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深入浅出PostgreSQL B-Tree索引结构

简介: PostgreSQL 的B-Tree索引页分为几种类别 meta page root page # btpo_flags=2 branch page # btpo_flags=0 leaf page # btpo_flags=1 如果即

PostgreSQL B-Tree是一种变种(high-concurrency B-tree management algorithm),算法详情请参考
src/backend/access/nbtree/README

PostgreSQL 的B-Tree索引页分为几种类别

meta page
root page         #  btpo_flags=2
branch page    #  btpo_flags=0
leaf page         #  btpo_flags=1

如果即是leaf又是root则  btpo_flags=3。  

其中meta page和root page是必须有的,meta page需要一个页来存储,表示指向root page的page id。
随着记录数的增加,一个root page可能存不下所有的heap item,就会有leaf page,甚至branch page,甚至多层的branch page。
一共有几层branch 和 leaf,就用btree page元数据的 level 来表示。
4

我们可以使用pageinspect插件,内窥B-Tree的结构。

层次可以从bt_page_stats的btpo得到,代表当前index page所处的层级。
注意层级并不是唯一的,例如btpo=3的层级,可能有分几个档。
打个比喻,腾讯的技术岗位级别T3,对应T3这个级别又有几个小的档位。和这里的含义差不多,只是没有区分小档位的值,但是后面我们能看到它的存在。
btpo=0级表示最底层,处于这个层级的index pages存储的items(ctid)是指向heap page的。


类别和层级不挂钩,类别里面又可以有多个层级,但是只有层级=0的index page存储的ctid内容才是指向heap page的; 其他层级index page存储的ctid内容都是指向同层级其他index page(双向链表),或者指下级的index page。
.1.
0层结构,只有meta和root页。
root页最多可以存储的item数,取决于索引字段数据的长度、以及索引页的大小。
1
例子

postgres=# create extension pageinspect;

postgres=# create table tab1(id int primary key, info text);
CREATE TABLE
postgres=# insert into tab1 select generate_series(1,100), md5(random()::text);
INSERT 0 100
postgres=# vacuum analyze tab1;
VACUUM

查看meta page,可以看到root page id = 1 。
索引的level = 0, 说明没有branch和leaf page。

postgres=# select * from bt_metap('tab1_pkey');
 magic  | version | root | level | fastroot | fastlevel 
--------+---------+------+-------+----------+-----------
 340322 |       2 |    1 |     0 |        1 |         0
(1 row)

根据root page id = 1查看root page的stats
btpo=0 说明已经到了最底层
btpo_flags=3,说明它既是leaf又是root页。

postgres=# select * from bt_page_stats('tab1_pkey',1);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     1 | l    |        100 |          0 |            16 |      8192 |      6148 |         0 |         0 |    0 |          3
(1 row)

btpo_prev和btpo_next分别表示该页的相邻页(branch page是双向链表)。

btpo_flags 可以在代码中查看(src/include/access/nbtree.h),一共有几个

/* 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 */

查看0级 page存储的ctid (即items)
0级ctid 表示存储的是 heap页的寻址。 (如果是多层结构,那么branch page中的ctid, 它表示的是同级btree页(链条项ctid)或者下级btree页的寻址) 。
当ctid指向heap时, data是对应的列值。(多级结构的data意义不一样,后面会讲)

postgres=# select * from bt_page_items('tab1_pkey',1);
 itemoffset |  ctid   | itemlen | nulls | vars |          data           
------------+---------+---------+-------+------+-------------------------
          1 | (0,1)   |      16 | f     | f    | 01 00 00 00 00 00 00 00
          2 | (0,2)   |      16 | f     | f    | 02 00 00 00 00 00 00 00
...
         99 | (0,99)  |      16 | f     | f    | 63 00 00 00 00 00 00 00
        100 | (0,100) |      16 | f     | f    | 64 00 00 00 00 00 00 00
(100 rows)

根据ctid 查看heap记录

postgres=# select * from tab1 where ctid='(0,100)';
 id  |               info               
-----+----------------------------------
 100 | 68b63c269ee8cc2d99fe204f04d0ffcb
(1 row)


.2.
1层结构,包括meta page, root page, leaf page.
2
例子

postgres=# truncate tab1;
TRUNCATE TABLE
postgres=# insert into tab1 select generate_series(1,1000), md5(random()::text);
INSERT 0 1000
postgres=# vacuum analyze tab1;
VACUUM

查看meta page,可以看到root page id = 3, 索引的level = 1。
level = 1 表示包含了leaf page。

postgres=# select * from bt_metap('tab1_pkey');
 magic  | version | root | level | fastroot | fastlevel 
--------+---------+------+-------+----------+-----------
 340322 |       2 |    3 |     1 |        3 |         1
(1 row)

根据root page id 查看root page的stats
btpo = 1 说明还没有到最底层(最底层btpo=0, 这种页里面存储的ctid才代表指向heap page的地址)
btpo_flags=2 说明这个页是root page

postgres=# select * from bt_page_stats('tab1_pkey',3);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     3 | r    |          3 |          0 |            13 |      8192 |      8096 |         0 |         0 |    1 |          2
(1 row)

查看root page存储的 leaf page items (指向leaf page)
一共3个leaf pages, data存储的是这个leaf page存储的最小值。

postgres=# select * from bt_page_items('tab1_pkey',3);
 itemoffset | ctid  | itemlen | nulls | vars |          data           
------------+-------+---------+-------+------+-------------------------
          1 | (1,1) |       8 | f     | f    | 
          2 | (2,1) |      16 | f     | f    | 6f 01 00 00 00 00 00 00
          3 | (4,1) |      16 | f     | f    | dd 02 00 00 00 00 00 00
(3 rows)

第一条为空,是因为这个leaf page是最左边的PAGE,不存最小值。
对于有右leaf page的leaf page,第一条存储的heap item为该页的右链路。
第二条才是起始ITEM。
另外需要注意,虽然在item里面只存储右链,leaf page还是双向链表,在stats能看到它的prev 和next page。
根据leaf page id查看stats
最左leaf page = 1
prev btpo 指向meta page

可以看到btpo = 0了,说明这个页是底层页。  
btpo_flags=1 说明是leaf page  
postgres=# select * from bt_page_stats('tab1_pkey',1);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     1 | l    |        367 |          0 |            16 |      8192 |       808 |         0 |         2 |    0 |          1
(1 row)

next btpo 指向meta page
最右leaf page = 4
btpo_flags=1 说明是leaf page

postgres=# select * from bt_page_stats('tab1_pkey',4);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     4 | l    |        268 |          0 |            16 |      8192 |      2788 |         2 |         0 |    0 |          1
(1 row)

中间leaf page = 2
btpo_flags=1 说明是leaf page

postgres=# select * from bt_page_stats('tab1_pkey',2);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     2 | l    |        367 |          0 |            16 |      8192 |       808 |         1 |         4 |    0 |          1
(1 row)

查看leaf page存储的 heap ctid (即heap items)
含右页的例子, index page 1
第一条为右链表的第一条item, 第二条才是起始item

postgres=# select * from bt_page_items('tab1_pkey',1);
 itemoffset |  ctid   | itemlen | nulls | vars |          data           
------------+---------+---------+-------+------+-------------------------
          1 | (3,7)   |      16 | f     | f    | 6f 01 00 00 00 00 00 00
          2 | (0,1)   |      16 | f     | f    | 01 00 00 00 00 00 00 00
          3 | (0,2)   |      16 | f     | f    | 02 00 00 00 00 00 00 00
...
        367 | (3,6)   |      16 | f     | f    | 6e 01 00 00 00 00 00 00
(367 rows)

不含右页的例子, index page 4
第一条就是起始ctid (即items)

postgres=# select * from bt_page_items('tab1_pkey',4);
 itemoffset |  ctid   | itemlen | nulls | vars |          data           
------------+---------+---------+-------+------+-------------------------
          1 | (6,13)  |      16 | f     | f    | dd 02 00 00 00 00 00 00
          2 | (6,14)  |      16 | f     | f    | de 02 00 00 00 00 00 00
...
        268 | (8,40)  |      16 | f     | f    | e8 03 00 00 00 00 00 00
(268 rows)

根据ctid 查看heap记录

postgres=#              select * from tab1 where ctid='(0,1)';
 id |               info               
----+----------------------------------
  1 | 6ebc6b77aebf5dd11621a2ed846c08c4
(1 row)


.3.
记录数超过1层结构的索引可以存储的记录数时,会分裂为2层结构,除了meta page和root page,还可能包含1层branch page以及1层leaf page。
如果是边界页(branch or leaf),那么其中一个方向没有PAGE,这个方向的链表信息都统一指向meta page。
3
例子

create table tbl1(id int primary key, info text);  
postgres=# select 285^2;
 ?column? 
----------
    81225
(1 row)
postgres=# insert into tab2 select trunc(random()*10000000), md5(random()::text) from generate_series(1,1000000) on conflict on constraint tab2_pkey do nothing;
INSERT 0 951379
postgres=# vacuum analyze tab2;
VACUUM

查看meta page,可以看到root page id = 412, 索引的level=2,即包括1级 branch 和 1级 leaf。

postgres=# select * from bt_metap('tab2_pkey');
 magic  | version | root | level | fastroot | fastlevel 
--------+---------+------+-------+----------+-----------
 340322 |       2 |  412 |     2 |      412 |         2
(1 row)

根据root page id 查看root page的stats
btpo = 2 当前在第二层,另外还表示下层是1
btpo_flags = 2 说明是root page

postgres=# select * from bt_page_stats('tab2_pkey', 412);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
   412 | r    |         11 |          0 |            15 |      8192 |      7936 |         0 |         0 |    2 |          2
(1 row)

查看root page存储的 branch page items (指向branch page)

postgres=# select * from bt_page_items('tab2_pkey', 412);
 itemoffset |   ctid   | itemlen | nulls | vars |          data           
------------+----------+---------+-------+------+-------------------------
          1 | (3,1)    |       8 | f     | f    | 
          2 | (2577,1) |      16 | f     | f    | e1 78 0b 00 00 00 00 00
          3 | (1210,1) |      16 | f     | f    | ec 3a 18 00 00 00 00 00
          4 | (2316,1) |      16 | f     | f    | de 09 25 00 00 00 00 00
          5 | (574,1)  |      16 | f     | f    | aa e8 33 00 00 00 00 00
          6 | (2278,1) |      16 | f     | f    | 85 90 40 00 00 00 00 00
          7 | (1093,1) |      16 | f     | f    | f6 e9 4e 00 00 00 00 00
          8 | (2112,1) |      16 | f     | f    | a3 60 5c 00 00 00 00 00
          9 | (411,1)  |      16 | f     | f    | b2 ea 6b 00 00 00 00 00
         10 | (2073,1) |      16 | f     | f    | db de 79 00 00 00 00 00
         11 | (1392,1) |      16 | f     | f    | df b0 8a 00 00 00 00 00
(11 rows)

根据branch page id查看stats
btpo = 1 当前在第一层 ,另外还表示下层是0
btpo_flags = 0 说明是branch page

postgres=# select * from bt_page_stats('tab2_pkey', 3);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     3 | i    |        254 |          0 |            15 |      8192 |      3076 |         0 |      2577 |    1 |          0
(1 row)

查看branch page存储的 leaf page ctid (指向leaf page)
只要不是最右边的页,第一条都代表右页的起始item。
第二条才是当前页的起始ctid
注意所有branch page的起始item对应的data都是空的。
也就是说它不存储当前branch page包含的所有leaf pages的索引字段内容的最小值。

postgres=# select * from bt_page_items('tab2_pkey', 3);
 itemoffset |   ctid   | itemlen | nulls | vars |          data           
------------+----------+---------+-------+------+-------------------------
          1 | (735,1)  |      16 | f     | f    | e1 78 0b 00 00 00 00 00
          2 | (1,1)    |       8 | f     | f    | 
          3 | (2581,1) |      16 | f     | f    | a8 09 00 00 00 00 00 00
          4 | (1202,1) |      16 | f     | f    | f8 13 00 00 00 00 00 00
...
        254 | (3322,1) |      16 | f     | f    | ee 6f 0b 00 00 00 00 00
(254 rows)

根据ctid 查看leaf page
btpo = 0 当前在第0层,即最底层,这里存储的是heap ctid
btpo_flags = 1 说明是leaf page

postgres=# select * from bt_page_stats('tab2_pkey', 1);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     1 | l    |        242 |          0 |            16 |      8192 |      3308 |         0 |      2581 |    0 |          1
(1 row)

postgres=# select * from bt_page_items('tab2_pkey', 1);
 itemoffset |    ctid    | itemlen | nulls | vars |          data           
------------+------------+---------+-------+------+-------------------------
          1 | (4985,16)  |      16 | f     | f    | a8 09 00 00 00 00 00 00
          2 | (7305,79)  |      16 | f     | f    | 01 00 00 00 00 00 00 00
          3 | (2757,120) |      16 | f     | f    | 09 00 00 00 00 00 00 00
...
        242 | (1329,101) |      16 | f     | f    | a0 09 00 00 00 00 00 00
(242 rows)

查看leaf page中包含的heap page items。
如果我们根据索引页结构的原理,能推算出来(7305,79)是最小值,取它就没错了。

postgres=# select * from tab2 where ctid='(7305,79)';
 id |               info               
----+----------------------------------
  1 | 18aaeb74c359355311ac825ae2aeb22a
(1 row)

postgres=# select min(id) from tab2;
 min 
-----
   1
(1 row)



.4.
多层结构,除了meta page,还可能包含多层branch page,以及一层leaf page。

4
例子

postgres=# create table tab3(id int primary key, info text);
CREATE TABLE
postgres=# insert into tab3 select generate_series(1, 100000000), md5(random()::text);  

查看meta page, 注意level,已经是3级了。

meta page
postgres=# select * from bt_metap('tab3_pkey');
 magic  | version |  root  | level | fastroot | fastlevel 
--------+---------+--------+-------+----------+-----------
 340322 |       2 | 116816 |     3 |   116816 |         3
(1 row)

btpo_flags=2 代表 root page
btpo = 3 代表第3层

postgres=# select * from bt_page_stats('tab3_pkey', 116816);
 blkno  | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
--------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
 116816 | r    |          3 |          0 |            13 |      8192 |      8096 |         0 |         0 |    3 |          2
(1 row)

postgres=# select * from bt_page_items('tab3_pkey', 116816);
 itemoffset |    ctid    | itemlen | nulls | vars |          data           
------------+------------+---------+-------+------+-------------------------
          1 | (412,1)    |       8 | f     | f    | 
          2 | (116815,1) |      16 | f     | f    | 5f 9e c5 01 00 00 00 00
          3 | (198327,1) |      16 | f     | f    | bd 3c 8b 03 00 00 00 00
(3 rows)

btpo_flags=0 代表 branch page
btpo = 2 代表第2层

postgres=# select * from bt_page_stats('tab3_pkey', 412);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
   412 | i    |        286 |          0 |            15 |      8192 |      2436 |         0 |    116815 |    2 |          0
(1 row)

postgres=# select * from bt_page_items('tab3_pkey', 412);
 itemoffset |   ctid    | itemlen | nulls | vars |          data           
------------+-----------+---------+-------+------+-------------------------
          1 | (81636,1) |      16 | f     | f    | 5f 9e c5 01 00 00 00 00  -- 这是指向当前层级右页的ctid
          2 | (3,1)     |       8 | f     | f    |    -- 注意第一条初始值是这
          3 | (411,1)   |      16 | f     | f    | 77 97 01 00 00 00 00 00
          4 | (698,1)   |      16 | f     | f    | ed 2e 03 00 00 00 00 00
...
        286 | (81350,1) |      16 | f     | f    | e9 06 c4 01 00 00 00 00
(286 rows)

btpo_flags=0 代表 branch page
btpo = 1 代表第1层

postgres=# select * from bt_page_stats('tab3_pkey', 3);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     3 | i    |        286 |          0 |            15 |      8192 |      2436 |         0 |       411 |    1 |          0
(1 row)

postgres=# select * from bt_page_items('tab3_pkey', 3);
 itemoffset |  ctid   | itemlen | nulls | vars |          data           
------------+---------+---------+-------+------+-------------------------
          1 | (287,1) |      16 | f     | f    | 77 97 01 00 00 00 00 00
          2 | (1,1)   |       8 | f     | f    | 
          3 | (2,1)   |      16 | f     | f    | 6f 01 00 00 00 00 00 00
          4 | (4,1)   |      16 | f     | f    | dd 02 00 00 00 00 00 00
...
        286 | (286,1) |      16 | f     | f    | 09 96 01 00 00 00 00 00
(286 rows)

btpo_flags=1 代表 leaf page
btpo = 0 代表第0层

postgres=# select * from bt_page_stats('tab3_pkey', 1);
 blkno | type | live_items | dead_items | avg_item_size | page_size | free_size | btpo_prev | btpo_next | btpo | btpo_flags 
-------+------+------------+------------+---------------+-----------+-----------+-----------+-----------+------+------------
     1 | l    |        367 |          0 |            16 |      8192 |       808 |         0 |         2 |    0 |          1
(1 row)

postgres=# select * from bt_page_items('tab3_pkey', 1);
 itemoffset |  ctid   | itemlen | nulls | vars |          data           
------------+---------+---------+-------+------+-------------------------
          1 | (3,7)   |      16 | f     | f    | 6f 01 00 00 00 00 00 00
          2 | (0,1)   |      16 | f     | f    | 01 00 00 00 00 00 00 00
          3 | (0,2)   |      16 | f     | f    | 02 00 00 00 00 00 00 00
...
        367 | (3,6)   |      16 | f     | f    | 6e 01 00 00 00 00 00 00
(367 rows)

通过第0层的ctid就可以获取到heap了.
heap tuple例子

postgres=# select * from tab3 where ctid='(0,1)';
 id |               info               
----+----------------------------------
  1 | 370ee1989a2b7f5d8a5b43243596d91f
(1 row)


如何解释explain analyze中的扫描了多少个btree page
实战例子1

postgres=# create table tbl1(id int primary key, info text);
CREATE TABLE
postgres=# insert into tbl1 select trunc(random()*10000000), md5(random()::text) from generate_series(1,5000000) on conflict on constraint tbl1_pkey do nothing;
INSERT 0 3934875
postgres=# select ctid,* from tbl1 limit 10;
  ctid  |   id    |               info               
--------+---------+----------------------------------
 (0,1)  | 2458061 | 5c91812b54bdcae602321dceaf22e276
 (0,2)  | 8577271 | fe8e7a8be0d71a94e13b1b5a7786010b
 (0,3)  | 4612744 | 56983e47f044b5a4655300e1868d2850
 (0,4)  | 3690167 | 4a5ec8abf67bc018dcc113be829a59da
 (0,5)  | 2646638 | 7686b47dcb94e56c11d69ec04d6017f3
 (0,6)  | 6023272 | 4779d9a849c8287490be9d37a27b4637
 (0,7)  | 7163674 | 35af37f479f48caa65033a5ef56cd75e
 (0,8)  | 4049257 | 12fa110d927c88dce0773b546cc600c6
 (0,9)  | 5815903 | 69ed9770ede59917d15ac2373ca8c797
 (0,10) | 4068194 | 738595f73670da7ede40aefa8cb3d00c
(10 rows)
postgres=# vacuum analyze tbl1;
VACUUM

首先我们需要了解索引的level,才能正确的判断需要扫描多少个index page才能取出1条记录。

postgres=# select * from bt_metap('tbl1_pkey');
 magic  | version | root | level | fastroot | fastlevel 
--------+---------+------+-------+----------+-----------
 340322 |       2 |  412 |     2 |      412 |         2
(1 row)

level = 2的btree应该长这样
6

.1. 以下查询,命中了1条记录,并且走的是index only scan。
读了4个INDEX PAGE, 包括1 meta page, 1 root page, 1 branch page, 1 leaf page.

postgres=#  explain (analyze,verbose,timing,costs,buffers) select id from tbl1 where id = 1;
                                                         QUERY PLAN                                                         
----------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using tbl1_pkey on public.tbl1  (cost=0.42..1.44 rows=1 width=4) (actual time=0.019..0.020 rows=1 loops=1)
   Output: id
   Index Cond: (tbl1.id = 1)
   Heap Fetches: 0
   Buffers: shared hit=4
 Planning time: 0.072 ms
 Execution time: 0.072 ms
(7 rows)

.2. 以下查询,命中了0条记录,并且走的是index only scan。
读了4个INDEX PAGE, 包括1 meta page, 1 root page, 1 branch page, 1 leaf page.
但是explain只算了3个,没有计算leaf page的那次,算个小BUG吧。

postgres=# explain (analyze,verbose,timing,costs,buffers) select id from tbl1 where id in (3);
                                                         QUERY PLAN                                                         
----------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using tbl1_pkey on public.tbl1  (cost=0.43..1.45 rows=1 width=4) (actual time=0.010..0.010 rows=0 loops=1)
   Output: id
   Index Cond: (tbl1.id = 3)
   Heap Fetches: 0
   Buffers: shared hit=3
 Planning time: 0.073 ms
 Execution time: 0.031 ms
(7 rows)

.3. 以下查询,命中了7条记录,并且走的是index only scan。
读了22个INDEX PAGE,
1 meta page + 7 * (1 root + 1 branch + 1 leaf) = 22
也就是说,每个value都扫了root,branch,leaf。

postgres=#  explain (analyze,verbose,timing,costs,buffers) select id from tbl1 where id in (1,2,3,4,100,1000,10000);
                                                         QUERY PLAN                                                          
-----------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using tbl1_pkey on public.tbl1  (cost=0.42..10.10 rows=7 width=4) (actual time=0.018..0.033 rows=7 loops=1)
   Output: id
   Index Cond: (tbl1.id = ANY ('{1,2,3,4,100,1000,10000}'::integer[]))
   Heap Fetches: 0
   Buffers: shared hit=22
 Planning time: 0.083 ms
 Execution time: 0.056 ms
(7 rows)

.4. 以下查询,命中了2条记录,并且走的是index only scan。
读了22个INDEX PAGE,
1 meta page + 7 * (1 root + 1 branch + 1 leaf) = 22
也就是说,每个value都扫了root,branch,leaf。

postgres=# explain (analyze,verbose,timing,costs,buffers) select id from tbl1 where id in (1,2,3,4,5,6,7);
                                                         QUERY PLAN                                                          
-----------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using tbl1_pkey on public.tbl1  (cost=0.43..10.13 rows=7 width=4) (actual time=0.039..0.046 rows=2 loops=1)
   Output: id
   Index Cond: (tbl1.id = ANY ('{1,2,3,4,5,6,7}'::integer[]))
   Heap Fetches: 0
   Buffers: shared hit=22
 Planning time: 0.232 ms
 Execution time: 0.086 ms
(7 rows)

.5. 以下查询结果和以上查询一样,也命中了3条记录,并且走的是index only scan。
但是只读了4个INDEX PAGE,
1 meta page + 1 root + 1 branch + 1 leaf

postgres=# explain (analyze,verbose,timing,costs,buffers) select id from tbl1 where id>0 and id <=7;
                                                         QUERY PLAN                                                         
----------------------------------------------------------------------------------------------------------------------------
 Index Only Scan using tbl1_pkey on public.tbl1  (cost=0.43..1.49 rows=3 width=4) (actual time=0.008..0.009 rows=2 loops=1)
   Output: id
   Index Cond: ((tbl1.id > 0) AND (tbl1.id <= 7))
   Heap Fetches: 0
   Buffers: shared hit=4
 Planning time: 0.127 ms
 Execution time: 0.028 ms
(7 rows)

对于第四个查询,扫描了22个块,这个查询,优化器有优化的空间,比如找到1和7作为边界值,在查询到第一个值时,就可以取到leaf page的下一个page的最小值,从而得到1,2,3,4,5,6,7的值在当前page就可以完全取到,不需要去重复扫描。

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