postgresql关于like%xxx%的优化操作

(编辑：jimmy 日期: 2024/11/10 浏览：2)

任何一个关系型数据库关于模糊匹配（like)的优化都是一件痛苦的事，相对而言，诸如like 'abc%'之类的还好一点，可以通过创建索引来优化，但对于like 'c%'之类的，真的就没有办法了。

这里介绍一种postgresql关于like 'c%'的优化方法，是基于全文检索的特性来实现的。

测试数据准备（环境centos6.5 + postgresql 9.6.1）。

postgres=# create table ts(id int,name text);
CREATE TABLE
postgres=# \d ts
Table "public.ts"
Column | Type  | Modifiers
--------+---------+-----------
id   | integer |
name  | text  |
postgres=# insert into ts select n,n||'_pjy' from generate_series(1,2000) n;
INSERT 0 2000
postgres=# insert into ts select n,n||'_mdh' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_lmm' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_syf' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_wbd' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_hhh' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_sjw' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_jjs' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_ymd' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_biu' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# insert into ts select n,n||'_dfl' from generate_series(1,2000000) n;
INSERT 0 2000000
postgres=# select count(*) from ts;
 count 
----------
 20002000
(1 row)

开始测试：

postgres=# explain analyze select * from ts where name like '%pjy%';
                        QUERY PLAN                        
-----------------------------------------------------------------------------------------------------------
 Seq Scan on ts (cost=0.00..358144.05 rows=2000 width=15) (actual time=0.006..1877.087 rows=2000 loops=1)
  Filter: (name ~~ '%pjy%'::text)
  Rows Removed by Filter: 20000000
 Planning time: 0.031 ms
 Execution time: 1877.178 ms
(5 rows)

关键一步：

postgres=# create index idx_name on ts using gin (to_tsvector('english',name));
CREATE INDEX
postgres=# vacuum analyze ts;
VACUUM
postgres=# \d ts
   Table "public.ts"
 Column | Type  | Modifiers
--------+---------+-----------
 id   | integer |
 name  | text  |
Indexes:
  "idx_name" gin (to_tsvector('english'::regconfig, name))
postgres=# explain analyze select * from ts where to_tsvector('english',name) @@ to_tsquery('pjy');
                           QUERY PLAN                           
---------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on ts (cost=39.75..8187.70 rows=2000 width=15) (actual time=0.016..0.016 rows=0 loops=1)
  Recheck Cond: (to_tsvector('english'::regconfig, name) @@ to_tsquery('pjy'::text))
  -> Bitmap Index Scan on idx_name (cost=0.00..39.25 rows=2000 width=0) (actual time=0.016..0.016 rows=0 loops=1)
     Index Cond: (to_tsvector('english'::regconfig, name) @@ to_tsquery('pjy'::text))
 Planning time: 0.094 ms
 Execution time: 0.036 ms
(6 rows)

大家可以看到，执行时间从2秒下降到了0.04毫秒！！！

关于pg的全文检索，tsvector和tsquery，这里就不详细介绍了，大家可以自己查阅手册。

补充：postgresql子查询优化(提升子查询)

问题背景

在开发项目过程中，客户要求使用gbase8s数据库(基于informix)，简单的分页页面响应很慢。排查发现分页sql是先查询出数据在外面套一层后再取多少条，如果去掉嵌套的一层，直接获取则很快。日常使用中postgresql并没有这样的操作也很快，这是为什么呢？

说明

在数据库实现早期，查询优化器对子查询一般采用嵌套执行的方式，即父查询中的每一行，都要执行一次子查询，这样子查询会执行很多次，效率非常低。

本篇主要讲postgresql针对子查询的优化。

项目中使用子查询的地方非常多，如何写出高效的sql，掌握子查询的优化是非常有必要的。

执行计划对比（gbase8s vs postgresql）:

gbase8s慢sql执行计划：

--gbase8s执行计划

SET EXPLAIN ON ; 
SET EXPLAIN FILE TO '/home/gbasedbt/sqexplain.out' ;
select skip 0 first 15 * from ( select * from T_SZGL_JDRY order by T_SZGL_JDRY.updatetime desc ) Estimated Cost: 3207 Estimated # of Rows Returned: 6172 "htmlcode">

select skip 0 first 15 * from T_SZGL_JDRY order by T_SZGL_JDRY.updatetime desc "htmlcode">

db_jcxxzypt=# explain select * from db_jcxx.t_jcxxzy_tjaj order by d_slrq limit 15 offset 0;                        QUERY PLAN                         ------------------------------------------------------------------------- Limit (cost=0.44..28.17 rows=15 width=879)  -> Index Scan using idx_ttjaj_dslrq on t_jcxxzy_tjaj (cost=0.44..32374439.85 rows=17507700 width=879) (2 rows) --子查询执行计划-嵌套一层 db_jcxxzypt=# explain db_jcxxzypt-# select * from ( db_jcxxzypt(# select * from db_jcxx.t_jcxxzy_tjaj order by d_slrq db_jcxxzypt(# )tab1 limit 15 offset 0;                        QUERY PLAN                         ------------------------------------------------------------------------- Limit (cost=0.44..28.32 rows=15 width=879)  -> Index Scan using idx_ttjaj_dslrq on t_jcxxzy_tjaj (cost=0.44..32374439.85 rows=17507700 width=879) (2 rows) "htmlcode">

db_jcxxzypt=# explain select * from t_jcxxzy_tjaj aj ,(select * from t_jcxxzy_ajdsr) dsr where dsr.c_ajbm = '1301020400000120090101';                   QUERY PLAN                    ------------------------------------------------------------------------- Nested Loop (cost=0.56..1252119.58 rows=17507700 width=1098)  -> Index Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr (cost=0.56..8.57 rows=1 width=219)     Index Cond: (c_ajbm = '1301020400000120090101'::bpchar)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1077034.00 rows=17507700 width=879) (4 rows) "htmlcode">

db_jcxxzypt=# explain select * from t_jcxxzy_tjaj aj where aj.c_ajbm in (select dsr.c_ajbm from t_jcxxzy_ajdsr dsr); 转化为： select * from t_jcxxzy_tjaj aj join t_jcxxzy_ajdsr dsr aj.c_ajbm = dsr.c_ajbm;                           QUERY PLAN                       ------------------------------------------------------------------------- Hash Semi Join (cost=362618.61..5537768.07 rows=7957409 width=879)  Hash Cond: (t_jcxxzy_tjaj.c_ajbm = t_jcxxzy_ajdsr.c_ajbm)  -> Seq Scan on t_jcxxzy_tjaj (cost=0.00..1077034.00 rows=17507700 width=879)  -> Hash (cost=237458.59..237458.59 rows=6817202 width=23)     -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr (cost=0.56..237458.59 rows=6817202 wi dth=23) (5 rows) --in等价于=any hash semi join表示执行的是两张表的hash半连接， 原始sql中没有(t_jcxxzy_tjaj.c_ajbm = t_jcxxzy_ajdsr.c_ajbm),表明此in子查询被优化，优化后采用hash semi join算法。 (2).相关子查询 --当加入条件where aj.d_slrq='2001-06-14'后不能提升子链接，如果把where aj.d_slrq ='2001-06-14'放到父查询 是支持子链接优化的 db_jcxxzypt=# explain db_jcxxzypt-# select * from t_jcxxzy_tjaj aj where c_ajbm in (select c_ajbm from t_jcxxzy_ajdsr dsr where aj.d_slrq='2001-06-14') ;                            QUERY PLAN                           ------------------------------------------------------------------------- Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..2227874766580.75 rows=8753850 width=879)  Filter: (SubPlan 1)  SubPlan 1   -> Result (cost=0.56..237458.59 rows=6817202 width=23)      One-Time Filter: (aj.d_slrq = '2001-06-14'::date)      -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr dsr (cost=0.56..237458.59 rows=6817 202 width=23) (6 rows (3). -- not in不能提升子链接 db_jcxxzypt=# explain select * from db_jcxx.t_jcxxzy_tjaj where c_ajbm not in (select c_ajbm from db_jcxx.t_jcxxzy_ajdsr);                           QUERY PLAN                           ------------------------------------------------------------------------- Seq Scan on t_jcxxzy_tjaj (cost=0.56..2875921362927.06 rows=8753850 width=879)  Filter: (NOT (SubPlan 1))  SubPlan 1   -> Materialize (cost=0.56..311489.60 rows=6817202 width=23)      -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr (cost=0.56..237458.59 rows=6817202 width=23) (5 rows) --not in与<>all含义相同


in子句存在不被优化的可能、当in子句中包含了主查询的表字段，和主查询有相关性时不能提升子链接。
exists子链接
--exists子链接


db_jcxxzypt=# explain
db_jcxxzypt-# select * from t_jcxxzy_tjaj aj where exists (select c_ajbm from t_jcxxzy_ajdsr dsr where aj.c_ajbm = dsr.c_ajbm);                            QUERY PLAN                           ------------------------------------------------------------------------- Hash Semi Join (cost=362618.61..5537768.07 rows=7957409 width=879)  Hash Cond: (aj.c_ajbm = dsr.c_ajbm)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1077034.00 rows=17507700 width=879)  -> Hash (cost=237458.59..237458.59 rows=6817202 width=23)     -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr dsr (cost=0.56..237458.59 rows=681720 2 width=23) (5 rows) -- 当加入where aj.c_xzdm = '150622'条件在子链接时，仍然支持上拉 db_jcxxzypt=# explain db_jcxxzypt-# select * from t_jcxxzy_tjaj aj where exists (select c_ajbm from t_jcxxzy_ajdsr dsr where aj.c_xzdm = '150622');                          QUERY PLAN                          ------------------------------------------------------------------------- Nested Loop Semi Join (cost=0.56..1361779.20 rows=5436 width=879)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1120803.25 rows=5436 width=879)     Filter: ((c_xzdm)::text = '150622'::text)  -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr dsr (cost=0.56..237458.59 rows=6817202 widt h=0) (4 rows) --exists子链接 db_jcxxzypt=# explain db_jcxxzypt-# select * from t_jcxxzy_tjaj aj where exists (select c_ajbm from t_jcxxzy_ajdsr dsr where dsr.c_ajbm='1101120300000120030101') db_jcxxzypt-# ;                        QUERY PLAN                        ------------------------------------------------------------------------- Result (cost=4.58..1077038.57 rows=17507700 width=879)  One-Time Filter: $0  InitPlan 1 (returns $0)   -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr dsr (cost=0.56..4.58 rows=1 width=0)      Index Cond: (c_ajbm = '1101120300000120030101'::bpchar)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1077034.00 rows=17507700 width=879) (6 rows) 


子查询只执行了一次，作为aj表的参数。


--not exists子链接 db_jcxxzypt=# explain db_jcxxzypt-# select * from t_jcxxzy_tjaj aj where not exists (select c_ajbm from t_jcxxzy_ajdsr dsr);                   QUERY PLAN                   ------------------------------------------------------------------------- Result (cost=0.04..1077034.04 rows=17507700 width=879)  One-Time Filter: (NOT $0)  InitPlan 1 (returns $0)   -> Seq Scan on t_jcxxzy_ajdsr dsr (cost=0.00..281210.02 rows=6817202 width=0)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1077034.00 rows=17507700 width=879) (5 rows) 


从执行计划上看，not exists子查询并没有被消除，子查询只是执行了一次，将结果作为aj表的参数。
in和exists都存在不被优化的可能，对于in和exists的选择，当父查询结果集小于子查询结果集则选择exists，如果父查询结果集大于子查询结果集选择in。
所有的all子链接都不支持上拉


db_jcxxzypt=# explain select * from db_jcxx.t_jcxxzy_tjaj where c_ajbm >all(select c_ajbm from db_jcxx.t_jcxxzy_ajdsr);                           QUERY PLAN                      ------------------------------------------------------------------------- Seq Scan on t_jcxxzy_tjaj (cost=0.56..2875921362927.06 rows=8753850 width=879)  Filter: (SubPlan 1)  SubPlan 1   -> Materialize (cost=0.56..311489.60 rows=6817202 width=23)      -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr (cost=0.56..237458.59 rows=6817202 width=23) (5 rows) "htmlcode">

db_jcxxzypt=#explain select * from db_jcxx.t_jcxxzy_tjaj where c_ajbm >some(select c_ajbm from db_jcxx.t_jcxxzy_ajdsr);                         QUERY PLAN                         ------------------------------------------------------------------------- - Nested Loop Semi Join (cost=0.56..11316607.35 rows=5835900 width=879)  -> Seq Scan on t_jcxxzy_tjaj (cost=0.00..1077034.00 rows=17507700 width=879)  -> Index Only Scan using idx_tajdsr_cajbm on t_jcxxzy_ajdsr (cost=0.56..64266.97 rows=2272401 width=23)     Index Cond: (c_ajbm < t_jcxxzy_tjaj.c_ajbm) (4 rows) "htmlcode">

db_jcxxzypt=# explain select * from t_jcxxzy_tjaj aj ,(select * from t_jcxxzy_ajdsr limit 10) dsr where dsr.c_ajbm = '1301020400000120090101';                     QUERY PLAN                     ------------------------------------------------------------------------- Nested Loop (cost=0.00..1252111.54 rows=17507700 width=1098)  -> Subquery Scan on dsr (cost=0.00..0.54 rows=1 width=219)     Filter: (dsr.c_ajbm = '1301020400000120090101'::bpchar)     -> Limit (cost=0.00..0.41 rows=10 width=219)        -> Seq Scan on t_jcxxzy_ajdsr (cost=0.00..281210.02 rows=6817202 width=219)  -> Seq Scan on t_jcxxzy_tjaj aj (cost=0.00..1077034.00 rows=17507700 width=879) (6 rows) "htmlcode">

#from_collapse_limit = 8


当from列表的对象少于from_collapse_limit时，优化器可以将子查询提升到上层进行JOIN，从而可能选择到更优的执行计划。


#join_collapse_limit = 8        # 1 disables collapsing of explicit
                    # JOIN clauses 当使用显示的JOIN时（除了full join），例如a join b join c join d，优化器可以重排JOIN的顺序，以产生更多的PLAN选择更优的执行计划。 如果join_collapse_limit=1，则不重排，使用SQL写法提供的顺序。 "htmlcode">

val>all(select...) to val>max(select...)
val<all(select...) to val<min(select...)
val>any(select...) to val>min(select...) val<any(select...) to val<max(select...) val>=all(select...) to val>=max(select...) val<=all(select...) to val<=min(select...) val>=any(select...) to val>=min(select...) val<=any(select...) to val<=max(select...) 


通常，聚集函数min(),max()的执行效率要比any、all效率高
相关子查询和非相关子查询
相关子查询子查询的执行依赖于外层父查询的一些属性值。子查询因依赖于父查询的参数，当父查询的参数改变时，子查询需要根据新参数值重新执行（查询优化器对相关子查询进行优化有一定意义），如：


select * from t_jcxxzy_tjaj aj where c_ajbm in (select c_ajbm from t_jcxxzy_ajdsr dsr where dsr.c_ajbm = aj.c_ajbm)/* 子查询语句中存在父查询的列 */


非相关子查询子查询的执行，不依赖于外层父查询的任何属性值。这样子查询具有独立性，可独自求解，形成一个子查询计划先于外层的查询求解，如：


select * from t_jcxxzy_tjaj aj where c_ajbm in (select c_ajbm from t_jcxxzy_ajdsr dsr where dsr.c_xzdm = '150622')/* 子查询语句中不存在父查询的属性 */


结束语
1.postgresql子查询的优化思路，子查询不用执行多次
2.优化器可以根据统计信息来选择不同的连接方法和不同的连接顺序
3.子查询中的连接条件，过滤条件分别变成了父查询的连接条件、过滤条件、优化器可以对这些条件进行下推、提高执行效率
4.将子查询优化为表连接后，子查询只需要执行一次、而优化器可以根据统计信息来选择不同的连接方式和连接顺序、子查询的连接条件和过滤条件分别变成父查询的条件。
5.这些查询中all是完全不支持上拉子子链接的，in和exists存在不被优化的可能
6.not exists虽然没有被上拉，但是被优化为只执行一次，相对于not in稍好
7.可使用等价改写的方式优化
8.可根据配置文件，固化子查询，以及表的连接顺序
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。如有错误或未考虑完全的地方，望不吝赐教。

        
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