本文简单介绍了在PG数据库B-Tree索引的物理存储结构中Special space部分，包括根节点、左右兄弟节点等相关索引结构信息，以及初步探讨了PG在物理存储上如何保证索引的有序性。
一、测试数据
测试数据同上一节，索引文件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、索引结构信息
索引物理存储结构在上一节已大体介绍，这里主要介绍索引的结构信息。通过pageinspect插件的bt_page_stats函数可以获得索引结构信息，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)
相关的数据结构如下：
//---------------------- 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 */
查询结果中，type=l（字母l），表示这个block（page）是leaf block，在这个block中有4个item（live_items=4），没有废弃的items（dead_items=0），没有left sibling（btpo_prev =0）也没有right sibling（btpo_next =0），也就是左右两边都没有同级节点。btpo是一个union，值为0，表示该page为叶子page，btpo_flags值为3即BTP_LEAF | BTP_ROOT，既是叶子page也是根page。
这些信息物理存储在先前介绍过的PageHeader中的special space中，共占用16个字节：
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
我们都知道，B-Tree索引是有序的，下面我们看看在物理存储结构上如何保证有序性。
插入数据，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=# -- 插入数据,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索引页
[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
插入数据，id=17
testdb=# -- 插入数据,id=17
testdb=# insert into t_index values(17,'4','d');
INSERT 0 1
testdb=# checkpoint;
CHECKPOINT
testdb=# -- 查看索引数据页头数据
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索引页
[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
索引的数据区，并没有按照大小顺序排序，\x11（17）在\x12（18）的后面（从尾部开始往前），但在索引页的头部ItemId区域是有序的，第5个ItemId（\x00209f90）指向的是17，而第6个ItemId（\x00209fa0）指向的是18，通过索引数据指针的有序保证索引有序性。
四、小结
小结一下，知识要点：
1、Special Space：介绍了索引PageHeaderData的Special Space的存储结构，通过Special Space可以获得B-Tree的root、左右sibling等信息；
2、有序性：索引Page通过索引项指针保证有序性。
最后：
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