PostgreSQL

F.22. pageinspect
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F.22. 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.

F.22.1. General Functions

[.term]#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)
[.term]#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.
[.term]#`+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.
[.term]#`+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.

F.22.2. Heap Functions

[.term]#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.
[.term]#`+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+`.
[.term]#`+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);

F.22.3. B-tree Functions

[.term]#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

[.term]#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

[.term]#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 [.quote]#“[.quote]#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 [.quote]#“[.quote]#minus infinity#”# item, with no actual value in its `+data+` field. Such an item does have a valid downlink in its `+ctid+` field, however.
[.term]#`+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.

F.22.4. BRIN Functions

[.term]#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

[.term]#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

[.term]#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)

[.term]#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.

F.22.5. GIN Functions

[.term]#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

[.term]#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)

[.term]#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)

F.22.6. Hash Functions

[.term]#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

[.term]#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

[.term]#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

[.term]#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

[.term]#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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