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Improve documentation of bt_page_items().

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Explain some of the funny conventions used in btree page items.

Peter Geoghegan and Jeff Janes
This commit is contained in:
Tom Lane 2015-03-12 14:18:26 -04:00
parent 443fd0540e
commit ebc0f5e01d
1 changed files with 16 additions and 2 deletions
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18
doc/src/sgml/pageinspect.sgml
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@ -192,6 +192,21 @@ test=# SELECT * FROM bt_page_items('pg_cast_oid_index', 1);
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>
@ -369,8 +384,7 @@ test=# SELECT * FROM gin_page_opaque_info(get_raw_page('gin_index', 2));
<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
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)"}