PostgreSQL大表count方法总结及优化探讨

PostgreSQL的MVCC是直接在原表通过增加新tuple来实现的，决定了它在大结果集count的时候性能不会太理想，因为需要对大结果集做可见性判断将会是一项繁重的工作，比如下面这种SQL：

select count(*) from big_tab;

单纯依靠DB进行优化，确实不是一件容易的事情。本文整理了count(*）的几种方式，并就提升count性能做初步探讨，效果不太理想，如果后续有朋友找到更好的方法，期待分享。

测试表及数据准备

创建一个测试表，并导入5kw的数据量：

akendb=# create table aken01(id int,name text,info text,primary key(id));
CREATE TABLE
akendb=# insert into aken01 select id,'aken_'||id,md5(random()::text) from generate_series(1,50000000) as id;
INSERT 0 50000000
akendb=# 
akendb=# \d+ aken01;
Table "public.aken01"
Column | Type | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+----------+--------------+-------------
id | integer | | not null | | plain | |
name | text | | | | extended | |
info | text | | | | extended | |
Indexes:
    "aken01_pkey" PRIMARY KEY, btree (id)
akendb=#

PostgreSQL大表count的方式

在执行count之前先analyze一下：

akendb=# analyze aken01;ANALYZETime: 536.959 ms

1.直接count原表统计。这是方式得到的结果是最真实的，但耗时较长：

akendb=# select count(*) from aken01;
  count   
---------- 
50000000
(1 row)
Time: 22097.741 ms (00:22.098)

2.通过统计信息统计

这种方式因为可以直接从系统表里面拿到数据，结果较快，但只是一个估计值，该方式可以有下面几种方法：

1）方法一：

akendb=# select n_live_tup as estimate_rows from pg_stat_all_tables where relname = 'aken01'; 
estimate_rows 
---------------
      50002378
(1 row)
Time: 13.768 ms
akendb=#

2）方法二：

akendb=#  select reltuples::bigint as estimate_rows from pg_class where relname = 'aken01'; 
estimate_rows 
---------------
      50002376
(1 row)
Time: 0.682 ms
akendb=#

3）方法三：

akendb=# select (reltuples/relpages) * (pg_relation_size('aken01')/(current_setting('block_size')::integer)) as rows from pg_class where relname = 'aken01';
rows
----------
50002376
(1 row)
Time: 2.457 ms
akendb=#

4）方法四：通过函数或执行计划信息统计

这里参考Michael Fuhr提供的方法，创建一个function从count语句的执行计划统计：

akendb=# CREATE FUNCTION count_estimate(query text) RETURNS integer AS
akendb-# $func$
akendb$# DECLARE
akendb$# rec record;
akendb$# rows integer;
akendb$# BEGIN
akendb$# FOR rec IN EXECUTE 'EXPLAIN ' || query LOOP
akendb$# rows := substring(rec."QUERY PLAN" FROM ' rows=([[:digit:]]+)');
akendb$# EXIT WHEN rows IS NOT NULL;
akendb$#     END LOOP;
akendb$# RETURN rows;
akendb$# END
akendb$# $func$ LANGUAGE plpgsql;
CREATE FUNCTION
Time: 1.023 ms
akendb=#

执行统计：

akendb=#SELECT count_estimate('select 1 FROM aken01');
count_estimate
----------------
50002376
(1 row)
Time: 3.226 ms
akendb=#

4.触发器统计

这种方式对表的dml事件创建一个触发器，然后通过计数表进行rows统计，这样能得到一个真实的统计值，但会拖慢dml的性能。

具体可参考A. Elein Mustain的文章：

http://www.varlena.com/GeneralBits/120.php

5.通过扩展插件统计

我们还可以通过pgstattuple这个扩展实现，这种能得到真实的结果，但效率比较慢：

1）创建扩展：

akendb=# create extension pgstattuple;
CREATE EXTENSION
akendb=# \dx
List of installed extensions
Name | Version | Schema | Description
--------------------+---------+------------+-------------------------------------------------------------------
pg_stat_error | 1.0 | public | track error code of all processes
pg_stat_log | 1.0 | public | track runtime execution statistics of all SQL statements executed
pg_stat_statements | 1.5 | public | track execution statistics of all SQL statements executed
pglogical | 2.2.1 | pglogical | PostgreSQL Logical Replication
pgstattuple | 1.5 | public | show tuple-level statistics
plpgsql | 1.0 | pg_catalog | PL/pgSQL procedural language
(6 rows)
akendb=#

2）执行统计

akendb=# select relname,(pgstattuple(relname)).tuple_count as rows from pg_class where relname='aken01';
relname | rows
---------+----------
aken01 | 50000000
(1 row)
Time: 10082.679 ms (00:10.083)
akendb=#
akendb=# insert into aken01(id,name,info) values(50000001,'aken01','aken');
INSERT 0 1
Time: 2.459 ms
akendb=# select relname,(pgstattuple(relname)).tuple_count as rows from pg_class where relname='aken01';
relname | rows
---------+----------
aken01 | 50000001
(1 row)
Time: 10327.826 ms (00:10.328)
akendb=#

优化尝试

对于数据库而言，SQL优化策略是尽量减少CPU的运算以及page的扫描数量。我们可以猜想，在没有任何谓词的count(*)查询，如果能有一个比原表小得多的索引覆盖到我们需要的数据，那么使用index-only scans的访问路径来替代seq scan，理论上应该是可以帮助我们实现优化的。

先来看最初的执行计划：

akendb=# explain (analyze,buffers,verbose) select count(*) from aken01;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=1140368.00..1140368.01 rows=1 width=8) (actual time=15423.918..15423.919 rows=1 loops=1)
Output: count(*)
Buffers: shared hit=156264 read=359104
I/O Timings: read=717.207
-> Seq Scan on public.aken01 (cost=0.00..1015368.00 rows=50000000 width=0) (actual time=0.034..7601.345 rows=50000001 loops=1)
Output: id, name, info
Buffers: shared hit=156264 read=359104
I/O Timings: read=717.207
Planning time: 0.074 ms
Execution time: 15423.970 ms
(10 rows)
akendb=#

上面这里走的全表扫描Seq Scan on table，主键索引约为表的1/4：

akendb=# SELECT pg_size_pretty(pg_relation_size('aken01'));
pg_size_pretty
----------------
4026 MB
(1 row)
akendb=# SELECT pg_size_pretty(pg_relation_size('aken01_pkey'));
pg_size_pretty
----------------
1071 MB
(1 row)
akendb=#

我们尝试一下index only scans的效果：

akendb=# set enable_seqscan=off;set enable_bitmapscan = off;set enable_tidscan = off;explain (analyze,buffers,verbose) select count(*) from aken01;
SET
SET
SET
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=1938751.56..1938751.57 rows=1 width=8) (actual time=27290.551..27290.551 rows=1 loops=1)
Output: count(*)
Buffers: shared read=651983
I/O Timings: read=1998.864
-> Index Only Scan using aken01_pkey on public.aken01 (cost=0.56..1813751.56 rows=50000000 width=0) (actual time=0.051..18871.965 rows=50000001 loops=1)
Output: id
Heap Fetches: 50000001
Buffers: shared read=651983
I/O Timings: read=1998.864
Planning time: 0.204 ms
Execution time: 27290.601 ms
(11 rows)

IO基本在shared buffer，但index only scan比Seq Scan效率更差了，因该idx无vm文件，依旧要做可见性判断。如果是Oracle的同学，通常针对大表的优化，在资源充足的情况，使用分区或并行技术通常会是一个比较有效的方法，这也是AP系统主要的优化手段，这里我们在PostgreSQL开启32个并行：

akendb=# set parallel_tuple_cost =0;
SET
akendb=# set parallel_tuple_cost =0;
SET
akendb=# set max_parallel_workers_per_gather =32;
SET
akendb=# alter table aken01 set (parallel_workers =32);
ALTER TABLE
akendb=# explain (analyze,buffers,verbose) select count(*) from aken01;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------
Finalize Aggregate (cost=535899.34..535899.35 rows=1 width=8) (actual time=6150.317..6150.318 rows=1 loops=1)
Output: count(*)
Buffers: shared hit=155144 read=360224
I/O Timings: read=1641.442
-> Gather (cost=535899.25..535899.26 rows=32 width=8) (actual time=6150.267..6160.640 rows=9 loops=1)
Output: (PARTIAL count(*))
Workers Planned: 32
Workers Launched: 8
Buffers: shared hit=155144 read=360224
I/O Timings: read=1641.442
-> Partial Aggregate (cost=534899.25..534899.26 rows=1 width=8) (actual time=6123.835..6123.835 rows=1 loops=9)
Output: PARTIAL count(*)
Buffers: shared hit=155144 read=360224
I/O Timings: read=1641.442
Worker 0: actual time=6078.874..6078.874 rows=1 loops=1
Buffers: shared hit=17161 read=40125
I/O Timings: read=158.571
Worker 1: actual time=6145.014..6145.014 rows=1 loops=1
Buffers: shared hit=16900 read=39533
I/O Timings: read=177.511
Worker 2: actual time=6145.094..6145.094 rows=1 loops=1
Buffers: shared hit=17266 read=40933
I/O Timings: read=93.361
Worker 3: actual time=6145.392..6145.392 rows=1 loops=1
Buffers: shared hit=17111 read=39107
I/O Timings: read=247.815
Worker 4: actual time=6079.581..6079.582 rows=1 loops=1
Buffers: shared hit=17405 read=39287
I/O Timings: read=90.894
Worker 5: actual time=6145.646..6145.646 rows=1 loops=1
Buffers: shared hit=17199 read=39618
I/O Timings: read=245.592
Worker 6: actual time=6145.679..6145.679 rows=1 loops=1
Buffers: shared hit=17273 read=41417
I/O Timings: read=219.668
Worker 7: actual time=6079.707..6079.707 rows=1 loops=1
Buffers: shared hit=16844 read=40581
I/O Timings: read=159.558
-> Parallel Seq Scan on public.aken01 (cost=0.00..530993.00 rows=1562500 width=0) (actual time=0.032..3845.940 rows=5555556 loops=9)
Buffers: shared hit=155144 read=360224
I/O Timings: read=1641.442
Worker 0: actual time=0.032..3666.180 rows=5557782 loops=1
Buffers: shared hit=17161 read=40125
I/O Timings: read=158.571
Worker 1: actual time=0.032..3601.889 rows=5475027 loops=1
Buffers: shared hit=16900 read=39533
I/O Timings: read=177.511
Worker 2: actual time=0.031..4047.629 rows=5646302 loops=1
Buffers: shared hit=17266 read=40933
I/O Timings: read=93.361
Worker 3: actual time=0.041..3998.200 rows=5454196 loops=1
Buffers: shared hit=17111 read=39107
I/O Timings: read=247.815
Worker 4: actual time=0.040..3864.664 rows=5500164 loops=1
Buffers: shared hit=17405 read=39287
I/O Timings: read=90.894
Worker 5: actual time=0.034..3586.671 rows=5512289 loops=1
Buffers: shared hit=17199 read=39618
I/O Timings: read=245.592
Worker 6: actual time=0.027..3921.528 rows=5693970 loops=1
Buffers: shared hit=17273 read=41417
I/O Timings: read=219.668
Worker 7: actual time=0.026..4269.655 rows=5571265 loops=1
Buffers: shared hit=16844 read=40581
I/O Timings: read=159.558
Planning time: 0.191 ms
Execution time: 6160.723 ms
(67 rows)
akendb=#
akendb=#select count(*) from aken01;
count
----------
50000001
(1 row)
Time: 4447.033 ms (00:04.447)
akendb=#

使用并行本质上是通过资源消耗替代时间消耗，在资源充足的情况下对SQL性能能起到提升作用，但资源不充足情况下，过大的并行度会加剧CPU的消耗，SQL效率则有可能变得更差。

附加：

查询不走索引的情况：
1、条件字段选择性弱，查出的结果集较大，不走索引；
查询结果集占数据行比例对执行计划的影响没有明确的数据界限，可能的比例界限在2%-3%之间
2、where条件等号两边字段类型不同，不走索引；（pg不受影响）
3、索引字段 is null 不走索引；
4、对于count(*)当索引字段有not null约束时走索引，否则不走索引；
5、like 后面的字符当首位为通配符时不走索引；
6、使用不等于操作符如：<>、!= 等不走索引；
7、索引字段前加了函数或参加了运算不走索引；
8，部分索引但查询条件包括不属于部分索引的数据
9、where条件中有or,且or条件两边字段不是都有索引
如果where条件都没有以上所述，那么考虑优化器分析的统计信息陈旧，需要更新这个表的统计信息或者重建索引
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版权声明：本文为CSDN博主「大妮哟」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/weixin_44847119/article/details/120187409