理解btree页面内部项目

Question

-- https://doxygen.postgresql.org/btreefuncs_8c_source.html#l00625
CREATE TABLE s2 AS  SELECT  g::int AS g FROM generate_series(1, 10000) g;
ALTER TABLE s2 ADD PRIMARY KEY (g);

SELECT root,level FROM bt_metap ('s2_pkey');

SELECT * FROM bt_page_stats ('s2_pkey', 3) gx

SELECT  itemoffset,itemlen,ctid
        ,pg_column_size(ctid) ctid_size,data
        ,pg_column_size(data) size_data
FROM    bt_page_items ('s2_pkey', 3)
ORDER BY    itemoffset LIMIT 10;

返回

 itemoffset | itemlen |  ctid  | ctid_size |          data           | size_data
------------+---------+--------+-----------+-------------------------+-----------
          1 |       8 | (1,0)  |         6 |                         |         1
          2 |      16 | (2,1)  |         6 | 6f 01 00 00 00 00 00 00 |        24
          3 |      16 | (4,1)  |         6 | dd 02 00 00 00 00 00 00 |        24
          4 |      16 | (5,1)  |         6 | 4b 04 00 00 00 00 00 00 |        24
          5 |      16 | (6,1)  |         6 | b9 05 00 00 00 00 00 00 |        24
          6 |      16 | (7,1)  |         6 | 27 07 00 00 00 00 00 00 |        24
          7 |      16 | (8,1)  |         6 | 95 08 00 00 00 00 00 00 |        24
          8 |      16 | (9,1)  |         6 | 03 0a 00 00 00 00 00 00 |        24
          9 |      16 | (10,1) |         6 | 71 0b 00 00 00 00 00 00 |        24
         10 |      16 | (11,1) |         6 | df 0c 00 00 00 00 00 00 |        24
(10 rows)

我想知道16项是什么意思？

Answer 1

itemlen只是索引条目的大小。ctid有8个字节的数据和6个字节，然后将其舍入到下一个8的倍数。所以大小是16个字节。

第一项是“高键”，它没有数据，因此大小为8。