場景 lower(name) like 'pf%'
create table users (id int primary key, name varchar(255));
Create or replace function random_string(length integer) returns text as
$$
declare
chars text[] := '{0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z}';
result text := '';
i integer := 0;
begin
if length 0 then
raise exception 'Given length cannot be less than 0';
end if;
for i in 1..length loop
result := result || chars[1+random()*(array_length(chars, 1)-1)];
end loop;
return result;
end;
$$ language plpgsql;
insert into users values(generate_series(1,50000), random_string(15));
普通bt：不走索引
pg_trgm模塊提供函數和操作符測定字母數字文本基于三元模型匹配的相似性，還有支持快速搜索相似字符串的索引操作符類。三元模型是一組從一個字符串中獲得的三個連續的字符。我們可以通過計數兩個字符串共享的三元模型的數量來測量它們的相似性。這個簡單的想法證明在測量許多自然語言詞匯的相似性時是非常有效的。
CREATE INDEX users_idx0 ON users (name);
全字匹配查詢（走索引）
explain select * from users where name='pfDNQVmhqDrF1EY';
QUERY PLAN
-------------------------------------------------------------------------
Index Scan using users_idx0 on users (cost=0.29..8.31 rows=1 width=20)
Index Cond: ((name)::text = 'pfDNQVmhqDrF1EY'::text)
(2 rows)
加函數全字匹配（不走索引）
explain select * from users where lower(name)='pfDNQVmhqDrF1EY';
QUERY PLAN
-----------------------------------------------------------
Seq Scan on users (cost=0.00..1069.00 rows=250 width=20)
Filter: (lower((name)::text) = 'pfDNQVmhqDrF1EY'::text)
(2 rows)
模糊匹配（不走索引）
explain select * from users where name like 'pf%';
QUERY PLAN
--------------------------------------------------------
Seq Scan on users (cost=0.00..944.00 rows=5 width=20)
Filter: ((name)::text ~~ 'pf%'::text)
explain select * from users where name like 'pf_';
QUERY PLAN
--------------------------------------------------------
Seq Scan on users (cost=0.00..944.00 rows=5 width=20)
Filter: ((name)::text ~~ 'pf_'::text)
字段帶函數的bt索引：函數走索引
drop index users_idx0;
CREATE INDEX users_dex1 ON users (lower(name));
加函數全字匹配（走索引）
explain select * from users where lower(name)='pfDNQVmhqDrF1EY';
QUERY PLAN
---------------------------------------------------------------------------
Bitmap Heap Scan on users (cost=6.23..324.34 rows=250 width=20)
Recheck Cond: (lower((name)::text) = 'pfDNQVmhqDrF1EY'::text)
-> Bitmap Index Scan on users_dex1 (cost=0.00..6.17 rows=250 width=0)
Index Cond: (lower((name)::text) = 'pfDNQVmhqDrF1EY'::text)
(4 rows)
模糊匹配（不走索引）
explain select * from users where lower(name) like 'pf%';
QUERY PLAN
-----------------------------------------------------------
Seq Scan on users (cost=0.00..1069.00 rows=250 width=20)
Filter: (lower((name)::text) ~~ 'pf%'::text)
(2 rows)
聲明操作符類的bt索引：like走索引
定義索引的同時可以為索引的每個字段聲明一個操作符類。
CREATE INDEX name ON table (column opclass [sort options] [, …]);
這個操作符類指明該索引用于該字段時要使用的操作符。
CREATE INDEX users_dex2 ON users (lower(name) varchar_pattern_ops);
模糊匹配（走索引）
explain select * from users where lower(name) like 'pf%';
QUERY PLAN
------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on users (cost=4.82..144.00 rows=5 width=20)
Filter: (lower((name)::text) ~~ 'pf%'::text)
-> Bitmap Index Scan on users_dex2 (cost=0.00..4.82 rows=53 width=0)
Index Cond: ((lower((name)::text) ~>=~ 'pf'::text) AND (lower((name)::text) ~~ 'pg'::text))
(4 rows)
場景2 name like '%pf%'
Create or replace function random_string(length integer) returns text as
$$
declare
chars text[] := '{0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z}';
result text := '';
i integer := 0;
begin
if length 0 then
raise exception 'Given length cannot be less than 0';
end if;
for i in 1..length loop
result := result || chars[1+random()*(array_length(chars, 1)-1)];
end loop;
return result;
end;
$$ language plpgsql;
create table users (id int primary key, name varchar(255));
insert into users values(generate_series(1,50000), random_string(15));
聲明操作符bt：不走索引
CREATE INDEX idx_name ON users USING btree (lower(name) varchar_pattern_ops);
explain (analyze true,format yaml, verbose true, buffers true) select * from users where lower(name) like '%pf%';\
QUERY PLAN
-----------------------------------------------------------
- Plan: +
Node Type: "Seq Scan" +
Parallel Aware: false +
Relation Name: "users" +
Schema: "public" +
Alias: "users" +
Startup Cost: 0.00 +
Total Cost: 1069.00 +
Plan Rows: 5 +
Plan Width: 20 +
Actual Startup Time: 0.320 +
Actual Total Time: 86.841 +
Actual Rows: 710 +
Actual Loops: 1 +
Output: +
- "id" +
- "name" +
Filter: "(lower((users.name)::text) ~~ '%pf%'::text)"+
Rows Removed by Filter: 49290 +
Shared Hit Blocks: 319 +
Shared Read Blocks: 0 +
Shared Dirtied Blocks: 0 +
Shared Written Blocks: 0 +
Local Hit Blocks: 0 +
Local Read Blocks: 0 +
Local Dirtied Blocks: 0 +
Local Written Blocks: 0 +
Temp Read Blocks: 0 +
Temp Written Blocks: 0 +
Planning Time: 0.188 +
Triggers: +
Execution Time: 86.975
聲明pg_trgm操作符bt：可以走索引
CREATE EXTENSION pg_trgm;
CREATE INDEX idx_users_name_trgm_gist ON users USING gist (name gist_trgm_ops);
explain (analyze true, verbose true, buffers true) select * from users where name like '%pf%';
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on public.users (cost=32.19..371.08 rows=505 width=20) (actual time=19.314..53.132 rows=193 loops=1)
Output: id, name
Recheck Cond: ((users.name)::text ~~ '%pf%'::text)
Rows Removed by Index Recheck: 49807
Heap Blocks: exact=319
Buffers: shared hit=972
-> Bitmap Index Scan on idx_users_name_trgm_gist (cost=0.00..32.06 rows=505 width=0) (actual time=19.175..19.175 rows=50000 loops=1)
Index Cond: ((users.name)::text ~~ '%pf%'::text)
Buffers: shared hit=653
Planning time: 0.188 ms
Execution time: 53.231 ms
(11 rows)
以上為個人經驗，希望能給大家一個參考，也希望大家多多支持腳本之家。如有錯誤或未考慮完全的地方，望不吝賜教。
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