path: root/doc/src/sgml/pageinspect.sgml

<!-- doc/src/sgml/pageinspect.sgml -->

<sect1 id="pageinspect" xreflabel="pageinspect">
 <title>pageinspect</title>

 <indexterm zone="pageinspect">
  <primary>pageinspect</primary>
 </indexterm>

 <para>
  The <filename>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.
 </para>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>get_raw_page(relname text, fork text, blkno int) returns bytea</function>
     <indexterm>
      <primary>get_raw_page</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>get_raw_page</function> reads the specified block of the named
      relation and returns a copy as a <type>bytea</> value.  This allows a
      single time-consistent copy of the block to be obtained.
      <replaceable>fork</replaceable> should be <literal>'main'</literal> for
      the main data fork, <literal>'fsm'</literal> for the free space map,
      <literal>'vm'</literal> for the visibility map, or <literal>'init'</literal>
      for the initialization fork.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>get_raw_page(relname text, blkno int) returns bytea</function>
    </term>

    <listitem>
     <para>
      A shorthand version of <function>get_raw_page</function>, for reading
      from the main fork.  Equivalent to
      <literal>get_raw_page(relname, 'main', blkno)</literal>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>page_header(page bytea) returns record</function>
     <indexterm>
      <primary>page_header</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>page_header</function> shows fields that are common to all
      <productname>PostgreSQL</> heap and index pages.
     </para>

     <para>
      A page image obtained with <function>get_raw_page</function> should be
      passed as argument.  For example:
<screen>
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
</screen>
      The returned columns correspond to the fields in the
      <structname>PageHeaderData</> struct.
      See <filename>src/include/storage/bufpage.h</> for details.
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>heap_page_items(page bytea) returns setof record</function>
     <indexterm>
      <primary>heap_page_items</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>heap_page_items</function> 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.
     </para>
     <para>
      A heap page image obtained with <function>get_raw_page</function> should
      be passed as argument.  For example:
<screen>
test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));
</screen>
      See <filename>src/include/storage/itemid.h</> and
      <filename>src/include/access/htup_details.h</> for explanations of the fields
      returned.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_metap(relname text) returns record</function>
     <indexterm>
      <primary>bt_metap</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>bt_metap</function> returns information about a B-tree
      index's metapage.  For example:
<screen>
test=# SELECT * FROM bt_metap('pg_cast_oid_index');
-[ RECORD 1 ]-----
magic     | 340322
version   | 2
root      | 1
level     | 0
fastroot  | 1
fastlevel | 0
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_page_stats(relname text, blkno int) returns record</function>
     <indexterm>
      <primary>bt_page_stats</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>bt_page_stats</function> returns summary information about
      single pages of B-tree indexes.  For example:
<screen>
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
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>bt_page_items(relname text, blkno int) returns setof record</function>
     <indexterm>
      <primary>bt_page_items</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>bt_page_items</function> returns detailed information about
      all of the items on a B-tree index page.  For example:
<screen>
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
</screen>
      In a B-tree leaf page, <structfield>ctid</> points to a heap tuple.
      In an internal page, the block number part of <structfield>ctid</>
      points to another page in the index itself, while the offset part
      (the second number) is ignored and is usually 1.
     </para>
     <para>
      Note that the first item on any non-rightmost page (any page with
      a non-zero value in the <structfield>btpo_next</> field) is the
      page's <quote>high key</quote>, meaning its <structfield>data</>
      serves as an upper bound on all items appearing on the page, while
      its <structfield>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>minus infinity</quote> item, with no actual value
      in its <structfield>data</> field.  Such an item does have a valid
      downlink in its <structfield>ctid</> field, however.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>brin_page_type(page bytea) returns text</function>
     <indexterm>
      <primary>brin_page_type</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>brin_page_type</function> returns the page type of the given
      <acronym>BRIN</acronym> index page, or throws an error if the page is
      not a valid <acronym>BRIN</acronym> page.  For example:
<screen>
test=# SELECT brin_page_type(get_raw_page('brinidx', 0));
 brin_page_type 
----------------
 meta
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>brin_metapage_info(page bytea) returns record</function>
     <indexterm>
      <primary>brin_metapage_info</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>brin_metapage_info</function> returns assorted information
      about a <acronym>BRIN</acronym> index metapage.  For example:
<screen>
test=# SELECT * FROM brin_metapage_info(get_raw_page('brinidx', 0));
   magic    | version | pagesperrange | lastrevmappage 
------------+---------+---------------+----------------
 0xA8109CFA |       1 |             4 |              2
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>brin_revmap_data(page bytea) returns setof tid</function>
     <indexterm>
      <primary>brin_revmap_data</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>brin_revmap_data</function> returns the list of tuple
      identifiers in a <acronym>BRIN</acronym> index range map page.
      For example:
<screen>
test=# SELECT * FROM brin_revmap_data(get_raw_page('brinidx', 2)) limit 5;
  pages  
---------
 (6,137)
 (6,138)
 (6,139)
 (6,140)
 (6,141)
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>brin_page_items(page bytea, index oid) returns setof record</function>
     <indexterm>
      <primary>brin_page_items</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>brin_page_items</function> returns the data stored in the
      <acronym>BRIN</acronym> data page.  For example:
<screen>
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}
</screen>
      The returned columns correspond to the fields in the
      <structname>BrinMemTuple</> and <structname>BrinValues</> structs.
      See <filename>src/include/access/brin_tuple.h</> for details.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>gin_metapage_info(page bytea) returns record</function>
     <indexterm>
      <primary>gin_metapage_info</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>gin_metapage_info</function> returns information about
      a <acronym>GIN</acronym> index metapage.  For example:
<screen>
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
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>gin_page_opaque_info(page bytea) returns record</function>
     <indexterm>
      <primary>gin_page_opaque_info</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>gin_page_opaque_info</function> returns information about
      a <acronym>GIN</acronym> index opaque area, like the page type.
      For example:
<screen>
test=# SELECT * FROM gin_page_opaque_info(get_raw_page('gin_index', 2));
 rightlink | maxoff |         flags
-----------+--------+------------------------
         5 |      0 | {data,leaf,compressed}
(1 row)
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>gin_leafpage_items(page bytea) returns setof record</function>
     <indexterm>
      <primary>gin_leafpage_items</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>gin_leafpage_items</function> returns information about
      the data stored in a <acronym>GIN</acronym> leaf page.  For example:
<screen>
 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)
</screen>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>fsm_page_contents(page bytea) returns text</function>
     <indexterm>
      <primary>fsm_page_contents</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      <function>fsm_page_contents</function> 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.
     </para>
     <para>
      See <filename>src/backend/storage/freespace/README</> for more
      information on the structure of an FSM page.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </sect2>

</sect1>