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.
get_raw_page(relname text, fork text, blkno int) returns bytea
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
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 int4) returns smallint
page_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 text
fsm_page_contents
shows the internal node structure of a 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_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 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 oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]
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
, attributes will be detoasted as needed. Default value is false
.
heap_page_item_attrs(page bytea, rel_oid regclass [, do_detoast bool]) returns setof record
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
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 | 3 root | 1 level | 0 fastroot | 1 fastlevel | 0 oldest_xact | 582 last_cleanup_num_tuples | 1000
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
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.
bt_page_items(page bytea) returns setof record
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 * FROM bt_page_items(get_raw_page('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
All the other details are the same as explained in the previous item.
brin_page_type(page bytea) returns text
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
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
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
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
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
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
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)
hash_page_type(page bytea) returns text
hash_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 record
hash_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 record
hash_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 int) returns record
hash_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 record
hash_metapage_info
returns information stored in 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 ]------------------------------------------------------------------------------- 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 | {0,0,0,0,0,0,1,1,1,1,1,1,1,1,3,4,4,4,45,55,58,59,508,567,628,704,1193,1202,1204} mapp | {65}
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