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。 **参考资料** [](https://www.cybertec-postgresql.com/en/b-tree-index-improvements-in-postgresql-v12/) [Make heap TID a tiebreaker nbtree index column.](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=dd299df8189bd00fbe54b72c64f43b6af2ffeccd)