在 PostgreSQL 中按字母顺序对字符串中的字母进行排序

Question

我目前正在使用这种方法在 PostgreSQL 中按字母顺序对字符串中的字母进行排序。还有其他有效的方法吗？

select string_agg(c, '') as s
from   (select unnest(regexp_split_to_array('ijsAafhareDbv', '')) as c 
        order  by c) as t; 

       s   
 --------------
 ADaabefhijrsv

Answer 1

如果你想要一个没有正则表达式的解决方案，你可以使用这个：

WITH t(s) AS (VALUES ('amfjwzeils'))
SELECT string_agg(substr(t.s, g.g, 1), ''
                  ORDER BY substr(t.s, g.g, 1)
                 )
FROM t
   CROSS JOIN LATERAL generate_series(1, length(t.s)) g;

 string_agg 
------------
 aefijlmswz
(1 row)

我将对哪种解决方案进行基准测试。

Answer 2

我创建了 3 个函数，一个使用我的查询，另一个使用 Laurenz 的查询，还有一个：我创建了一个 Python(plpythonu) 函数用于排序。最后，我创建了一个包含 100000 行的 table（我现在是在我的 Mac 笔记本电脑上完成的）每个包含一个随机的 15 个字符的字符串，在此 Link

中使用 random_string 函数生成

create table t as select random_string(15) as s FROM generate_series(1,100000);

这是 3 个函数。

CREATE or REPLACE FUNCTION sort1(x TEXT) RETURNS TEXT AS $$
select string_agg(c, '') as s
from   (select unnest(regexp_split_to_array(, '')) as c 
        order  by c) as t;
$$ LANGUAGE SQL IMMUTABLE;


CREATE or REPLACE FUNCTION sort2(x TEXT) RETURNS TEXT AS $$
WITH t(s) AS (VALUES ())
SELECT string_agg(substr(t.s, g.g, 1), ''
                  ORDER BY substr(t.s, g.g, 1)
                 )
FROM t
   CROSS JOIN LATERAL generate_series(1, length(t.s)) g;

$$ LANGUAGE SQL IMMUTABLE;


create language plpythonu;
CREATE or REPLACE FUNCTION pysort(x text)
  RETURNS text
AS $$
  return ''.join(sorted(x))
$$ LANGUAGE plpythonu IMMUTABLE;

这些是所有三个 EXPLAIN ANALYSE 的结果。

knayak=# EXPLAIN ANALYSE select sort1(s)  FROM t;
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Seq Scan on t  (cost=0.00..26541.00 rows=100000 width=32) (actual time=0.266..7097.740 rows=100000 loops=1)
 Planning time: 0.119 ms
 Execution time: 7106.871 ms
(3 rows)

knayak=# EXPLAIN ANALYSE select sort2(s)  FROM t;
                                                 QUERY PLAN                                                  
-------------------------------------------------------------------------------------------------------------
 Seq Scan on t  (cost=0.00..26541.00 rows=100000 width=32) (actual time=0.418..7012.935 rows=100000 loops=1)
 Planning time: 0.270 ms
 Execution time: 7021.587 ms
(3 rows)

knayak=# EXPLAIN ANALYSE select pysort(s) FROM t;
                                                 QUERY PLAN                                                 
------------------------------------------------------------------------------------------------------------
 Seq Scan on t  (cost=0.00..26541.00 rows=100000 width=32) (actual time=0.060..389.729 rows=100000 loops=1)
 Planning time: 0.048 ms
 Execution time: 395.760 ms
(3 rows)

根据这个分析，事实证明 - Python sort 是最快的，前两个之间没有显着差异。需要实时检查性能尽管在我们的系统中有巨大的 tables。

Answer 3

在 C 中实现的功能 比我们用 LANGUAGE sql 或 plpgsql 实现的任何功能快得多 。所以以微弱优势赢得表演比赛。

但是 plpythonu 是一种 不受信任的 过程语言。它不是默认安装的，只有超级用户才能使用不受信任的语言创建函数。您需要了解安全隐患。不受信任的语言在大多数云服务上根本不可用。
The current manual (quote from pg 10):

PL/Python is only available as an “untrusted” language, meaning it does not offer any way of restricting what users can do in it and is therefore named plpythonu. A trusted variant plpython might become available in the future if a secure execution mechanism is developed in Python. The writer of a function in untrusted PL/Python must take care that the function cannot be used to do anything unwanted, since it will be able to do anything that could be done by a user logged in as the database administrator. Only superusers can create functions in untrusted languages such as plpythonu.

您测试的 SQL 功能没有得到很好的优化。有一千零一种 提高性能的方法，但是：

演示

-- func to create random strings
CREATE OR REPLACE FUNCTION f_random_string(int)
  RETURNS text AS
$func$
SELECT array_to_string(ARRAY(
   SELECT substr('0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz', (ceil(random()*62))::int, 1)
   FROM generate_series(1, )
   ), '')
$func$  LANGUAGE sql VOLATILE;

-- test tbl with 100K rows
CREATE TABLE tbl(str text);
INSERT INTO tbl
SELECT f_random_string(15)
FROM   generate_series(1, 100000) g;

VACUUM ANALYZE tbl;

-- 1: your test function 1 (inefficient)
CREATE OR REPLACE FUNCTION sort1(text)  RETURNS text AS
$func$  -- your test function 1 (very inefficient)
SELECT string_agg(c, '')
FROM  (SELECT unnest(regexp_split_to_array(, '')) AS c ORDER  BY c) t;
$func$ LANGUAGE sql IMMUTABLE;

-- 2: your test function 2 ( inefficient)
CREATE OR REPLACE FUNCTION sort2(text)  RETURNS text AS
$func$
WITH t(s) AS (VALUES ())
SELECT string_agg(substr(t.s, g.g, 1), '' ORDER BY substr(t.s, g.g, 1))
FROM   t
CROSS  JOIN LATERAL generate_series(1, length(t.s)) g;
$func$  LANGUAGE sql IMMUTABLE;

-- 3: remove pointless CTE from sort2
CREATE OR REPLACE FUNCTION sort3(text)  RETURNS text AS
$func$
SELECT string_agg(substr(, g, 1), '' ORDER BY substr(, g, 1))
FROM   generate_series(1, length()) g;
$func$  LANGUAGE sql IMMUTABLE;

-- 4: use unnest instead of calling substr N times
CREATE OR REPLACE FUNCTION sort4(text)  RETURNS text AS
$func$
SELECT string_agg(c, '' ORDER BY c)
FROM   unnest(string_to_array(, NULL)) c
$func$  LANGUAGE sql IMMUTABLE;

-- 5: ORDER BY in subquery
CREATE OR REPLACE FUNCTION sort5(text)  RETURNS text AS
$func$
SELECT string_agg(c, '')
FROM  (
   SELECT c
   FROM   unnest(string_to_array(, NULL)) c
   ORDER  BY c
   ) sub
$func$  LANGUAGE sql IMMUTABLE;

-- 6: SRF in SELECT list
CREATE OR REPLACE FUNCTION sort6(text)  RETURNS text AS
$func$
SELECT string_agg(c, '')
FROM  (SELECT unnest(string_to_array(, NULL)) c ORDER BY 1) sub
$func$  LANGUAGE sql IMMUTABLE;

-- 7: ARRAY constructor instead of aggregate func
CREATE OR REPLACE FUNCTION sort7(text)  RETURNS text AS
$func$
SELECT array_to_string(ARRAY(SELECT unnest(string_to_array(, NULL)) c ORDER BY c), '')
$func$  LANGUAGE sql IMMUTABLE;

-- 8: The same with COLLATE "C"
CREATE OR REPLACE FUNCTION sort8(text)  RETURNS text AS
$func$
SELECT array_to_string(ARRAY(SELECT unnest(string_to_array( COLLATE "C", NULL)) c ORDER BY c), '')
$func$  LANGUAGE sql IMMUTABLE;

SELECT str, sort1(str), sort2(str), sort3(str), sort4(str), sort5(str), sort6(str), sort7(str), sort8(str) FROM tbl LIMIT 1;  -- result sample

str             | sort1           | sort2           | sort3           | sort4           | sort5           | sort6           | sort7           | sort8          
:-------------- | :-------------- | :-------------- | :-------------- | :-------------- | :-------------- | :-------------- | :-------------- | :--------------
tUkmori4D1rHhI1 | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DhHiIkmorrtU | 114DHIUhikmorrt

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort1(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.053 ms                                                                  |
| Execution time: 2742.904 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort2(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.105 ms                                                                  |
| Execution time: 2579.397 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort3(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.079 ms                                                                  |
| Execution time: 2191.228 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort4(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.075 ms                                                                  |
| Execution time: 2194.780 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort5(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.083 ms                                                                  |
| Execution time: 1902.829 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort6(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.075 ms                                                                  |
| Execution time: 1866.407 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort7(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.067 ms                                                                  |
| Execution time: 1863.713 ms                                                              |

EXPLAIN (ANALYZE, TIMING OFF) SELECT sort8(str) FROM tbl;

| QUERY PLAN                                                                               |
| :--------------------------------------------------------------------------------------- |
| Seq Scan on tbl  (cost=0.00..26541.00 rows=100000 width=32) (actual rows=100000 loops=1) |
| Planning time: 0.074 ms                                                                  |
| Execution time: 1569.376 ms                                                              |

db<>fiddle here

最后一种没有COLLATION规则排序，严格按照字符的字节值排序，这样便宜很多。但是您可能需要不同语言环境的排序顺序，也可能不需要。

The manual about COLLATION expressions.

在 PostgreSQL 中按字母顺序对字符串中的字母进行排序

To sort the letters in a string alphabetically in PostgreSQL

sql

postgresql

plpgsql

postgresql-performance

演示