背景
PolarDB 的云原生存算分离架构, 具备低廉的数据存储、高效扩展弹性、高速多机并行计算能力、高速数据搜索和处理; PolarDB与计算算法结合, 将实现双剑合璧, 推动业务数据的价值产出, 将数据变成生产力.
本文将介绍PolarDB 开源版通过pg_rational插件支持Stern-Brocot trees , 实现高效自定义顺序和调整顺序需求.
测试环境为macos+docker, polardb部署请参考如何用 PolarDB 证明巴菲特的投资理念 - 包括PolarDB简单部署。
pg_rational for PolarDB
pg_rational扩展使用 Stern-Brocot 树找到有效的中间点作为最低项的分数。它可以根据任何实际应用的需要继续在分数之间进行更深的拆分。实现高效自定义顺序和调整顺序需求.
pg_rational特性：
Stores fractions in exactly 64 bits (same size as float)
Written in C for high performance
Detects and halts arithmetic overflow for correctness
Uses native CPU instructions for fast overflow detection
Defers GCD calculation until requested or absolutely required
Supports btree and hash indices
Implements Stern-Brocot trees for finding intermediate points
Coercion from integer/bigint/tuple
Custom aggregate
安装pg_rational
git clone --depth 1 https://github.com/begriffs/pg_rational
cd pg_rational/
USE_PGXS=1 make
USE_PGXS=1 make install
export PGHOST=127.0.0.1
USE_PGXS=1 make installcheck
/home/postgres/tmp_basedir_polardb_pg_1100_bld/lib/pgxs/src/makefiles/../../src/test/regress/pg_regress --inputdir=./ --bindir='/home/postgres/tmp_basedir_polardb_pg_1100_bld/bin' --dbname=contrib_regression pg_rational_test
(using postmaster on 127.0.0.1, default port)
============== dropping database "contrib_regression" ==============
DROP DATABASE
============== creating database "contrib_regression" ==============
CREATE DATABASE
ALTER DATABASE
============== running regression test queries ==============
test pg_rational_test ... ok
==========================================================
All 1 tests passed.
POLARDB:
All 1 tests, 0 tests in ignore, 0 tests in polar ignore.
========================================================== 加载pg_rational插件
psql
psql (11.9)
Type "help" for help.
postgres=# create extension pg_rational;
CREATE EXTENSION 基本操作, pg_rational可以和浮点、整型相互转换.
-- fractions are precise
-- this would not work with a float type
select 1::rational / 3 * 3 = 1;
-- => t
-- provides the usual operations, e.g.
select '1/3'::rational + '2/7';
-- => 13/21
-- helper "ratt' type to coerce from tuples
select 1 + (i,i+1)::ratt from generate_series(1,5) as i;
-- => 3/2, 5/3, 7/4, 9/5, 11/6
-- simplify if desired
select rational_simplify('36/12');
-- => 3/1
-- convert float to rational
select 0.263157894737::float::rational;
-- => 5/19
-- convert rational to float
select '-1/2'::rational::float;
-- => -0.5 调整顺序测试, 不需要指定值, 只需要指定你要插入到哪两个rational value之间, pg_rational扩展使用 Stern-Brocot 树找到有效的中间点作为最低项的分数。从而实现了快速的顺序调整.
postgres=# create sequence todos_seq;
CREATE SEQUENCE
postgres=# create table todos (
prio rational unique
default nextval('todos_seq')::float8::rational,
what text not null
);
CREATE TABLE
postgres=# insert into todos (what) values
postgres-# ('install extension'),
postgres-# ('read about it'),
postgres-# ('try it'),
postgres-# ('profit?');
INSERT 0 4
postgres=#
-- put "try" between "install" and "read"
postgres=# select * from todos order by prio asc;
prio | what
------+-------------------
1/1 | install extension
2/1 | read about it
3/1 | try it
4/1 | profit?
(4 rows)
-- put "read" back between "install" and "try"
postgres=# update todos
postgres-# set prio = rational_intermediate(1,2) -- 1为install extension, 2为read about it. 根据Stern-Brocot 树找到1,2之间的3/2分数.
postgres-# where prio = 3;
UPDATE 1
postgres=# select * from todos order by prio asc;
prio | what
------+-------------------
1/1 | install extension
3/2 | try it
2/1 | read about it
4/1 | profit?
(4 rows)
postgres=# update todos
postgres-# set prio = rational_intermediate(1,'3/2') -- 1为install extension, 3/2为try it. 根据Stern-Brocot 树找到1,3/2之间的4/3分数.
postgres-# where prio = 2;
UPDATE 1
postgres=# select * from todos order by prio asc;
prio | what
------+-------------------
1/1 | install extension
4/3 | read about it
3/2 | try it
4/1 | profit?
(4 rows) 
