openGauss每日一练第17天 | 学习心得体会

索引是一个指向表中数据的指针。一个数据库中的索引与一本书的索引目录是非常相似的。
索引可以用来提高数据库查询性能，但是不恰当的使用将导致数据库性能下降。

1.创建表，在表中创建索引

--为表test的testnum列创建一个索引
su - omm
gsql -r

drop table if exists test;
create table test(id serial primary key,testnum serial);

create index idx_test_testnum on test(testnum);

--查看索引
\di
omm=# \di
                      List of relations
 Schema |       Name       | Type  | Owner | Table | Storage 
--------+------------------+-------+-------+-------+---------
 public | idx_test_testnum | index | omm   | test  | 
 public | test_pkey        | index | omm   | test  | 
(2 rows)

2.通过hint使用索引

--测试准备，创建表customer，并插入数据
CREATE TABLE customer
(
ca_address_sk integer NOT NULL ,
ca_address_id character(16),
ca_street_number character(10) ,
ca_street_name character varying(60) ,
ca_street_type character(15) ,
ca_suite_number character(10) ,
ca_city character varying(60) ,
ca_county character varying(30) ,
ca_state character(2) ,
ca_zip character(10) ,
ca_country character varying(20) ,
ca_gmt_offset numeric(5,2) ,
ca_location_type character(20)
);
insert into customer values
(1, 'AAAAAAAABAAAAAAA', '18', 'Jackson', 'Parkway', 'Suite 280', 'Fairfield', 'Maricopa County', 'AZ', '86192' ,'United States', -7.00, 'condo'),
(2, 'AAAAAAAACAAAAAAA', '362', 'Washington 6th', 'RD', 'Suite 80', 'Fairview', 'Taos County', 'NM', '85709', 'United States', -7.00, 'condo'),
(3, 'AAAAAAAADAAAAAAA', '585', 'Dogwood Washington', 'Circle', 'Suite Q', 'Pleasant Valley', 'York County', 'PA', '12477', 'United States', -5.00, 'single family');

--创建索引

create index customer_idx on customer(ca_address_sk);

omm=# \di customer_idx 

omm=#                      List of relations
 Schema |     Name     | Type  | Owner |  Table   | Storage 
--------+--------------+-------+-------+----------+---------
 public | customer_idx | index | omm   | customer | 
(1 row)

--通过hint强制使用索引，查看执行计划

EXPLAIN SELECT /*+ indexscan(customer customer_idx ) */ 
 * FROM customer WHERE ca_address_sk<100;
                                  QUERY PLAN                                   
-------------------------------------------------------------------------------
 [Bypass]
 Index Scan using customer_idx on customer  (cost=0.00..8.27 rows=1 width=788)
   Index Cond: (ca_address_sk < 100)
(3 rows)
EXPLAIN SELECT * FROM customer WHERE ca_address_sk<100;

--正常查询
omm=# EXPLAIN SELECT * FROM customer WHERE ca_address_sk<100;
                        QUERY PLAN                        
----------------------------------------------------------
 Seq Scan on customer  (cost=0.00..1.04 rows=1 width=788)
   Filter: (ca_address_sk < 100)
(2 rows)

3.rename索引

ALTER INDEX idx_test_testnum RENAME TO idx_test_testnum_new;

omm=# \di idx_test_testnum_new                                                
                        List of relations
 Schema |         Name         | Type  | Owner | Table | Storage 
--------+----------------------+-------+-------+-------+---------
 public | idx_test_testnum_new | index | omm   | test  | 
(1 row)

4.重建索引

--重建一个单独索引
ALTER INDEX  idx_test_testnum_new REBUILD;

REINDEX INDEX  idx_test_testnum_new;
--重建所有索引
reindex table test;

5.移动索引到其他表空间

--创建表空间myindex_ts：
CREATE TABLESPACE myindex_ts RELATIVE LOCATION 'tablespace/myindex_ts1';

--将索引idx_test_testnum_new移动到表空间myindex_ts：
ALTER INDEX idx_test_testnum_new SET TABLESPACE myindex_ts;

--查看索引所在的表空间
select * from pg_indexes where tablename = 'test';
 schemaname | tablename |      indexname       | tablespace |                                       indexdef            
 public     | test      | test_pkey            |            | CREATE UNIQUE INDEX test_pkey ON test USING btree (id) TAB
LESPACE pg_default
 public     | test      | idx_test_testnum_new | myindex_ts | CREATE INDEX idx_test_testnum_new ON test USING btree (tes
tnum) TABLESPACE myindex_ts
(2 rows)
--或
select * from pg_indexes where indexname = 'idx_test_testnum_new';
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