00 00 00 | f | (1,2) | {"(1,2)","(1,19)"}
6 | (16,8292) | 616 | f | f | 28 00 00 00 00 00 00 00 | f | (2,24) | {"(2,24)","(4,11)"}
7 | (16,8292) | 616 | f | f | 29 00 00 00 00 00 00 00 | f | (2,17) | {"(2,17)","(11,2)"}
8 | (16,8292) | 616 | f | f | 2a 00 00 00 00 00 00 00 | f | (0,25) | {"(0,25)","(3,20)"}
9 | (16,8292) | 616 | f | f | 2b 00 00 00 00 00 00 00 | f | (0,10) | {"(0,10)","(0,14)"}
10 | (16,8292) | 616 | f | f | 2c 00 00 00 00 00 00 00 | f | (1,3) | {"(1,3)","(3,9)"}
11 | (16,8292) | 616 | f | f | 2d 00 00 00 00 00 00 00 | f | (6,28) | {"(6,28)","(11,1)"}
12 | (16,8292) | 616 | f | f | 2e 00 00 00 00 00 00 00 | f | (0,27) | {"(0,27)","(1,13)"}
13 | (16,8292) | 616 | f | f | 2f 00 00 00 00 00 00 00 | f | (4,17) | {"(4,17)","(4,21)"}
(13 rows)
This is a B-tree leaf page. All tuples that point to the table
happen to be posting list tuples (all of which store a total of
100 6 byte TIDs). There is also a high key tuple
at itemoffset number 1.
ctid is used to store encoded
information about each tuple in this example, though leaf page
tuples often store a heap TID directly in the
ctid field instead.
tids is the list of TIDs stored as a
posting list.
In an internal page (not shown), the block number part of
ctid is a downlink,
which is a block number of another page in the index itself.
The offset part (the second number) of
ctid stores encoded information about
the tuple, such as the number of columns present (suffix
truncation may have removed unneeded suffix columns). Truncated
columns are treated as having the value minus
infinity.
htid shows a heap TID for the tuple,
regardless of the underlying tuple representation. This value
may match ctid, or may be decoded
from the alternative representations used by posting list tuples
and tuples from internal pages. Tuples in internal pages
usually have the implementation level heap TID column truncated
away, which is represented as a NULL
htid value.
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 does not point to
another block. Also, on internal pages, the first real data
item (the first item that is not a high key) reliably has every
column truncated away, leaving no actual value in its
data field. Such an item does have a
valid downlink in its ctid field,
however.
For more details about the structure of B-tree indexes, see
. For more details about
deduplication and posting lists, see .
bt_page_items(page bytea) returns setof recordbt_page_items
It is also possible to pass a page to bt_page_items
as a bytea value. A page image obtained
with get_raw_page should be passed as argument. So
the last example could also be rewritten like this:
test=# SELECT itemoffset, ctid, itemlen, nulls, vars, data, dead, htid, tids[0:2] AS some_tids
FROM bt_page_items(get_raw_page('tenk2_hundred', 5));
itemoffset | ctid | itemlen | nulls | vars | data | dead | htid | some_tids
------------+-----------+---------+-------+------+-------------------------+------+--------+---------------------
1 | (16,1) | 16 | f | f | 30 00 00 00 00 00 00 00 | | |
2 | (16,8292) | 616 | f | f | 24 00 00 00 00 00 00 00 | f | (1,6) | {"(1,6)","(10,22)"}
3 | (16,8292) | 616 | f | f | 25 00 00 00 00 00 00 00 | f | (1,18) | {"(1,18)","(4,22)"}
4 | (16,8292) | 616 | f | f | 26 00 00 00 00 00 00 00 | f | (4,18) | {"(4,18)","(6,17)"}
5 | (16,8292) | 616 | f | f | 27 00 00 00 00 00 00 00 | f | (1,2) | {"(1,2)","(1,19)"}
6 | (16,8292) | 616 | f | f | 28 00 00 00 00 00 00 00 | f | (2,24) | {"(2,24)","(4,11)"}
7 | (16,8292) | 616 | f | f | 29 00 00 00 00 00 00 00 | f | (2,17) | {"(2,17)","(11,2)"}
8 | (16,8292) | 616 | f | f | 2a 00 00 00 00 00 00 00 | f | (0,25) | {"(0,25)","(3,20)"}
9 | (16,8292) | 616 | f | f | 2b 00 00 00 00 00 00 00 | f | (0,10) | {"(0,10)","(0,14)"}
10 | (16,8292) | 616 | f | f | 2c 00 00 00 00 00 00 00 | f | (1,3) | {"(1,3)","(3,9)"}
11 | (16,8292) | 616 | f | f | 2d 00 00 00 00 00 00 00 | f | (6,28) | {"(6,28)","(11,1)"}
12 | (16,8292) | 616 | f | f | 2e 00 00 00 00 00 00 00 | f | (0,27) | {"(0,27)","(1,13)"}
13 | (16,8292) | 616 | f | f | 2f 00 00 00 00 00 00 00 | f | (4,17) | {"(4,17)","(4,21)"}
(13 rows)
All the other details are the same as explained in the previous item.
BRIN Functionsbrin_page_type(page bytea) returns textbrin_page_typebrin_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 recordbrin_metapage_infobrin_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 tidbrin_revmap_databrin_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 recordbrin_page_itemsbrin_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 Functionsgin_metapage_info(page bytea) returns recordgin_metapage_infogin_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 recordgin_page_opaque_infogin_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 recordgin_leafpage_itemsgin_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)
GiST Functionsgist_page_opaque_info(page bytea) returns recordgist_page_opaque_infogist_page_opaque_info returns information from
a GiST index page's opaque area, such as the NSN,
rightlink and page type.
For example:
test=# SELECT * FROM gist_page_opaque_info(get_raw_page('test_gist_idx', 2));
lsn | nsn | rightlink | flags
-----+-----+-----------+--------
0/1 | 0/0 | 1 | {leaf}
(1 row)
gist_page_items(page bytea, index_oid regclass) returns setof recordgist_page_itemsgist_page_items returns information about
the data stored in a page of a GiST index. For example:
test=# SELECT * FROM gist_page_items(get_raw_page('test_gist_idx', 0), 'test_gist_idx');
itemoffset | ctid | itemlen | dead | keys
------------+-----------+---------+------+-------------------------------
1 | (1,65535) | 40 | f | (p)=("(185,185),(1,1)")
2 | (2,65535) | 40 | f | (p)=("(370,370),(186,186)")
3 | (3,65535) | 40 | f | (p)=("(555,555),(371,371)")
4 | (4,65535) | 40 | f | (p)=("(740,740),(556,556)")
5 | (5,65535) | 40 | f | (p)=("(870,870),(741,741)")
6 | (6,65535) | 40 | f | (p)=("(1000,1000),(871,871)")
(6 rows)
gist_page_items_bytea(page bytea) returns setof recordgist_page_items_bytea
Same as gist_page_items, but returns the key data
as a raw bytea blob. Since it does not attempt to decode
the key, it does not need to know which index is involved. For
example:
test=# SELECT * FROM gist_page_items_bytea(get_raw_page('test_gist_idx', 0));
itemoffset | ctid | itemlen | dead | key_data
------------+-----------+---------+------+-----------------------------------------&zwsp;-------------------------------------------
1 | (1,65535) | 40 | f | \x00000100ffff28000000000000c0644000000000&zwsp;00c06440000000000000f03f000000000000f03f
2 | (2,65535) | 40 | f | \x00000200ffff28000000000000c0744000000000&zwsp;00c074400000000000e064400000000000e06440
3 | (3,65535) | 40 | f | \x00000300ffff28000000000000207f4000000000&zwsp;00207f400000000000d074400000000000d07440
4 | (4,65535) | 40 | f | \x00000400ffff28000000000000c0844000000000&zwsp;00c084400000000000307f400000000000307f40
5 | (5,65535) | 40 | f | \x00000500ffff28000000000000f0894000000000&zwsp;00f089400000000000c884400000000000c88440
6 | (6,65535) | 40 | f | \x00000600ffff28000000000000208f4000000000&zwsp;00208f400000000000f889400000000000f88940
7 | (7,65535) | 40 | f | \x00000700ffff28000000000000408f4000000000&zwsp;00408f400000000000288f400000000000288f40
(7 rows)
Hash Functionshash_page_type(page bytea) returns texthash_page_typehash_page_type returns page type of
the given HASH index page. For example:
test=# SELECT hash_page_type(get_raw_page('con_hash_index', 0));
hash_page_type
----------------
metapage
hash_page_stats(page bytea) returns setof recordhash_page_statshash_page_stats returns information about
a bucket or overflow page of a HASH index.
For example:
test=# SELECT * FROM hash_page_stats(get_raw_page('con_hash_index', 1));
-[ RECORD 1 ]---+-----------
live_items | 407
dead_items | 0
page_size | 8192
free_size | 8
hasho_prevblkno | 4096
hasho_nextblkno | 8474
hasho_bucket | 0
hasho_flag | 66
hasho_page_id | 65408
hash_page_items(page bytea) returns setof recordhash_page_itemshash_page_items returns information about
the data stored in a bucket or overflow page of a HASH
index page. For example:
test=# SELECT * FROM hash_page_items(get_raw_page('con_hash_index', 1)) LIMIT 5;
itemoffset | ctid | data
------------+-----------+------------
1 | (899,77) | 1053474816
2 | (897,29) | 1053474816
3 | (894,207) | 1053474816
4 | (892,159) | 1053474816
5 | (890,111) | 1053474816
hash_bitmap_info(index oid, blkno bigint) returns recordhash_bitmap_infohash_bitmap_info shows the status of a bit
in the bitmap page for a particular overflow page of HASH
index. For example:
test=# SELECT * FROM hash_bitmap_info('con_hash_index', 2052);
bitmapblkno | bitmapbit | bitstatus
-------------+-----------+-----------
65 | 3 | t
hash_metapage_info(page bytea) returns recordhash_metapage_infohash_metapage_info returns information stored
in the meta page of a HASH index. For example:
test=# SELECT magic, version, ntuples, ffactor, bsize, bmsize, bmshift,
test-# maxbucket, highmask, lowmask, ovflpoint, firstfree, nmaps, procid,
test-# regexp_replace(spares::text, '(,0)*}', '}') as spares,
test-# regexp_replace(mapp::text, '(,0)*}', '}') as mapp
test-# FROM hash_metapage_info(get_raw_page('con_hash_index', 0));
-[ RECORD 1 ]-------------------------------------------------&zwsp;------------------------------
magic | 105121344
version | 4
ntuples | 500500
ffactor | 40
bsize | 8152
bmsize | 4096
bmshift | 15
maxbucket | 12512
highmask | 16383
lowmask | 8191
ovflpoint | 28
firstfree | 1204
nmaps | 1
procid | 450
spares | {0,0,0,0,0,0,1,1,1,1,1,1,1,1,3,4,4,4,45,55,58,59,&zwsp;508,567,628,704,1193,1202,1204}
mapp | {65}