为什么PREPARE语句不尊重INDEXing

Question

帮我理解为什么使用预处理语句评估的查询不尊重索引。

\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 

预处理语句

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。

• 为什么表扫描？
• 为什么索引不被使用？

在其他说明。

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) 

问：一个select查询适当的索引击败倒手准备的语句查询，为什么呢？

问：对于上述简单查询，使用PREPARED语句没有优势吗？

只是一个头的查询10_000记录的差异是如此之高，我不得不放弃准备好的声明。

查询10_000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) 

机器细节。

Postgres Version : 9.5.9 
Mac OS: 10.12.5 

Answer 1

的问题是，你准备语句的参数是character型（同bpchar）的，因此使用该类型等于运算符，因为它与character varying规定不能使用的索引。

如果将准备好的语句的参数更改为varchar，它应该按预期工作。