diff options
| -rw-r--r-- | doc/src/sgml/pageinspect.sgml | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/doc/src/sgml/pageinspect.sgml b/doc/src/sgml/pageinspect.sgml index 6f51dc682df..313cbaea308 100644 --- a/doc/src/sgml/pageinspect.sgml +++ b/doc/src/sgml/pageinspect.sgml @@ -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)"} |