Sql 为什么PREPARE语句不尊重索引
帮助我理解为什么使用prepared语句计算的查询不尊重索引Sql 为什么PREPARE语句不尊重索引,sql,postgresql,query-performance,Sql,Postgresql,Query Performance,帮助我理解为什么使用prepared语句计算的查询不尊重索引 \d+ lerg; Table "public.lerg" Column | Type | Modifiers | Storage | Stats target | Descriptio
\d+ lerg;
Table "public.lerg"
Column | Type | Modifiers | Storage | Stats target | Description
------------+-----------------------------+---------------------------------------------------+----------+--------------+-------------
id | bigint | not null default nextval('lerg_id_seq'::regclass) | plain | |
lata | character varying | not null | extended | |
npa | character varying(3) | | extended | |
nxx | character varying(3) | | extended | |
block | character varying(1) | | extended | |
ocn | character varying | | extended | |
created_at | timestamp without time zone | not null default now() | plain | |
Indexes:
"lerg_pkey" PRIMARY KEY, btree (id)
"lerg_npa_nxx_block" UNIQUE, btree (npa, nxx, block)
LERG
表中的记录总数
select count(*) from lerg;
=> 199846
准备好的声明
prepare fetch_lata(char(3), char(3), char(1)) as select lata from lerg where npa=$1 and nxx=$2 and block=$3;
执行语句
explain analyze execute fetch_lata('365','406','A');
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Seq Scan on lerg (cost=0.00..5163.31 rows=1 width=6) (actual time=0.014..27.530 rows=1 loops=1)
Filter: (((npa)::bpchar = '365'::bpchar) AND ((nxx)::bpchar = '406'::bpchar) AND ((block)::bpchar = 'A'::bpchar))
Rows Removed by Filter: 199845
Execution time: 27.560 ms
(4 rows)
无法理解
- 为什么要扫描表格李>
- 为什么不使用索引
explain analyze select lata from lerg where npa='365' and nxx='406' and block='A';
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Index Scan using lerg_npa_nxx_block on lerg (cost=0.42..8.44 rows=1 width=6) (actual time=0.016..0.016 rows=1 loops=1)
Index Cond: (((npa)::text = '365'::text) AND ((nxx)::text = '406'::text) AND ((block)::text = 'A'::text))
Planning time: 0.081 ms
Execution time: 0.033 ms
(4 rows)
Q.Aselect
query与适当的索引比手工编制语句查询要好,为什么
Q.使用PREPARED
语句进行上述简单查询没有任何好处
仅仅领先一步,查询10000记录的差异如此之大,以至于我不得不放弃准备好的声明
explain analyze execute fetch_lata('365','406','A');
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Seq Scan on lerg (cost=0.00..5163.31 rows=1 width=6) (actual time=0.014..27.530 rows=1 loops=1)
Filter: (((npa)::bpchar = '365'::bpchar) AND ((nxx)::bpchar = '406'::bpchar) AND ((block)::bpchar = 'A'::bpchar))
Rows Removed by Filter: 199845
Execution time: 27.560 ms
(4 rows)
查询10000条记录的结果
user system total real
pg_without_prepared_statmt 0.050000 0.080000 0.130000 ( 0.935051)
pg_with_prepared_statmt 0.090000 0.110000 0.200000 ( 5.707693)
机器详细信息
Postgres Version : 9.5.9
Mac OS: 10.12.5
问题是,您准备的语句的参数类型为
character
(与bpchar
相同),因此将使用该类型的相等运算符,并且无法使用索引,因为它是使用字符变量定义的
如果将prepared语句的参数更改为varchar
,则它应该可以正常工作