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F.21. 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.21.1. Functions

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 |        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 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

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 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)
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.

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