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.

General Functions
-----------------

`get_raw_page(relname text, fork text, blkno bigint) 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](#storage-fsm), `'vm'` for the [visibility map](#storage-vm), 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 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 |        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 smallint` page\_checksum

:   `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

:   `fsm_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 Functions
--------------

`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.

    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\_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`, 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

:   `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);

`heap_tuple_infomask_flags(t_infomask integer, t_infomask2 integer) returns record` heap\_tuple\_infomask\_flags

:   `heap_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 Functions
----------------

`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                   | 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 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    | 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 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 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
    [???](#btree-structure). For more details about deduplication and
    posting lists, see [???](#btree-deduplication).

`bt_page_items(page bytea) returns setof record` bt\_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 Functions
--------------

`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 Functions
-------------

`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)

GiST Functions
--------------

`gist_page_opaque_info(page bytea) returns record` gist\_page\_opaque\_info

:   `gist_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 record` gist\_page\_items

:   `gist_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)=((166,166))
                  2 | (2,65535) |      40 | f    | (p)=((332,332))
                  3 | (3,65535) |      40 | f    | (p)=((498,498))
                  4 | (4,65535) |      40 | f    | (p)=((664,664))
                  5 | (5,65535) |      40 | f    | (p)=((830,830))
                  6 | (6,65535) |      40 | f    | (p)=((996,996))
                  7 | (7,65535) |      40 | f    | (p)=((1000,1000))
        (7 rows)

`gist_page_items_bytea(page bytea) returns setof record` gist\_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    | \x00000100ffff28000000000000c0644000000000ZWSP00c06440000000000000f03f000000000000f03f
                  2 | (2,65535) |      40 | f    | \x00000200ffff28000000000000c0744000000000ZWSP00c074400000000000e064400000000000e06440
                  3 | (3,65535) |      40 | f    | \x00000300ffff28000000000000207f4000000000ZWSP00207f400000000000d074400000000000d07440
                  4 | (4,65535) |      40 | f    | \x00000400ffff28000000000000c0844000000000ZWSP00c084400000000000307f400000000000307f40
                  5 | (5,65535) |      40 | f    | \x00000500ffff28000000000000f0894000000000ZWSP00f089400000000000c884400000000000c88440
                  6 | (6,65535) |      40 | f    | \x00000600ffff28000000000000208f4000000000ZWSP00208f400000000000f889400000000000f88940
                  7 | (7,65535) |      40 | f    | \x00000700ffff28000000000000408f4000000000ZWSP00408f400000000000288f400000000000288f40
        (7 rows)

Hash Functions
--------------

`hash_page_type(page bytea) returns text` hash\_page\_type

:   `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

:   `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

:   `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 bigint) returns record` hash\_bitmap\_info

:   `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

:   `hash_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,ZWSP508,567,628,704,1193,1202,1204}
        mapp      | {65}
