postgresql – 为什么真空分析改变查询计划而分析没有？

我想在Postgres中利用仅索引扫描的强大功能,并尝试使用一个表：

CREATE TABLE dest.contexts
(
  id integer NOT NULL,phrase_id integer NOT NULL,lang character varying(5) NOT NULL,ranking_value double precision,index_min integer,index_max integer,case_sensitive boolean,is_enabled boolean,is_to_sync boolean NOT NULL DEFAULT true
);

insert into dest.contexts select * from source.contexts;

alter table dest.contexts
  add constraint pk_contexts primary key (id,phrase_id,lang);

 CREATE INDEX idx_contexts_
  ON dest.contexts
  USING btree
  (id,is_enabled,lang,ranking_value,index_min,index_max,case_sensitive);

索引涵盖了我想在下一个查询中使用的所有列：

explain analyze
select ranking_value,case_sensitive
from dest.contexts
where id = 456 and is_enabled

我在创建后立即检查计划：

Bitmap Heap Scan on contexts  (cost=4.41..31.46 rows=12 width=17) (actual time=0.045..0.045 rows=0 loops=1)
  Recheck Cond: (id = 456)
  Filter: is_enabled
  ->  Bitmap Index Scan on idx_contexts_  (cost=0.00..4.40 rows=12 width=0) (actual time=0.038..0.038 rows=0 loops=1)
        Index Cond: ((id = 456) AND (is_enabled = true))
Planning time: 0.631 ms
Execution time: 0.093 ms

这很奇怪,但还行……

几秒钟后它变成了另一个(autovacuum？)

Index Scan using pk_contexts on contexts  (cost=0.28..17.93 rows=6 width=17) (actual time=0.027..0.027 rows=0 loops=1)
  Index Cond: (id = 456)
  Filter: is_enabled
Planning time: 0.185 ms
Execution time: 0.070 ms

我尝试强制它使用仅索引扫描：

analyze dest.contexts

但它没有改变任何东西.然后我做

vacuum verbose analyze dest.contexts;

INFO:  vacuuming "dest.contexts"
INFO:  index "pk_contexts" Now contains 4845 row versions in 21 pages
DETAIL:  0 index row versions were removed.
0 index pages have been deleted,0 are currently reusable.
cpu 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  index "idx_contexts_" Now contains 4845 row versions in 37 pages
DETAIL:  0 index row versions were removed.
0 index pages have been deleted,0 are currently reusable.
cpu 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  "contexts": found 0 removable,4845 nonremovable row versions in 41 out of 41 pages
DETAIL:  0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
cpu 0.00s/0.00u sec elapsed 0.00 sec.
INFO:  analyzing "dest.contexts"
INFO:  "contexts": scanned 41 of 41 pages,containing 4845 live rows and 0 dead rows; 4845 rows in sample,4845 estimated total rows

在这里,我终于得到了我想要的东西：

Index Only Scan using idx_contexts_ on contexts  (cost=0.28..4.40 rows=6 width=17) (actual time=0.014..0.014 rows=0 loops=1)
  Index Cond: ((id = 456) AND (is_enabled = true))
  Filter: is_enabled
  Heap Fetches: 0
Planning time: 0.247 ms
Execution time: 0.052 ms

以下是问题：

>为什么不分析教它使用大的“全覆盖”指数？
>为什么真空分析会这样做？
>我的桌子从头开始用一个大插件填充.真空为什么要做任何事情？在我看来,那里没有任何真空.

Analyze负责收集统计数据,并分析对查询计划和计划成本的选择有影响.

索引不包含有关元组的可见性信息.因此,虽然select中的所有列都在索引中,但也会应用读取数据页和对结果集进行过滤.如果页面中显示所有元组,Postgres不需要额外的过滤操作. Postgres使用可见性图(vm)来实现这一点.

真空使用并更新可见性图,同时回收死元组.因此,如果可能,vacuum会更改查询计划以使用仅索引扫描.

https://www.postgresql.org/docs/9.6/static/sql-vacuum.html

https://www.postgresql.org/docs/9.6/static/storage-vm.html

原文地址：https://www.jb51.cc/postgresql/192076.html