479 lines
15 KiB
Plaintext
479 lines
15 KiB
Plaintext
<!-- doc/src/sgml/pageinspect.sgml -->
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<sect1 id="pageinspect" xreflabel="pageinspect">
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<title>pageinspect</title>
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<indexterm zone="pageinspect">
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<primary>pageinspect</primary>
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</indexterm>
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<para>
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The <filename>pageinspect</> module provides functions that allow you to
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inspect the contents of database pages at a low level, which is useful for
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debugging purposes. All of these functions may be used only by superusers.
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</para>
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<sect2>
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<title>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<function>get_raw_page(relname text, fork text, blkno int) returns bytea</function>
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<indexterm>
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<primary>get_raw_page</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>get_raw_page</function> reads the specified block of the named
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relation and returns a copy as a <type>bytea</> value. This allows a
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single time-consistent copy of the block to be obtained.
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<replaceable>fork</replaceable> should be <literal>'main'</literal> for
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the main data fork, <literal>'fsm'</literal> for the free space map,
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<literal>'vm'</literal> for the visibility map, or <literal>'init'</literal>
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for the initialization fork.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>get_raw_page(relname text, blkno int) returns bytea</function>
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</term>
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<listitem>
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<para>
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A shorthand version of <function>get_raw_page</function>, for reading
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from the main fork. Equivalent to
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<literal>get_raw_page(relname, 'main', blkno)</literal>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>page_header(page bytea) returns record</function>
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<indexterm>
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<primary>page_header</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>page_header</function> shows fields that are common to all
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<productname>PostgreSQL</> heap and index pages.
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</para>
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<para>
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A page image obtained with <function>get_raw_page</function> should be
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passed as argument. For example:
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<screen>
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test=# SELECT * FROM page_header(get_raw_page('pg_class', 0));
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lsn | checksum | flags | lower | upper | special | pagesize | version | prune_xid
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-----------+----------+--------+-------+-------+---------+----------+---------+-----------
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0/24A1B50 | 1 | 1 | 232 | 368 | 8192 | 8192 | 4 | 0
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</screen>
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The returned columns correspond to the fields in the
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<structname>PageHeaderData</> struct.
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See <filename>src/include/storage/bufpage.h</> for details.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>heap_page_items(page bytea) returns setof record</function>
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<indexterm>
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<primary>heap_page_items</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>heap_page_items</function> shows all line pointers on a heap
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page. For those line pointers that are in use, tuple headers as well
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as tuple raw data are also shown. All tuples are shown, whether or not
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the tuples were visible to an MVCC snapshot at the time the raw page
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was copied.
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</para>
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<para>
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A heap page image obtained with <function>get_raw_page</function> should
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be passed as argument. For example:
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<screen>
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test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));
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</screen>
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See <filename>src/include/storage/itemid.h</> and
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<filename>src/include/access/htup_details.h</> for explanations of the fields
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returned.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>tuple_data_split(rel_oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]</function>
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<indexterm>
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<primary>tuple_data_split</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>tuple_data_split</function> splits tuple data into attributes
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in the same way as backend internals.
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<screen>
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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));
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</screen>
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This function should be called with the same arguments as the return
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attributes of <function>heap_page_items</function>.
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</para>
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<para>
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If <parameter>do_detoast</parameter> is <literal>true</literal>,
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attribute that will be detoasted as needed. Default value is
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<literal>false</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>heap_page_item_attrs(rel_oid, t_data bytea, [, do_detoast bool]) returns bytea[]</function>
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<indexterm>
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<primary>heap_page_item_attrs</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>heap_page_item_attrs</function> is equivalent to
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<function>heap_page_items</function> except that it returns
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tuple raw data as an array of attributes that can optionally
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be detoasted by <parameter>do_detoast</parameter> which is
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<literal>false</literal> by default.
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</para>
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<para>
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A heap page image obtained with <function>get_raw_page</function> should
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be passed as argument. For example:
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<screen>
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test=# SELECT * FROM heap_page_item_attrs(get_raw_page('pg_class', 0), 'pg_class'::regclass);
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>bt_metap(relname text) returns record</function>
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<indexterm>
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<primary>bt_metap</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>bt_metap</function> returns information about a B-tree
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index's metapage. For example:
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<screen>
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test=# SELECT * FROM bt_metap('pg_cast_oid_index');
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-[ RECORD 1 ]-----
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magic | 340322
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version | 2
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root | 1
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level | 0
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fastroot | 1
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fastlevel | 0
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>bt_page_stats(relname text, blkno int) returns record</function>
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<indexterm>
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<primary>bt_page_stats</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>bt_page_stats</function> returns summary information about
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single pages of B-tree indexes. For example:
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<screen>
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test=# SELECT * FROM bt_page_stats('pg_cast_oid_index', 1);
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-[ RECORD 1 ]-+-----
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blkno | 1
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type | l
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live_items | 256
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dead_items | 0
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avg_item_size | 12
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page_size | 8192
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free_size | 4056
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btpo_prev | 0
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btpo_next | 0
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btpo | 0
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btpo_flags | 3
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>bt_page_items(relname text, blkno int) returns setof record</function>
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<indexterm>
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<primary>bt_page_items</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>bt_page_items</function> returns detailed information about
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all of the items on a B-tree index page. For example:
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<screen>
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test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
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itemoffset | ctid | itemlen | nulls | vars | data
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------------+---------+---------+-------+------+-------------
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1 | (0,1) | 12 | f | f | 23 27 00 00
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2 | (0,2) | 12 | f | f | 24 27 00 00
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3 | (0,3) | 12 | f | f | 25 27 00 00
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4 | (0,4) | 12 | f | f | 26 27 00 00
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5 | (0,5) | 12 | f | f | 27 27 00 00
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6 | (0,6) | 12 | f | f | 28 27 00 00
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7 | (0,7) | 12 | f | f | 29 27 00 00
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8 | (0,8) | 12 | f | f | 2a 27 00 00
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</screen>
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In a B-tree leaf page, <structfield>ctid</> points to a heap tuple.
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In an internal page, the block number part of <structfield>ctid</>
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points to another page in the index itself, while the offset part
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(the second number) is ignored and is usually 1.
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</para>
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<para>
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Note that the first item on any non-rightmost page (any page with
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a non-zero value in the <structfield>btpo_next</> field) is the
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page's <quote>high key</quote>, meaning its <structfield>data</>
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serves as an upper bound on all items appearing on the page, while
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its <structfield>ctid</> field is meaningless. Also, on non-leaf
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pages, the first real data item (the first item that is not a high
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key) is a <quote>minus infinity</quote> item, with no actual value
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in its <structfield>data</> field. Such an item does have a valid
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downlink in its <structfield>ctid</> field, however.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>brin_page_type(page bytea) returns text</function>
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<indexterm>
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<primary>brin_page_type</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>brin_page_type</function> returns the page type of the given
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<acronym>BRIN</acronym> index page, or throws an error if the page is
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not a valid <acronym>BRIN</acronym> page. For example:
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<screen>
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test=# SELECT brin_page_type(get_raw_page('brinidx', 0));
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brin_page_type
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----------------
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meta
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>brin_metapage_info(page bytea) returns record</function>
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<indexterm>
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<primary>brin_metapage_info</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>brin_metapage_info</function> returns assorted information
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about a <acronym>BRIN</acronym> index metapage. For example:
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<screen>
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test=# SELECT * FROM brin_metapage_info(get_raw_page('brinidx', 0));
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magic | version | pagesperrange | lastrevmappage
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------------+---------+---------------+----------------
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0xA8109CFA | 1 | 4 | 2
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>brin_revmap_data(page bytea) returns setof tid</function>
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<indexterm>
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<primary>brin_revmap_data</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>brin_revmap_data</function> returns the list of tuple
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identifiers in a <acronym>BRIN</acronym> index range map page.
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For example:
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<screen>
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test=# SELECT * FROM brin_revmap_data(get_raw_page('brinidx', 2)) limit 5;
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pages
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---------
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(6,137)
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(6,138)
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(6,139)
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(6,140)
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(6,141)
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>brin_page_items(page bytea, index oid) returns setof record</function>
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<indexterm>
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<primary>brin_page_items</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>brin_page_items</function> returns the data stored in the
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<acronym>BRIN</acronym> data page. For example:
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<screen>
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test=# SELECT * FROM brin_page_items(get_raw_page('brinidx', 5),
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'brinidx')
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ORDER BY blknum, attnum LIMIT 6;
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itemoffset | blknum | attnum | allnulls | hasnulls | placeholder | value
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------------+--------+--------+----------+----------+-------------+--------------
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137 | 0 | 1 | t | f | f |
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137 | 0 | 2 | f | f | f | {1 .. 88}
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138 | 4 | 1 | t | f | f |
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138 | 4 | 2 | f | f | f | {89 .. 176}
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139 | 8 | 1 | t | f | f |
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139 | 8 | 2 | f | f | f | {177 .. 264}
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</screen>
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The returned columns correspond to the fields in the
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<structname>BrinMemTuple</> and <structname>BrinValues</> structs.
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See <filename>src/include/access/brin_tuple.h</> for details.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>gin_metapage_info(page bytea) returns record</function>
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<indexterm>
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<primary>gin_metapage_info</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>gin_metapage_info</function> returns information about
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a <acronym>GIN</acronym> index metapage. For example:
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<screen>
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test=# SELECT * FROM gin_metapage_info(get_raw_page('gin_index', 0));
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-[ RECORD 1 ]----+-----------
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pending_head | 4294967295
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pending_tail | 4294967295
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tail_free_size | 0
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n_pending_pages | 0
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n_pending_tuples | 0
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n_total_pages | 7
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n_entry_pages | 6
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n_data_pages | 0
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n_entries | 693
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version | 2
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>gin_page_opaque_info(page bytea) returns record</function>
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<indexterm>
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<primary>gin_page_opaque_info</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>gin_page_opaque_info</function> returns information about
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a <acronym>GIN</acronym> index opaque area, like the page type.
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For example:
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<screen>
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test=# SELECT * FROM gin_page_opaque_info(get_raw_page('gin_index', 2));
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rightlink | maxoff | flags
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-----------+--------+------------------------
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5 | 0 | {data,leaf,compressed}
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(1 row)
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>gin_leafpage_items(page bytea) returns setof record</function>
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<indexterm>
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<primary>gin_leafpage_items</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>gin_leafpage_items</function> returns information about
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the data stored in a <acronym>GIN</acronym> leaf page. For example:
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<screen>
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test=# SELECT first_tid, nbytes, tids[0:5] as some_tids
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FROM gin_leafpage_items(get_raw_page('gin_test_idx', 2));
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first_tid | nbytes | some_tids
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-----------+--------+----------------------------------------------------------
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(8,41) | 244 | {"(8,41)","(8,43)","(8,44)","(8,45)","(8,46)"}
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(10,45) | 248 | {"(10,45)","(10,46)","(10,47)","(10,48)","(10,49)"}
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(12,52) | 248 | {"(12,52)","(12,53)","(12,54)","(12,55)","(12,56)"}
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(14,59) | 320 | {"(14,59)","(14,60)","(14,61)","(14,62)","(14,63)"}
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(167,16) | 376 | {"(167,16)","(167,17)","(167,18)","(167,19)","(167,20)"}
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(170,30) | 376 | {"(170,30)","(170,31)","(170,32)","(170,33)","(170,34)"}
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(173,44) | 197 | {"(173,44)","(173,45)","(173,46)","(173,47)","(173,48)"}
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(7 rows)
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</screen>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>fsm_page_contents(page bytea) returns text</function>
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<indexterm>
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<primary>fsm_page_contents</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>fsm_page_contents</function> shows the internal node structure
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of a FSM page. The output is a multiline string, with one line per
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node in the binary tree within the page. Only those nodes that are not
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zero are printed. The so-called "next" pointer, which points to the
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next slot to be returned from the page, is also printed.
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</para>
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<para>
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See <filename>src/backend/storage/freespace/README</> for more
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information on the structure of an FSM page.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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</sect1>
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