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postgres/doc/src/sgml/pageinspect.sgml
David Rowley 04539e73fa Use the correct article for abbreviations 
We've accumulated quite a mix of instances of "an SQL" and "a SQL" in the
documents.  It would be good to be a bit more consistent with these.

The most recent version of the SQL standard I looked at seems to prefer
"an SQL".  That seems like a good lead to follow, so here we change all
instances of "a SQL" to become "an SQL".  Most instances correctly use
"an SQL" already, so it also makes sense to use the dominant variation in
order to minimise churn.

Additionally, there were some other abbreviations that needed to be
adjusted. FSM, SSPI, SRF and a few others.  Also fix some pronounceable,
abbreviations to use "a" instead of "an".  For example, "a SASL" instead
of "an SASL".

Here I've only adjusted the documents and error messages.  Many others
still exist in source code comments.  Translator hint comments seem to be
the biggest culprit.  It currently does not seem worth the churn to change
these.

Discussion: https://postgr.es/m/CAApHDvpML27UqFXnrYO1MJddsKVMQoiZisPvsAGhKE_tsKXquw%40mail.gmail.com
2021-06-11 13:38:04 +12:00

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<!-- doc/src/sgml/pageinspect.sgml -->
<sect1 id="pageinspect" xreflabel="pageinspect">
<title>pageinspect</title>
<indexterm zone="pageinspect">
<primary>pageinspect</primary>
</indexterm>
<para>
The <filename>pageinspect</filename> 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>General Functions</title>
<variablelist>
<varlistentry>
<term>
<function>get_raw_page(relname text, fork text, blkno bigint) 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</type> 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 bigint) 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</productname> 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 | 0 | 1 | 232 | 368 | 8192 | 8192 | 4 | 0
</screen>
The returned columns correspond to the fields in the
<structname>PageHeaderData</structname> struct.
See <filename>src/include/storage/bufpage.h</filename> for details.
</para>
<para>
The <structfield>checksum</structfield> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>page_checksum(page bytea, blkno bigint) returns smallint</function>
<indexterm>
<primary>page_checksum</primary>
</indexterm>
</term>
<listitem>
<para>
<function>page_checksum</function> computes the checksum for the page, as if
it was located at the given block.
</para>
<para>
A page image obtained with <function>get_raw_page</function> should be
passed as argument. For example:
<screen>
test=# SELECT page_checksum(get_raw_page('pg_class', 0), 0);
page_checksum
---------------
13443
</screen>
Note that the checksum depends on the block number, so matching block
numbers should be passed (except when doing esoteric debugging).
</para>
<para>
The checksum computed with this function can be compared with
the <structfield>checksum</structfield> result field of the
function <function>page_header</function>. If data checksums are
enabled for this instance, then the two values should be equal.
</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 an FSM page. For example:
<screen>
test=# SELECT fsm_page_contents(get_raw_page('pg_class', 'fsm', 0));
</screen>
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</filename> for more
information on the structure of an FSM page.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Heap Functions</title>
<variablelist>
<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 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.
</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</filename> and
<filename>src/include/access/htup_details.h</filename> for explanations of the fields
returned.
</para>
<para>
The <function>heap_tuple_infomask_flags</function> function can be
used to unpack the flag bits of <structfield>t_infomask</structfield>
and <structfield>t_infomask2</structfield> for heap tuples.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>tuple_data_split(rel_oid oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]</function>
<indexterm>
<primary>tuple_data_split</primary>
</indexterm>
</term>
<listitem>
<para>
<function>tuple_data_split</function> splits tuple data into attributes
in the same way as backend internals.
<screen>
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));
</screen>
This function should be called with the same arguments as the return
attributes of <function>heap_page_items</function>.
</para>
<para>
If <parameter>do_detoast</parameter> is <literal>true</literal>,
attributes will be detoasted as needed. Default value is
<literal>false</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>heap_page_item_attrs(page bytea, rel_oid regclass [, do_detoast bool]) returns setof record</function>
<indexterm>
<primary>heap_page_item_attrs</primary>
</indexterm>
</term>
<listitem>
<para>
<function>heap_page_item_attrs</function> is equivalent to
<function>heap_page_items</function> except that it returns
tuple raw data as an array of attributes that can optionally
be detoasted by <parameter>do_detoast</parameter> which is
<literal>false</literal> by default.
</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_item_attrs(get_raw_page('pg_class', 0), 'pg_class'::regclass);
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>heap_tuple_infomask_flags(t_infomask integer, t_infomask2 integer) returns record</function>
<indexterm>
<primary>heap_tuple_infomask_flags</primary>
</indexterm>
</term>
<listitem>
<para>
<function>heap_tuple_infomask_flags</function> decodes the
<structfield>t_infomask</structfield> and
<structfield>t_infomask2</structfield> returned by
<function>heap_page_items</function> 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:
<screen>
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;
</screen>
This function should be called with the same arguments as the return
attributes of <function>heap_page_items</function>.
</para>
<para>
Combined flags are displayed for source-level macros that take into
account the value of more than one raw bit, such as
<literal>HEAP_XMIN_FROZEN</literal>.
</para>
<para>
See <filename>src/include/access/htup_details.h</filename> for
explanations of the flag names returned.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>B-Tree Functions</title>
<variablelist>
<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 | 4
root | 1
level | 0
fastroot | 1
fastlevel | 0
last_cleanup_num_delpages | 0
last_cleanup_num_tuples | 230
allequalimage | f
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>bt_page_stats(relname text, blkno bigint) 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 | 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
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>bt_page_items(relname text, blkno bigint) 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 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)
</screen>
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 <quote>high key</quote> tuple
at <literal>itemoffset</literal> number 1.
<structfield>ctid</structfield> is used to store encoded
information about each tuple in this example, though leaf page
tuples often store a heap TID directly in the
<structfield>ctid</structfield> field instead.
<structfield>tids</structfield> is the list of TIDs stored as a
posting list.
</para>
<para>
In an internal page (not shown), the block number part of
<structfield>ctid</structfield> is a <quote>downlink</quote>,
which is a block number of another page in the index itself.
The offset part (the second number) of
<structfield>ctid</structfield> 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 <quote>minus
infinity</quote>.
</para>
<para>
<structfield>htid</structfield> shows a heap TID for the tuple,
regardless of the underlying tuple representation. This value
may match <structfield>ctid</structfield>, 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
<structfield>htid</structfield> value.
</para>
<para>
Note that the first item on any non-rightmost page (any page with
a non-zero value in the <structfield>btpo_next</structfield> field) is the
page's <quote>high key</quote>, meaning its <structfield>data</structfield>
serves as an upper bound on all items appearing on the page, while
its <structfield>ctid</structfield> 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
<structfield>data</structfield> field. Such an item does have a
valid downlink in its <structfield>ctid</structfield> field,
however.
</para>
<para>
For more details about the structure of B-tree indexes, see
<xref linkend="btree-structure"/>. For more details about
deduplication and posting lists, see <xref
linkend="btree-deduplication"/>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>bt_page_items(page bytea) returns setof record</function>
<indexterm>
<primary>bt_page_items</primary>
</indexterm>
</term>
<listitem>
<para>
It is also possible to pass a page to <function>bt_page_items</function>
as a <type>bytea</type> value. A page image obtained
with <function>get_raw_page</function> should be passed as argument. So
the last example could also be rewritten like this:
<screen>
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)
</screen>
All the other details are the same as explained in the previous item.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>BRIN Functions</title>
<variablelist>
<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</structname> and <structname>BrinValues</structname> structs.
See <filename>src/include/access/brin_tuple.h</filename> for details.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>GIN Functions</title>
<variablelist>
<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>
</variablelist>
</sect2>
<sect2>
<title>GiST Functions</title>
<variablelist>
<varlistentry>
<term>
<function>gist_page_opaque_info(page bytea) returns record</function>
<indexterm>
<primary>gist_page_opaque_info</primary>
</indexterm>
</term>
<listitem>
<para>
<function>gist_page_opaque_info</function> returns information from
a <acronym>GiST</acronym> index page's opaque area, such as the NSN,
rightlink and page type.
For example:
<screen>
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)
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>gist_page_items(page bytea, index_oid regclass) returns setof record</function>
<indexterm>
<primary>gist_page_items</primary>
</indexterm>
</term>
<listitem>
<para>
<function>gist_page_items</function> returns information about
the data stored in a page of a <acronym>GiST</acronym> index. For example:
<screen>
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)
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>gist_page_items_bytea(page bytea) returns setof record</function>
<indexterm>
<primary>gist_page_items_bytea</primary>
</indexterm>
</term>
<listitem>
<para>
Same as <function>gist_page_items</function>, but returns the key data
as a raw <type>bytea</type> blob. Since it does not attempt to decode
the key, it does not need to know which index is involved. For
example:
<screen>
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 | \x00000100ffff28000000000000c0644000000000&zwsp;00c06440000000000000f03f000000000000f03f
2 | (2,65535) | 40 | f | \x00000200ffff28000000000000c0744000000000&zwsp;00c074400000000000e064400000000000e06440
3 | (3,65535) | 40 | f | \x00000300ffff28000000000000207f4000000000&zwsp;00207f400000000000d074400000000000d07440
4 | (4,65535) | 40 | f | \x00000400ffff28000000000000c0844000000000&zwsp;00c084400000000000307f400000000000307f40
5 | (5,65535) | 40 | f | \x00000500ffff28000000000000f0894000000000&zwsp;00f089400000000000c884400000000000c88440
6 | (6,65535) | 40 | f | \x00000600ffff28000000000000208f4000000000&zwsp;00208f400000000000f889400000000000f88940
7 | (7,65535) | 40 | f | \x00000700ffff28000000000000408f4000000000&zwsp;00408f400000000000288f400000000000288f40
(7 rows)
</screen>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Hash Functions</title>
<variablelist>
<varlistentry>
<term>
<function>hash_page_type(page bytea) returns text</function>
<indexterm>
<primary>hash_page_type</primary>
</indexterm>
</term>
<listitem>
<para>
<function>hash_page_type</function> returns page type of
the given <acronym>HASH</acronym> index page. For example:
<screen>
test=# SELECT hash_page_type(get_raw_page('con_hash_index', 0));
hash_page_type
----------------
metapage
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>hash_page_stats(page bytea) returns setof record</function>
<indexterm>
<primary>hash_page_stats</primary>
</indexterm>
</term>
<listitem>
<para>
<function>hash_page_stats</function> returns information about
a bucket or overflow page of a <acronym>HASH</acronym> index.
For example:
<screen>
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
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>hash_page_items(page bytea) returns setof record</function>
<indexterm>
<primary>hash_page_items</primary>
</indexterm>
</term>
<listitem>
<para>
<function>hash_page_items</function> returns information about
the data stored in a bucket or overflow page of a <acronym>HASH</acronym>
index page. For example:
<screen>
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
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>hash_bitmap_info(index oid, blkno bigint) returns record</function>
<indexterm>
<primary>hash_bitmap_info</primary>
</indexterm>
</term>
<listitem>
<para>
<function>hash_bitmap_info</function> shows the status of a bit
in the bitmap page for a particular overflow page of <acronym>HASH</acronym>
index. For example:
<screen>
test=# SELECT * FROM hash_bitmap_info('con_hash_index', 2052);
bitmapblkno | bitmapbit | bitstatus
-------------+-----------+-----------
65 | 3 | t
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>hash_metapage_info(page bytea) returns record</function>
<indexterm>
<primary>hash_metapage_info</primary>
</indexterm>
</term>
<listitem>
<para>
<function>hash_metapage_info</function> returns information stored
in the meta page of a <acronym>HASH</acronym> index. For example:
<screen>
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,&zwsp;508,567,628,704,1193,1202,1204}
mapp | {65}
</screen>
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
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