本节简单介绍了PostgreSQL 12 B-tree的改进：索引出现很多重复值时提升性能，减少空间占用。
PG 11
创建数据表，创建索引
[local]:5110 xdb@testdb=# drop table rel;
DROP TABLE
Time: 130.868 ms
[local]:5110 xdb@testdb=# CREATE TABLE rel (
xdb@testdb(# aid bigint NOT NULL,
xdb@testdb(# bid bigint NOT NULL
xdb@testdb(# );
CREATE TABLE
Time: 16.041 ms
[local]:5110 xdb@testdb=#
[local]:5110 xdb@testdb=# ALTER TABLE rel
xdb@testdb-# ADD CONSTRAINT rel_pkey PRIMARY KEY (aid, bid);
ALTER TABLE
Time: 5.236 ms
[local]:5110 xdb@testdb=#
[local]:5110 xdb@testdb=# CREATE INDEX rel_bid_idx ON rel (bid);
CREATE INDEX
Time: 1.838 ms
[local]:5110 xdb@testdb=#
[local]:5110 xdb@testdb=# INSERT INTO rel (aid, bid)
xdb@testdb-# SELECT i, i / 10000
xdb@testdb-# FROM generate_series(1, 20000000) AS i;
INSERT 0 20000000
Time: 152699.275 ms (02:32.699)
[local]:5110 xdb@testdb=#
[local]:5110 xdb@testdb=#
查看索引信息
[local]:5110 xdb@testdb=#
[local]:5110 xdb@testdb=# \d rel
Table "public.rel"
Column | Type | Collation | Nullable | Default
--------+--------+-----------+----------+---------
aid | bigint | | not null |
bid | bigint | | not null |
Indexes:
"rel_pkey" PRIMARY KEY, btree (aid, bid)
"rel_bid_idx" btree (bid)
[local]:5110 xdb@testdb=# \di+ rel_pkey
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+----------+-------+-------+-------+--------+-------------
public | rel_pkey | index | xdb | rel | 602 MB |
(1 row)
[local]:5110 xdb@testdb=# \di+ rel_bid_idx
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+-------------+-------+-------+-------+--------+-------------
public | rel_bid_idx | index | xdb | rel | 545 MB |
(1 row)
PG 12
创建数据表，创建索引
[local:/run/pg12]:5120 pg12@testdb=# \timing on
Timing is on.
[local:/run/pg12]:5120 pg12@testdb=# drop table rel;
DROP TABLE
Time: 279.144 ms
[local:/run/pg12]:5120 pg12@testdb=# CREATE TABLE rel (
pg12@testdb(# aid bigint NOT NULL,
pg12@testdb(# bid bigint NOT NULL
pg12@testdb(# );
CREATE TABLE
Time: 1.579 ms
[local:/run/pg12]:5120 pg12@testdb=#
[local:/run/pg12]:5120 pg12@testdb=# ALTER TABLE rel
pg12@testdb-# ADD CONSTRAINT rel_pkey PRIMARY KEY (aid, bid);
ALTER TABLE
Time: 3.450 ms
[local:/run/pg12]:5120 pg12@testdb=#
[local:/run/pg12]:5120 pg12@testdb=# CREATE INDEX rel_bid_idx ON rel (bid);
CREATE INDEX
Time: 1.201 ms
[local:/run/pg12]:5120 pg12@testdb=#
[local:/run/pg12]:5120 pg12@testdb=# INSERT INTO rel (aid, bid)
pg12@testdb-# SELECT i, i / 10000
pg12@testdb-# FROM generate_series(1, 20000000) AS i;
INSERT 0 20000000
Time: 124503.212 ms (02:04.503)
[local:/run/pg12]:5120 pg12@testdb=#
查看索引信息
[local:/run/pg12]:5120 pg12@testdb=# \di+ rel_pkey
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+----------+-------+-------+-------+--------+-------------
public | rel_pkey | index | pg12 | rel | 601 MB |
(1 row)
[local:/run/pg12]:5120 pg12@testdb=# \di+ rel_bid_idx
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+-------------+-------+-------+-------+--------+-------------
public | rel_bid_idx | index | pg12 | rel | 408 MB |
(1 row)
[local:/run/pg12]:5120 pg12@testdb=#
可以看到PK没有太大的变化，但有很多重复值的bid列索引则有明显的变化，比PG 11少了25%的空间。
原理
PG 11 vs PG 12
从上面两个图可以看出，PG 11的索引leaf page发生在middle，而PG 12发生在rightmost，middle叶子节点的page稠密度明显要比PG 11好很多。