案例：建立合适的索引

现象描述

查询与销售部所有员工的信息：

1. SELECT staff_id,first_name,last_name,employment_id,state_name,city
2. FROM staffs,sections,states,places
3. WHERE sections.section_name='Sales'
4. AND staffs.section_id = sections.section_id
5. AND sections.place_id = places.place_id
6. AND places.state_id = states.state_id
7. ORDER BY staff_id;

优化分析

在优化前，没有创建places.place_id和states.state_id索引，执行计划如下：

1. QUERY PLAN
2. ---------------------------------------------------------------------------------------------------
3. Sort  (cost=129.74..131.18 rows=576 width=136)
4. Sort Key: staffs.staff_id
5. ->  Hash Join  (cost=70.54..103.33 rows=576 width=136)
6. Hash Cond: (states.state_id = places.state_id)
7. ->  Seq Scan on states  (cost=0.00..22.38 rows=1238 width=36)
8. ->  Hash  (cost=69.38..69.38 rows=93 width=108)
9. ->  Hash Join  (cost=42.41..69.38 rows=93 width=108)
10. Hash Cond: (places.place_id = sections.place_id)
11. ->  Seq Scan on places  (cost=0.00..21.67 rows=1167 width=40)
12. ->  Hash  (cost=42.21..42.21 rows=16 width=76)
13. ->  Hash Join  (cost=24.66..42.21 rows=16 width=76)
14. Hash Cond: (staffs.section_id = sections.section_id)
15. ->  Seq Scan on staffs  (cost=0.00..15.37 rows=537 width=76)
16. ->  Hash  (cost=24.59..24.59 rows=6 width=8)
17. ->  Seq Scan on sections  (cost=0.00..24.59 rows=6 width=8)
18. Filter: (section_name = 'Sales'::text)
19. (16 rows)

建议在places.place_id和states.state_id列上建立2个索引，执行计划如下：

1. QUERY PLAN
2. -----------------------------------------------------------------------------------------------------------
3. Sort  (cost=119.76..121.20 rows=576 width=136)
4. Sort Key: staffs.staff_id
5. ->  Hash Join  (cost=70.14..93.35 rows=576 width=136)
6. Hash Cond: (staffs.section_id = sections.section_id)
7. ->  Seq Scan on staffs  (cost=0.00..15.37 rows=537 width=76)
8. ->  Hash  (cost=67.43..67.43 rows=217 width=68)
9. ->  Nested Loop  (cost=24.66..67.43 rows=217 width=68)
10. ->  Hash Join  (cost=24.66..51.06 rows=35 width=40)
11. Hash Cond: (places.place_id = sections.place_id)
12. ->  Seq Scan on places  (cost=0.00..21.67 rows=1167 width=40)
13. ->  Hash  (cost=24.59..24.59 rows=6 width=8)
14. ->  Seq Scan on sections  (cost=0.00..24.59 rows=6 width=8)
15. Filter: (section_name = 'Sales'::text)
16. ->  Index Scan using states_state_id_idx on states  (cost=0.00..0.41 rows=6 width=36)
17. Index Cond: (state_id = places.state_id)
18. (15 rows)