PostgreSQL Page頁結構解析（6）- B-Tree索引儲存結構#2

本文簡單介紹了在PG資料庫B-Tree索引的物理儲存結構中Special space部分，包括根節點、左右兄弟節點等相關索引結構資訊，以及初步探討了PG在物理儲存上如何保證索引的有序性。

一、測試資料

測試資料同上一節，索引檔案raw data：

[xdb@localhost utf8db]$ hexdump -C base/16477/26637
00000000  01 00 00 00 20 5d 0e db  00 00 00 00 40 00 f0 1f  |.... ]......@...|
00000010  f0 1f 04 20 00 00 00 00  62 31 05 00 03 00 00 00  |... ....b1......|
00000020  01 00 00 00 00 00 00 00  01 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 f0 bf  |................|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001ff0  00 00 00 00 00 00 00 00  00 00 00 00 08 00 00 00  |................|
00002000  01 00 00 00 98 5c 0e db  00 00 00 00 28 00 b0 1f  |.....\......(...|
00002010  f0 1f 04 20 00 00 00 00  e0 9f 20 00 d0 9f 20 00  |... ...... ... .|
00002020  c0 9f 20 00 b0 9f 20 00  b0 9f 20 00 00 00 00 00  |.. ... ... .....|
00002030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00003fb0  00 00 00 00 04 00 10 00  10 00 00 00 00 00 00 00  |................|
00003fc0  00 00 00 00 03 00 10 00  08 00 00 00 00 00 00 00  |................|
00003fd0  00 00 00 00 02 00 10 00  04 00 00 00 00 00 00 00  |................|
00003fe0  00 00 00 00 01 00 10 00  02 00 00 00 00 00 00 00  |................|
00003ff0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 00 00  |................|
00004000

二、索引結構資訊

索引物理儲存結構在上一節已大體介紹，這裡主要介紹索引的結構資訊。通過pageinspect外掛的bt_page_stats函式可以獲得索引結構資訊，包括root/leaf page,next & previous page：

testdb=# select * from bt_page_stats('pk_t_index',1);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     1 | l    |          4 |          0 |            16 |      8192 |      8068 |         0 |         0 |    0 |          3
(1 row)

相關的資料結構如下：

//---------------------- src/include/access/nbtree.h
/*
 *  BTPageOpaqueData -- At the end of every page, we store a pointer
 *  to both siblings in the tree.  This is used to do forward/backward
 *  index scans.  The next-page link is also critical for recovery when
 *  a search has navigated to the wrong page due to concurrent page splits
 *  or deletions; see src/backend/access/nbtree/README for more info.
 *
 *  In addition, we store the page's btree level (counting upwards from
 *  zero at a leaf page) as well as some flag bits indicating the page type
 *  and status.  If the page is deleted, we replace the level with the
 *  next-transaction-ID value indicating when it is safe to reclaim the page.
 *
 *  We also store a "vacuum cycle ID".  When a page is split while VACUUM is
 *  processing the index, a nonzero value associated with the VACUUM run is
 *  stored into both halves of the split page.  (If VACUUM is not running,
 *  both pages receive zero cycleids.)  This allows VACUUM to detect whether
 *  a page was split since it started, with a small probability of false match
 *  if the page was last split some exact multiple of MAX_BT_CYCLE_ID VACUUMs
 *  ago.  Also, during a split, the BTP_SPLIT_END flag is cleared in the left
 *  (original) page, and set in the right page, but only if the next page
 *  to its right has a different cycleid.
 *
 *  NOTE: the BTP_LEAF flag bit is redundant since level==0 could be tested
 *  instead.
 */

typedef struct BTPageOpaqueData
{
    BlockNumber btpo_prev;      /* left sibling, or P_NONE if leftmost */
    BlockNumber btpo_next;      /* right sibling, or P_NONE if rightmost */
    union
    {
        uint32      level;      /* tree level --- zero for leaf pages */
        TransactionId xact;     /* next transaction ID, if deleted */
    }           btpo;
    uint16      btpo_flags;     /* flag bits, see below */
    BTCycleId   btpo_cycleid;   /* vacuum cycle ID of latest split */
} BTPageOpaqueData;

typedef BTPageOpaqueData *BTPageOpaque;

/* Bits defined in btpo_flags */
#define BTP_LEAF        (1 << 0)    /* leaf page, i.e. not internal page */
#define BTP_ROOT        (1 << 1)    /* root page (has no parent) */
#define BTP_DELETED     (1 << 2)    /* page has been deleted from tree */
#define BTP_META        (1 << 3)    /* meta-page */
#define BTP_HALF_DEAD   (1 << 4)    /* empty, but still in tree */
#define BTP_SPLIT_END   (1 << 5)    /* rightmost page of split group */
#define BTP_HAS_GARBAGE (1 << 6)    /* page has LP_DEAD tuples */
#define BTP_INCOMPLETE_SPLIT (1 << 7)   /* right sibling's downlink is missing */

查詢結果中，type=l（字母l），表示這個block（page）是leaf block，在這個block中有4個item（live_items=4），沒有廢棄的items（dead_items=0），沒有left sibling（btpo_prev =0）也沒有right sibling（btpo_next =0），也就是左右兩邊都沒有同級節點。btpo是一個union，值為0，表示該page為葉子page，btpo_flags值為3即BTP_LEAF | BTP_ROOT，既是葉子page也是根page。
這些資訊物理儲存在先前介紹過的PageHeader中的special space中，共佔用16個位元組：

testdb=# select * from page_header(get_raw_page('pk_t_index',1));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid 
------------+----------+-------+-------+-------+---------+----------+---------+-----------
 1/DB0E5C98 |        0 |     0 |    40 |  8112 |    8176 |     8192 |       4 |         0
(1 row)

testdb=# select 8192+8176;
 ?column? 
----------
    16368
(1 row)

[xdb@localhost utf8db]$ hexdump -C base/16477/26637 -s 16368 -n 16
00003ff0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 00 00  |................|
00004000

三、索引有序性

我們都知道，B-Tree索引是有序的，下面我們看看在物理儲存結構上如何保證有序性。
插入資料，id=18

testdb=# select * from page_header(get_raw_page('pk_t_index',1));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid 
------------+----------+-------+-------+-------+---------+----------+---------+-----------
 1/DB0E5C98 |        0 |     0 |    40 |  8112 |    8176 |     8192 |       4 |         0
(1 row)

testdb=# -- 插入資料,id=18
testdb=# insert into t_index values(18,'4','d');
INSERT 0 1
testdb=# select * from page_header(get_raw_page('pk_t_index',1));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid 
------------+----------+-------+-------+-------+---------+----------+---------+-----------
 1/DB0E6498 |        0 |     0 |    44 |  8096 |    8176 |     8192 |       4 |         0
(1 row)
-- dump索引頁
[xdb@localhost utf8db]$ hexdump -C base/16477/26637 -s 8192
00002000  01 00 00 00 98 64 0e db  00 00 00 00 2c 00 a0 1f  |.....d......,...|
00002010  f0 1f 04 20 00 00 00 00  e0 9f 20 00 d0 9f 20 00  |... ...... ... .|
00002020  c0 9f 20 00 b0 9f 20 00  a0 9f 20 00 00 00 00 00  |.. ... ... .....|
00002030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00003fa0  00 00 00 00 05 00 10 00  12 00 00 00 00 00 00 00  |................|
00003fb0  00 00 00 00 04 00 10 00  10 00 00 00 00 00 00 00  |................|
00003fc0  00 00 00 00 03 00 10 00  08 00 00 00 00 00 00 00  |................|
00003fd0  00 00 00 00 02 00 10 00  04 00 00 00 00 00 00 00  |................|
00003fe0  00 00 00 00 01 00 10 00  02 00 00 00 00 00 00 00  |................|
00003ff0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 00 00  |................|
00004000

插入資料，id=17

testdb=# -- 插入資料,id=17
testdb=# insert into t_index values(17,'4','d');
INSERT 0 1
testdb=# checkpoint;
CHECKPOINT
testdb=# -- 檢視索引資料頁頭資料
testdb=# select * from page_header(get_raw_page('pk_t_index',1));
    lsn     | checksum | flags | lower | upper | special | pagesize | version | prune_xid 
------------+----------+-------+-------+-------+---------+----------+---------+-----------
 1/DB0E6808 |        0 |     0 |    48 |  8080 |    8176 |     8192 |       4 |         0
(1 row)
-- dump索引頁
[xdb@localhost utf8db]$ hexdump  -C base/16477/26637 -s 8192
00002000  01 00 00 00 08 68 0e db  00 00 00 00 30 00 90 1f  |.....h......0...|
00002010  f0 1f 04 20 00 00 00 00  e0 9f 20 00 d0 9f 20 00  |... ...... ... .|
00002020  c0 9f 20 00 b0 9f 20 00  90 9f 20 00 a0 9f 20 00  |.. ... ... ... .|
00002030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00003f90  00 00 00 00 06 00 10 00  11 00 00 00 00 00 00 00  |................|
00003fa0  00 00 00 00 05 00 10 00  12 00 00 00 00 00 00 00  |................|
00003fb0  00 00 00 00 04 00 10 00  10 00 00 00 00 00 00 00  |................|
00003fc0  00 00 00 00 03 00 10 00  08 00 00 00 00 00 00 00  |................|
00003fd0  00 00 00 00 02 00 10 00  04 00 00 00 00 00 00 00  |................|
00003fe0  00 00 00 00 01 00 10 00  02 00 00 00 00 00 00 00  |................|
00003ff0  00 00 00 00 00 00 00 00  00 00 00 00 03 00 00 00  |................|
00004000

索引的資料區，並沒有按照大小順序排序，\x11（17）在\x12（18）的後面（從尾部開始往前），但在索引頁的頭部ItemId區域是有序的，第5個ItemId（\x00209f90）指向的是17，而第6個ItemId（\x00209fa0）指向的是18，通過索引資料指標的有序保證索引有序性。

四、小結

小結一下，知識要點：
1、Special Space：介紹了索引PageHeaderData的Special Space的儲存結構，通過Special Space可以獲得B-Tree的root、左右sibling等資訊；
2、有序性：索引Page通過索引項指標保證有序性。

最後：

Don’t Be Afraid To Explore Beneath The Surface

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